DELL COMPUTERS

Dell, Inc. is a multinational company based in Round Rock, Texas which develops, manufactures, sells, and supports personal computers and other computer-related products. As of 2008, Dell employs more than 88,000 people worldwide.[3]
Dell grew during the 1980s and 1990s to become (for a time) the largest seller of PCs and servers. As of 2008, it held the second spot in computer-sales within the industry behind the Hewlett-Packard Company. The company currently sells personal computers, servers, data storage devices, network switches, software, computer peripherals and televisions.
In 2006, Fortune magazine ranked Dell as the 25th-largest company in the Fortune 500 list, 8th on its annual Top 20 list of the most-admired companies in the United States. In 2007 Dell ranked 34th and 8th respectively on the equivalent lists for the year. A 2006 publication identified Dell as one of 38 high-performance companies in the S&P 500 which had consistently out-performed the market over

POCKET PC'S

A Pocket PC, abbreviated P/PC or PPC, is a hardware specification for a handheld-sized computer (Personal digital assistant) that runs the Microsoft Windows Mobile operating system. It may have the capability to run an alternative operating system like NetBSD, Linux, Android or others. It has many of the capabilities of modern desktop PCs.
Currently there are thousands of applications for handhelds adhering to the Microsoft Pocket PC specification, many of which are freeware. Some of these devices also include mobile phone features. Microsoft compliant Pocket PCs can also be used with many other add-ons like GPS receivers, barcode readers, RFID readers, and cameras.
In 2007, with the advent of Windows Mobile 6, Microsoft dropped the name Pocket PC in favor of a new naming scheme. Devices without an integrated phone are called Windows Mobile Classic instead of Pocket PC. Devices with an integrated phone and a touch screen are called Windows Mobile
OS versions
Main article: Windows Mobile
Windows Mobile 6.1
Microsoft's current release is Windows Mobile 6.1, one of the major changes from WM6 is the introduction of Instant messaging-like texting. Press release on April 1st 2008.

Windows Mobile 6
Microsoft's Windows Mobile 6, internally code-named "Crossbow'". It was officially released by Microsoft on February 12, 2007.

Windows Mobile 5
Windows Mobile 5.0 marked the convergence of the Phone Edition and Professional Edition operating systems into one system that contains both phone and PDA capabilities. A 'Phone' application was included in the OS, and all PIM applications were updated to interface with it. Windows Mobile 5.0 was compatible with Microsoft's Smartphone operating system and was capable of running Smartphone applications.
Pocket PCs running previous versions of the operating system generally stored user-installed applications and data in RAM, which meant that if the battery was depleted the device would lose all of its data. Windows Mobile 5.0 solved this problem by storing all user data in persistent (flash) memory, leaving the RAM to be used only for running applications, as it would be on a desktop computer. As a result, Windows Mobile 5.0 Pocket PCs generally had a greater amount of flash memory, and a smaller amount of RAM, compared to earlier devices

Windows Mobile 2003
Windows Mobile 2003 consisted of the Windows CE.NET 4.2 operating system bundled with scaled-down versions of many popular desktop applications, including Microsoft Outlook, Internet Explorer, Word, Excel, Windows Media Player, and others.
Windows Mobile 2003 Second Edition added native landscape support as well as other fixes and changes to those features already present in the original release of Windows Mobile 2003.

Pocket PC 2000 and 2002
Pocket PC 2002 (launched October 2001) and Pocket PC 2000 (launched April 2000) both ran Windows CE 3.0 underneath.

BIPOLAR JUBCTION TRANSISTER

BJT redirects here. For the Japanese language proficiency test, see Business Japanese Proficiency Test.
A bipolar junction transistor (BJT) is a type of transistor. It is a three-terminal device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons and holes.
An NPN transistor can be considered as two diodes with a shared anode region. In typical operation, the emitter–base junction is forward biased and the base–collector junction is reverse biased. In an NPN transistor, for example, when a positive voltage is applied to the base–emitter junction, the equilibrium between thermally generated carriers and the repelling electric field of the depletion region becomes unbalanced, allowing thermally excited electrons to inject into the base region. These electrons wander (or "diffuse") through the base from the region of high concentration near the emitter towards the region of low concentration near the collector. The electrons in the base are called minority carriers because the base is doped p-type which would make holes the majority carrier in the base.
The base region of the transistor must be made thin, so that carriers can diffuse across it in much less time than the semiconductor's minority carrier lifetime, to minimize the percentage of carriers that recombine before reaching the collector–base junction. To ensure this, the thickness of the base is much less than the diffusion length of the electrons. The collector–base junction is reverse-biased, so little electron injection occurs from the collector to the base, but electrons that diffuse through the base towards the collector are swept into the collector by the electric field in the depletion region of the collector–base juncti
Voltage, current, and charge control
The collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the base–emitter voltage (voltage control). These views are related by the current–voltage relation of the base–emitter junction, which is just the usual exponential current–voltage curve of a p-n junction (diode).[1]
The physical explanation for collector current is the amount of minority-carrier charge in the base region.[1][2][3] Detailed models of transistor action, such as the Gummel–Poon model, account for the distribution of this charge explicitly to explain transistor behavior more exactly. The charge-control view easily handles photo-transistors, where minority carriers in the base region are created by the absorption of photons, and handles the dynamics of turn-off, or recovery time, which depends on charge in the base region recombining. However, since base charge is not a signal that is visible at the terminals, the current- and voltage-control views are usually used in circuit design and analysis.
In analog circuit design, the current-control view is sometimes used since it is approximately linear. That is, the collector current is approximately βF times the base current. Some basic circuits can be designed by assuming that the emitter–base voltage is approximately constant, and that collector current is beta times the base current. However, to accurately and reliably design production bjt circuits, the voltage-control (for example, Ebers–Moll) model is required[1] The voltage-control model requires an exponential function to be taken into account, but when it is linearized such that the transistor can be modelled as a transconductance, as in the Ebers–Moll model, design for circuits such as differential amplifiers again becomes a mostly linear problem, so the voltage-control view is often preferred. For translinear circuits, in which the exponential I–V curve is key to the operation, the transistors are usually modelled as voltage controlled with transconductance proportional to collector current. In general, transistor level circuit design is performed using SPICE or a comparable analogue circuit simulator, so model complexity is usually not of much concern to the designer.

Transistor 'alpha' and 'beta'
The proportion of electrons able to cross the base and reach the collector is a measure of the BJT efficiency. The heavy doping of the emitter region and light doping of the base region cause many more electrons to be injected from the emitter into the base than holes to be injected from the base into the emitter. The common-emitter current gain is represented by βF or hfe. It is approximately the ratio of the DC collector current to the DC base current in forward-active region, and is typically greater than 100. Another important parameter is the common-base current gain, αF. The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.98 and 0.998. Alpha and beta are more precisely related by the following identities (NPN transistor):

Structure

Simplified cross section of a planar NPN bipolar junction transistor

Die of a KSY34 high-frequency NPN transistor, base and emitter connected via bonded wires
A BJT consists of three differently doped semiconductor regions, the emitter region, the base region and the collector region. These regions are, respectively, p type, n type and p type in a PNP, and n type, p type and n type in a NPN transistor. Each semiconductor region is connected to a terminal, appropriately labeled: emitter (E), base (B) and collector (C).
The base is physically located between the emitter and the collector and is made from lightly doped, high resistivity material. The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape being collected, thus making the resulting value of α very close to unity, and so, giving the transistor a large β. A cross section view of a BJT indicates that the collector–base junction has a much larger area than the emitter–base junction.
The bipolar junction transistor, unlike other transistors, is usually not a symmetrical device. This means that interchanging the collector and the emitter makes the transistor leave the forward active mode and start to operate in reverse mode. Because the transistor's internal structure is usually optimized to forward-mode operation, interchanging the collector and the emitter makes the values of α and β in reverse operation much smaller than those found in forward operation; often the α of the reverse mode is lower than 0.5. The lack of symmetry is primarily due to the doping ratios of the emitter and the collector. The emitter is heavily doped, while the collector is lightly doped, allowing a large reverse bias voltage to be applied before the collector–base junction breaks down. The collector–base junction is reverse biased in normal operation. The reason the emitter is heavily doped is to increase the emitter injection efficiency: the ratio of carriers injected by the emitter to those injected by the base. For high current gain, most of the carriers injected into the emitter–base junction must come from the emitter.
The low-performance "lateral" bipolar transistors sometimes used in CMOS processes are sometimes designed symmetrically, that is, with no difference between forward and backward operation.
Small changes in the voltage applied across the base–emitter terminals causes the current that flows between the emitter and the collector to change significantly. This effect can be used to amplify the input voltage or current. BJTs can be thought of as voltage-controlled current sources, but are more simply characterized as current-controlled current sources, or current amplifiers, due to the low impedance at the base.
Early transistors were made from germanium but most modern BJTs are made from silicon. A significant minority are also now made from gallium arsenide, especially for very high speed applications (see HBT, below).

NPN

The symbol of an NPN Bipolar Junction Transistor.
NPN is one of the two types of bipolar transistors, in which the letters "N" and "P" refer to the majority charge carriers inside the different regions of the transistor. Most bipolar transistors used today are NPN, because electron mobility is higher than hole mobility in semiconductors, allowing greater currents and faster operation.
NPN transistors consist of a layer of P-doped semiconductor (the "base") between two N-doped layers. A small current entering the base in common-emitter mode is amplified in the collector output. In other terms, an NPN transistor is "on" when its base is pulled high relative to the emitter.
The arrow in the NPN transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode.
One mnemonic device for identifying the symbol for the NPN transistor is "not pointing in".[5]

PNP
The other type of BJT is the PNP with the letters "P" and "N" referring to the majority charge carriers inside the different regions of the transistor.

The symbol of a PNP Bipolar Junction Transistor.
PNP transistors consist of a layer of N-doped semiconductor between two layers of P-doped material. A small current leaving the base in common-emitter mode is amplified in the collector output. In other terms, a PNP transistor is "on" when its base is pulled low relative to the emitter.
The arrow in the PNP transistor symbol is on the emitter leg and points in the direction of the conventional current flow when the device is in forward active mode.
One mnemonic device for identifying the symbol for the PNP transistor is "points in proudly".[5]
Heterojunction bipolar transistor

Bands in graded heterojunction npn bipolar transistor. Barriers indicated for electrons to move from emitter to base, and for holes to be injected backward from base to emitter; Also, grading of bandgap in base assists electron transport in base region; Light colors indicate depleted regions
The heterojunction bipolar transistor (HBT) is an improvement of the BJT that can handle signals of very high frequencies up to several hundred GHz. It is common nowadays in ultrafast circuits, mostly RF systems.[6][7] Heterojunction transistors have different semiconductors for the elements of the transistor. Usually the emitter is composed of a larger bandgap material than the base. The figure shows that this difference in bandgap allows the barrier for holes to inject backward into the base, denoted in figure as Δφp, to be made large, while the barrier for electrons to inject into the base Δφn is made low. This barrier arrangement helps reduce minority carrier injection from the base when the emitter-base junction is under forward bias, and thus reduces base current and increases emitter injection efficiency.
The improved injection of carriers into the base allows the base to have a higher doping level, resulting in lower resistance to access the base electrode. In the more traditional BJT, also referred to as homojunction BJT, the efficiency of carrier injection from the emitter to the base is primarily determined by the doping ratio between the emitter and base, which means the base must be lightly doped to obtain high injection efficiency, making its resistance relatively high. In addition, higher doping in the base can improve figures of merit like the Early voltage by lessening base narrowing.
The grading of composition in the base, for example, by progressively increasing the amount of germanium in a SiGe transistor, causes a gradient in bandgap in the neutral base, denoted in the figure by ΔφG, providing a "built-in" field that assists electron transport across the base. That drift component of transport aids the normal diffusive transport, increasing the frequency response of the transistor by shortening the transit time across the base.
Two commonly used HBTs are silicon–germanium and aluminum gallium arsenide, though a wide variety of semiconductors may be used for the HBT structure. HBT structures are usually grown by epitaxy techniques like MOCVD and MBE.

RANDOM ACCESS MEMORY

Random access memory (usually known by its acronym, RAM) is a type of computer data storage. Today it takes the form of integrated circuits that allow the stored data to be accessed in any order, i.e. at random. The word random thus refers to the fact that any piece of data can be returned in a constant time, regardless of its physical location and whether or not it is related to the previous piece of data.[1]
This contrasts with storage mechanisms such as tapes, magnetic discs and optical discs, which rely on the physical movement of the recording medium or a reading head. In these devices, the movement takes longer than the data transfer, and the retrieval time varies depending on the physical location of the next item.
Types of RAM
Modern types of writable RAM generally store a bit of data in either the state of a flip-flop, as in SRAM (static RAM), or as a charge in a capacitor (or transistor gate), as in DRAM (dynamic RAM), EPROM, EEPROM and Flash. Some types have circuitry to detect and/or correct random faults called memory errors in the stored data, using parity bits or error correction codes. RAM of the read-only type, ROM, instead uses a metal mask to permanently enable/disable selected transistors, instead of storing a charge in them.
As both SRAM and DRAM are volatile, other forms of computer storage, such as disks and magnetic tapes, have been used as "permanent" storage in traditional computers. Many newer products instead rely on flash memory to maintain data between sessions of use: examples include PDAs, small music players, mobile phones, synthesizers, advanced calculators, industrial instrumentaion and robotics, and many other types of products; even certain categories of personal computers, such as the OLPC XO-1, Asus Eee PC, and others, have begun replacing magnetic disk with so called flash drives (similar to fast memory cards equipped with an IDE or SATA interface).
There are two basic types of flash memory: the NOR type, which is capable of true random access, and the NAND type, which is not; the former is therefore often used in place of ROM, while the latter is used in most memory cards and solid-state drives, due to a lower price.

Memory hierarchy

One module of 128MB NEC SD-RAM.
Many computer systems have a memory hierarchy consisting of CPU registers, on-die SRAM caches, external caches, DRAM, paging systems, and virtual memory or swap space on a hard drive. This entire pool of memory may be referred to as "RAM" by many developers, even though the various subsystems can have very different access times, violating the original concept behind the random access term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank, rank, channel, or interleave organization of the components make the access time variable, although not to the extent that rotating storage media or a tape is variable. (Generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom.)
In most modern personal computers, the RAM comes in easily upgraded form of modules called memory modules or DRAM modules about the size of a few sticks of chewing gum. These can quickly be replaced should they become damaged or too small for current purposes. As suggested above, smaller amounts of RAM (mostly SRAM) are also integrated in the CPU and other ICs on the motherboard, as well as in hard-drives, CD-ROMs, and several other parts of the computer system.

Swapping
If a computer becomes low on RAM during intensive application cycles, the computer can resort to swapping. In this case, the computer temporarily uses hard drive space as additional memory. Constantly relying on this type of backup memory is called thrashing, which is generally undesirable because it lowers overall system performance. In order to reduce the dependency on swapping, more RAM can be installed.

Other uses of the term
Other physical devices with read/write capability can have "RAM" in their names: for example, DVD-RAM. "Random access" is also the name of an indexing method: hence, disk storage is often called "random access" because the reading head can move relatively quickly from one piece of data to another, and does not have to read all the data in between. However the final "M" is crucial: "RAM" (provided there is no additional term as in "DVD-RAM") always refers to a solid-state device.

"RAM disks"
Software can "partition" a portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a RAM disk. Unless the memory used is non-volatile, a RAM disk loses the stored data when the computer is shut down. However, volatile memory can retain its data when the computer is shut down if it has a separate power source, usually a battery.

Recent developments
Several new types of non-volatile RAM, which will preserve data while powered down, are under development. The technologies used include carbon nanotubes and the magnetic tunnel effect. In summer 2003, a 128 KB magnetic RAM chip manufactured with 0.18 µm technology was introduced. The core technology of MRAM is based on the magnetic tunnel effect. In June 2004, Infineon Technologies unveiled a 16 MB prototype again based on 0.18 µm technology. Nantero built a functioning carbon nanotube memory prototype 10 GB array in 2004. Whether some of these technologies will be able to eventually take a significant market share from either DRAM, SRAM, or flash-memory technology, remains to be seen however.
In 2006, "Solid-state drives" (based on flash memory) with capacities exceeding 150 gigabytes and speeds far exceeding traditional disks have become available. This development has started to blur the definition between traditional random access memory and "disks", dramatically reducing the difference in performance.

Memory wall
The "memory wall" is the growing disparity of speed between CPU and memory outside the CPU chip. An important reason for this disparity is the limited communication bandwidth beyond chip boundaries. From 1986 to 2000, CPU speed improved at an annual rate of 55% while memory speed only improved at 10%. Given these trends, it was expected that memory latency would become an overwhelming bottleneck in computer performance. [2]
Currently, CPU speed improvements have slowed significantly partly due to major physical barriers and partly because current CPU designs have already hit the memory wall in some sense. Intel summarized these causes in their Platform 2015 documentation (PDF):
“First of all, as chip geometries shrink and clock frequencies rise, the transistor leakage current increases, leading to excess power consumption and heat (more on power consumption below). Secondly, the advantages of higher clock speeds are in part negated by memory latency, since memory access times have not been able to keep pace with increasing clock frequencies. Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster (due to the so-called Von Neumann bottleneck), further undercutting any gains that frequency increases might otherwise buy. In addition, partly due to limitations in the means of producing inductance within solid state devices, resistance-capacitance (RC) delays in signal transmission are growing as feature sizes shrink, imposing an additional bottleneck that frequency increases don't address.”
The RC delays in signal transmission were also noted in Clock Rate versus IPC: The End of the Road for Conventional Microarchitectures which projects a maximum of 12.5% average annual CPU performance improvement between 2000 and 2014. The data on Intel Processors clearly shows a slowdown in performance improvements in recent processors. However, Intel's new processors, Core 2 Duo (codenamed Conroe) show a significant improvement over previous Pentium 4 processors; due to a more efficient architecture, performance increased while clock rate actually decreased.

MOTHERBOARD

A motherboard is the central or primary printed circuit board (PCB) making up a complex electronic system, such as a modern computer. It is also known as a mainboard, baseboard, system board, planar board, or, on Apple computers, a logic board, and is sometimes abbreviated casually as mobo.[1]
Most motherboards produced today are designed for so-called IBM-compatible computers, which held over 96% of the global personal computer market in 2005.[2] Motherboards for IBM-compatible computers are specifically covered in the PC motherboard article.
A motherboard, like a backplane, provides the electrical connections by which the other components of the system communicate, but unlike a backplane also contains the central processing unit and other subsystems such as real time clock, and some peripheral interfaces Components and functions

The 2004 K7VT4A Pro[3] motherboard by ASRock. The chipset on this board consists of northbridge and southbridge chips.
The motherboard of a typical desktop consists of a large printed circuit board. It holds electronic components and interconnects, as well as physical connectors (sockets, slots, and headers) into which other computer components may be inserted or attached.
Most motherboards include, at a minimum:
sockets (or slots) in which one or more microprocessors (CPUs) are installed[4]
slots into which the system's main memory is installed (typically in the form of DIMM modules containing DRAM chips)
a chipset which forms an interface between the CPU's front-side bus, main memory, and peripheral buses
non-volatile memory chips (usually Flash ROM in modern motherboards) containing the system's firmware or BIOS
a clock generator which produces the system clock signal to synchronize the various components
slots for expansion cards (these interface to the system via the buses supported by the chipset)
power connectors and circuits, which receive electrical power from the computer power supply and distribute it to the CPU, chipset, main memory, and expansion cards.[5]

The Octek Jaguar V motherboard from 1993.[6] This board has 6 ISA slots but few onboard peripherals, as evidenced by the lack of external connectors.
Additionally, nearly all motherboards include logic and connectors to support commonly-used input devices, such as PS/2 connectors for a mouse and keyboard. Early personal computers such as the Apple II or IBM PC included only this minimal peripheral support on the motherboard. Occasionally video interface hardware was also integrated into the motherboard; for example on the Apple II, and rarely on IBM-comatible computers such as the IBM PC Jr. Additional peripherals such as disk controllers and serial ports were provided as expansion cards.
Given the high thermal design power of high-speed computer CPUs and components, modern motherboards nearly always include heatsinks and mounting points for fans to dissipate excess heat.

Integrated peripherals

Diagram of a modern motherboard, which supports many on-board peripheral functions as well as several expansion slots.
With the steadily declining costs and size of integrated circuits, it is now possible to include support for many peripherals on the motherboard. By combining many functions on one PCB, the physical size and total cost of the system may be reduced; highly-integrated motherboards are thus especially popular in small form factor and budget computers.
For example, the ECS RS485M-M,[7] a typical modern budget motherboard for computers based on AMD processors, has on-board support for a very large range of peripherals:
disk controllers for a floppy disk drive, up to 2 PATA drives, and up to 6 SATA drives (including RAID 0/1 support)
integrated ATI Radeon graphics controller supporting 2D and 3D graphics, with VGA and TV output
integrated sound card supporting 8-channel (7.1) audio and S/PDIF output
fast Ethernet network controller for 10/100 Mbit networking
USB 2.0 controller supporting up to 12 USB ports
IrDA controller for infrared data communication (e.g. with an IrDA enabled Cellular Phone or Printer)
temperature, voltage, and fan-speed sensors that allow software to monitor the health of computer components
Expansion cards to support all of these functions would have cost hundreds of dollars even a decade ago, however as of April 2007 such highly-integrated motherboards are available for as little as $30 in the USA.
Temperature and reliability
Motherboards are generally air cooled with heat sinks often mounted on larger chips, such as the northbridge, in modern motherboards. Passive cooling, or a single fan mounted on the power supply, was sufficient for many desktop computer CPUs until the late 1990s; since then, most have required CPU fans mounted on their heatsinks, due to rising clock speeds and power consumption. Most motherboards have connectors for additional case fans as well. Newer motherboards have integrated temperature sensors to detect motherboard and CPU temperatures, and controllable fan connectors which the BIOS or operating system can use to regulate fan speed.
Some small form factor computers and home theater PCs designed for quiet and energy-efficient operation boast fan-less designs. This typically requires the use of a low-power CPU, as well as careful layout of the motherboard and other components to allow for heat sink placement.
A 2003 study[8] found that some spurious computer crashes and general reliability issues, ranging from screen image distortions to I/O read/write errors, can be attributed not to software or peripheral hardware but to aging capacitors on PC motherboards. Ultimately this was shown to be the result of a faulty electrolyte formulation.[9]
For more information on premature capacitor failure on PC motherboards, see capacitor plague.
Motherboards use electrolytic capacitors to filter the DC power distributed around the board. These capacitors age at a temperature-dependent rate, as their water based electrolytes slowly evaporate. This can lead to loss of capacitance and subsequent motherboard malfunctions due to voltage instabilities. While most capacitors are rated for 2000 hours of operation at 105 °C,[10] their expected design life roughly doubles for every 10 °C below this. At 45 °C a lifetime of 15 years can be expected. This appears reasonable for a computer motherboard, however many manufacturers have delivered substandard capacitors, which significantly reduce this life expectancy. Inadequate case cooling and elevated temperatures easily exacerbate this problem. It is possible, but tedious and time-consuming, to find and replace failed capacitors on PC motherboards; it is less expensive to buy a new motherboard than to pay for such a repair.

iPHONES

The iPhone is an Internet-enabled multimedia mobile phone designed and marketed by Apple Inc. It has a multi-touch screen with virtual keyboard and buttons, but a minimal amount of hardware input. The iPhone's functions include those of a camera phone and portable media player (equivalent to the iPod) in addition to text messaging and visual voicemail. It also offers Internet services including e-mail, web browsing, and local Wi-Fi connectivity. The first generation phone hardware was quad-band GSM with EDGE; the second generation uses UMTS and HSDPA.[29]
Screen and interface
The 9 cm (3.5 in) liquid crystal display (320×480 px at 6.3 px/mm, 160 ppi) HVGA touchscreen with scratch-resistant glass[33] is specifically created for use with a finger, or multiple fingers for multi-touch sensing. Because the screen is a capacitive touchscreen, bare skin is required; a stylus or a normal glove prevents the necessary electrical conductivity.[34][35][36][37]
Almost all input is given through the touch screen, which understands complex gestures using multi-touch. The iPhone user interface enables the user to move the content itself up or down by a touch-drag motion of the finger. For example, zooming in and out of web pages and photos is done by placing two fingers on the screen and spreading them farther apart or closer together. Similarly, scrolling through a long list in a menu works as if the list is pasted on the outer surface of a wheel: the wheel can be "spun" by sliding a finger over the display from bottom to top (or vice versa). In either case, the list continues to move based on the flicking motion of the finger, slowly decelerating as if affected by friction. In this way, the interface simulates the physics of a real 3D object. There are other visual effects, such as horizontally sliding sub-selections and co-selections from right and left, vertically sliding system menus from the bottom (e.g. favorites, keyboard), and menus and widgets that turn around to allow settings to be configured on their back sides.
The display responds to three sensors. A proximity sensor shuts off the display and touchscreen when the iPhone is brought near the face to save battery power and to prevent inadvertent inputs from the user's face and ears. An ambient light sensor adjusts the display brightness which in turn saves battery power. A 3-axis accelerometer senses the orientation of the phone and changes the screen accordingly.[38] Photo browsing, web browsing, and music playing support both upright and left or right widescreen orientations, while videos play in only one widescreen orientation.[citation needed]
A software update allowed the first generation iPhone to use cell towers and Wi-Fi networks to locate itself despite lacking a hardware GPS. The iPhone 3G includes A-GPS but also uses cell towers and Wi-Fi for location finding.
A single "home" hardware button below the display brings up the main menu. Subselections are made via the touchscreen. The iPhone utilizes a full-paged display, with context-specific submenus at the top and/or bottom of each page, sometimes depending on screen orientation. Detail pages display the equivalent of a "Back" button to return to the parent menu.
The iPhone has three physical switches on its sides: wake/sleep, volume up/down, and ringer on/off. These are made of plastic on the original iPhone and metal on the iPhone 3G. All other multimedia and phone operations are done via the touchscreen.

Audio
The iPhone's headphones are similar to those of most current smartphones, incorporating a microphone. A multipurpose button in the microphone can be used to play or pause music, skip tracks, and answer or end phone calls without touching the iPhone. The 3.5 mm TRS connector for the headphones is located on the top left corner. The headphone socket on the original iPhone is recessed into the casing and is narrow when compared to some headphone jacks, making it incompatible with most headphones without the use of an adapter.[39] The iPhone 3G has a flush mounted headphone socket.
Wireless earpieces that use Bluetooth technology to communicate with the iPhone are sold separately. It does not support stereo audio.
The loudspeaker is used both for handsfree operations and media playback, but does not support voice recording.
Composite or component video at up to 576i and stereo audio can be output from the dock connector using an adapter sold by Apple.[40]

Battery
The iPhone features a built-in rechargeable battery that is not user-replaceable, similar to existing iPods, but dissimilar to most existing cellular phones.[41][42] If the battery prematurely reaches the end of its life time, the phone can be returned to Apple and replaced for free while still in warranty,[43] one year at purchase and extended to two years with AppleCare. The cost of having Apple provide a new battery and replace it when the iPhone is out of warranty is, in the United States, US$79 and US$6.95 for shipping.[44]
Since July 2007 third party battery packs have been available[45] at a much lower price than Apple's own battery replacement program. These kits often include a small screwdriver and an instruction leaflet, but as with many newer iPod models the battery in the original iPhone has been soldered in. Therefore a soldering iron is required to install the new battery. This is not the case with the iPhone 3G as it uses a different battery fitted with a connector[46]
The original iPhone's battery was stated to be capable of providing up to seven hours of video, six hours of web browsing, eight hours of talk time, 24 hours of music or up to 250 hours on standby.[33] Apple's site says that the battery life "is designed to retain up to 80% of its original capacity after 400 full charge and discharge cycles",[47] which is comparable to the iPod batteries.
The iPhone 3G's battery is stated to be capable of providing up to seven hours of video, six hours of web browsing on Wi-Fi or five on 3G, ten hours of 2G talk time, or five on 3G, 24 hours of music, or 300 hours of standby.[27]
The Foundation for Taxpayer and Consumer Rights, a consumer advocate group, has sent a complaint to Apple and AT&T over the fee that consumers have to pay to have the battery replaced.[48] Though the battery replacement service and its pricing was not made known to buyers until the day the product was launched,[48][49] a similar service had been well established for the iPods by Apple and various third party service providers.

SIM card

The original iPhone's SIM card slot shown as open, with ejected SIM card.
The SIM card is located in a slot at the top of the device, which can be ejected with a paperclip or a SIM card ejection tool which is included with the iPhone 3G.[50] The iPhone is usually sold with a simlock preventing the use of SIM cards from different mobile networks.

Storage
The iPhone was initially released with two options for internal storage size; either a 4 GB or 8 GB flash drive (manufactured by Samsung) model was available. On September 5, 2007, Apple announced they were discontinuing the 4 GB models.[51] On February 5, 2008, Apple announced the addition of a 16 GB model to the iPhone lineup.[52] The iPhone does not contain any memory card slots for expanded storage.

Included items and accessories
Both the iPhone and the iPhone 3G came with a series of included accessories and items when purchased.

Items common to both versions
Appropriate documentation
Stereo headset with microphone
Dock connector to USB cable (standard USB cable for connection)
Cleaning/polishing cloth

Original iPhone
Dock
Standard USB power adapter

iPhone 3G
SIM ejector tool
"Mini" USB power adapter (U.S. model)
Standard USB power adapter (European model)

Software
Main article: iPhone OS
iPhone OS is the operating system that runs on the iPhone and iPod touch. It is based on a variant of the same basic Mach kernel that is found in Mac OS X. iPhone OS includes the software component "Core Animation" from Mac OS X v10.5 which, together with the PowerVR MBX 3D hardware, is responsible for the smooth animations used in its user interface. The operating system takes up considerably less than half a GB of the device's total 8 GB or 16 GB storage.[53] It will be capable of supporting bundled and future applications from Apple.
Like an iPod, the iPhone is managed with iTunes version 7.3 or later, which is compatible with Mac OS X version 10.4.10 or later, and 32-bit Windows XP or Vista.[54] The release of iTunes 7.6 expanded this support to include 64-bit versions of XP and Vista,[55] and a workaround has been discovered for previous 64-bit Windows operating systems.[56]
The iPhone' applications can not simply be copied from Mac OS X and have to be written and compiled specifically for the iPhone. Additionally, the Safari web browser supports web applications written with AJAX, which, by design, are platform agnostic applications.

Applications
See also: iPhone SDK

The photo display application
There are several applications located on the "Home" screen: Text (SMS messaging), Calendar, Photos, Camera, YouTube, Stocks, Maps (Google Maps), Weather, Clock, Calculator, Notes, Settings, and iTunes (store). Four other applications, docked at the base of the screen, delineate the iPhone's main purposes: Phone, Mail, Safari, and iPod.[57]
The YouTube application streams videos over Wi-Fi, 2G, or 3G after encoding them using the open H.264 codec, to which YouTube has converted about 10,000 videos. As a result, the YouTube application on iPhone can currently view only a certain selection of videos from the site.[58]
At WWDC 2007 on June 11, 2007 Apple announced that the iPhone will support third-party "applications" via the Safari web browser that share the look and feel of the iPhone interface. On October 17, 2007, Steve Jobs, in an open letter posted to Apple's "Hot News" weblog, announced that a software development kit (SDK) would be made available to third-party developers in February 2008. Due to security concerns and Jobs' praise of Nokia's digital signature system, it was suggested that Apple would adopt a similar method. The SDK will also allow application development for the iPod touch.[59] The iPhone SDK was officially announced on March 6, 2008, at the Apple Town Hall facility.[60] The SDK will allow developers to develop native applications for the iPhone and iPod touch, as well as test them in an "iPhone simulator". However, loading an application onto the devices is only possible after paying a Apple membership fee. Developers are free to set any price for their applications to be distributed through the App Store, of which they will receive a 70 percent share[61]. Developers can also opt to release the application for free and will not pay any costs to release or distribute the application beyond the membership fee. The SDK is available immediately, while the launch of applications will require waiting until a firmware update on This update will be free for iPhone users and there will be a charge for iPod touch owners.
Many Safari "applications" and un-signed native applications are also available.The ability to install native applications onto the iPhone outside of the App Store will not be supported by Apple. Such native applications could be broken by any software update, but Apple has stated it will not design software updates specifically to break native applications other than applications that perform SIM unlocking.

MOBILE PHONES

The mobile phone (also called a wireless phone or cellular phone)[1] is a short-range, portable electronic device used for mobile voice or data communication over a network of specialized base stations known as cell sites. In addition to the standard voice function of a telephone, current mobile phones may support many additional services, and accessories, such as SMS for text messaging, email, packet switching for access to the Internet, gaming, bluetooth, infrared, camera with video recorder and MMS for sending and receiving photos and video. Most current mobile phones connect to a cellular network of base stations (cell sites), which is in turn interconnected to the public switched telephone network (PSTN) (the exception is satellite phones).
Cellular systems
See also: Cellular frequencies

Mobile phone tower
Mobile phones send and receive radio signals with any number of cell site base stations fitted with microwave antennas. These sites are usually mounted on a tower, pole or building, located throughout populated areas, then connected to a cabled communication network and switching system. The phones have a low-power transceiver that transmits voice and data to the nearest cell sites, normally not more than 8 to 13 km (approximately 5 to 8 miles) away.
When the mobile phone or data device is turned on, it registers with the mobile telephone exchange, or switch, with its unique identifiers, and can then be alerted by the mobile switch when there is an incoming telephone call. The handset constantly listens for the strongest signal being received from the surrounding base stations, and is able to switch seamlessly between sites. As the user moves around the network, the "handoffs" are performed to allow the device to switch sites without interrupting the call.
Cell sites have relatively low-power (often only one or two watts) radio transmitters which broadcast their presence and relay communications between the mobile handsets and the switch. The switch in turn connects the call to another subscriber of the same wireless service provider or to the public telephone network, which includes the networks of other wireless carriers. Many of these sites are camouflaged to blend with existing environments, particularly in scenic areas.
The dialogue between the handset and the cell site is a stream of digital data that includes digitized audio (except for the first generation analog networks). The technology that achieves this depends on the system which the mobile phone operator has adopted. The technologies are grouped by generation. The first-generation systems started in 1979 with Japan, are all analog and include AMPS and NMT. Second-generation systems, started in 1991 in Finland, are all digital and include GSM, CDMA and TDMA.
The nature of cellular technology renders many phones vulnerable to 'cloning': anytime a cell phone moves out of coverage (for example, in a road tunnel), when the signal is re-established, the phone sends out a 're-connect' signal to the nearest cell-tower, identifying itself and signalling that it is again ready to transmit. With the proper equipment, it's possible to intercept the re-connect signal and encode the data it contains into a 'blank' phone -- in all respects, the 'blank' is then an exact duplicate of the real phone and any calls made on the 'clone' will be charged to the original account.
Third-generation (3G) networks, which are still being deployed, began in Japan in 2001. They are all digital, and offer high-speed data access in addition to voice services and include W-CDMA (known also as UMTS), and CDMA2000 EV-DO. China will launch a third generation technology on the TD-SCDMA standard. Operators use a mix of predesignated frequency bands determined by the network requirements and local regulations.
In an effort to limit the potential harm from having a transmitter close to the user's body, the first fixed/mobile cellular phones that had a separate transmitter, vehicle-mounted antenna, and handset (known as car phones and bag phones) were limited to a maximum 3 watts Effective Radiated Power. Modern handheld cellphones which must have the transmission antenna held inches from the user's skull are limited to a maximum transmission power of 0.6 watts ERP. Regardless of the potential biological effects, the reduced transmission range of modern handheld phones limits their usefulness in rural locations as compared to car/bag phones, and handhelds require that cell towers be spaced much closer together to compensate for their lack of transmission power.
Some handhelds include an optional auxiliary antenna port on the back of the phone, which allows it to be connected to a large external antenna and a 3 watt cellular booster. Alternately in fringe-reception areas, a cellular repeater may be used, which uses a long distance high-gain dish antenna or yagi antenna to communicate with a cell tower far outside of normal range, and a repeater to rebroadcast on a small short-range local antenna that allows any cellphone within a few meters to function properly.

Handsets
Nokia is currently the world's largest manufacturer of mobile phones, with a global device market share of approximately 40% in 2008. Other major mobile phone manufacturers (in order of market share) include Samsung (14%), Motorola (14%), Sony Ericsson (9%) and LG (7%).[4] These manufacturers account for over 80% of all mobile phones sold and produce phones for sale in most countries.
Other manufacturers include Apple Inc., Audiovox (now UTStarcom), Benefon, BenQ-Siemens, CECT, High Tech Computer Corporation (HTC), Fujitsu, Kyocera, Mitsubishi Electric, NEC, Neonode, Panasonic (Matsushita Electric), Pantech Curitel, Philips, Research In Motion, Sagem, Sanyo, Sharp, Siemens, Sierra Wireless, SK Teletech, Sonim Technologies, T&A Alcatel, Huawei, Trium and Toshiba. There are also specialist communication systems related to (but distinct from) mobile phones.
There are several categories of mobile phones, from basic phones to feature phones such as musicphones and cameraphones, to smartphones. The first smartphone was the Nokia 9000 Communicator in 1996 which incorporated PDA functionality to the basic mobile phone at the time. As miniaturization and increased processing power of microchips has enabled ever more features to be added to phones, the concept of the smartphone has evolved, and what was a high-end smartphone five years ago, is a standard phone today. Several phone series have been introduced to address a given market segment, such as the RIM Blackberry focusing on enterprise/corporate customer email needs; the SonyEricsson Walkman series of musicphones and Cybershot series of cameraphones; and the Nokia N-Series of multimedia phones. The Apple iPhone is another example of a multimedia smartphone.
Main article: Mobile phone features
Mobile phones often have features beyond sending text messages and making voice calls, including Internet browsing, music (MP3) playback, memo recording, personal organizer functions, e-mail, instant messaging, built-in cameras and camcorders, ringtones, games, radio, Push-to-Talk (PTT), infrared and Bluetooth connectivity, call registers, ability to watch streaming video or download video for later viewing, video calling and serve as a wireless modem for a PC, and soon will also serve as a console of sorts to online games and other high quality games. The total value of mobile data services exceeds the value of paid services on the Internet, and was worth 31 billion dollars in 2006 (source Informa).[citation needed] The largest categories of mobile services are music, picture downloads, videogaming, adult entertainment, gambling, video/TV.

Applications
The most commonly used data application on mobile phones is SMS text messaging, with 74% of all mobile phone users as active users (over 2.4 billion out of 3.3 billion total subscribers at the end of 2007). SMS text messaging was worth over 100 billion dollars in annual revenues in 2007 and the worldwide average of messaging use is 2.6 SMS sent per day per person across the whole mobile phone subscriber base. (source Informa 2007). The first SMS text message was sent from a computer to a mobile phone in 1992 in the UK, while the first person-to-person SMS from phone to phone was sent in Finland in 1993.
The other non-SMS data services used by mobile phones were worth 31 Billion dollars in 2007, and were led by mobile music, downloadable logos and pictures, gaming, gambling, adult entertainment and advertising (source: Informa 2007). The first downloadable mobile content was sold to a mobile phone in Finland in 1998, when Radiolinja (now Elisa) introduced the downloadable ringing tone service. In 1999 Japanese mobile operator NTT DoCoMo introduced its mobile internet service, i-Mode, which today is the world's largest mobile internet service and roughly the same size as Google in annual revenues.
The first mobile news service, delivered via SMS, was launched in Finland in 2000. Mobile news services are expanding with many organizations providing "on-demand" news services by SMS. Some also provide "instant" news pushed out by SMS. Mobile telephony also facilitates activism and public journalism being explored by Reuters and Yahoo![5] and small independent news companies such as Jasmine News in Sri Lanka. Companies like Monster[6] are starting to offer mobile services such as job search and career advice. Consumer applications are on the rise and include everything from information guides on local activities and events to mobile coupons and discount offers one can use to save money on purchases. Even tools for creating websites for mobile phones are increasingly becoming available, e.g. Mobilemo.
Mobile payments were first trialled in Finland in 1998 when two coca cola machines in Espoo were enabled to work with SMS payments. Eventually the idea spread and in 1999 the Philippines launched the first commercial mobile payments systems, on the mobile operators Globe and Smart. Today mobile payments ranging from mobile banking to mobile credit cards to mobile commerce are very widely used in Asia and Africa, and in selected European markets. For example in the Philippines it is not unusual to have your whole paycheck paid to the mobile account. In Kenya the limit of money transfers from one mobile banking account to another is one million US dollars. In India paying utility bills with mobile gains a 5% discount. In Estonia the government found criminals collecting cash parking fees, so the government declared that only mobile payments via SMS were valid for parking and today all parking fees in Estonia are handled via mobile and the crime involved in the activity has vanished.
Mobile Applications are developed using the Six M's (previously Five M's) service-development theory created by the author Tomi Ahonen with Joe Barrett of Nokia and Paul Golding of Motorola. The Six M's are Movement (location), Moment (time), Me (personalization), Multi-user (community), Money (payments) and Machines (automation). The Six M's / Five M's theory is widely referenced in the telecoms applications literature and used by most major industry players. The first book to discuss the theory was Services for UMTS by Ahonen & Barrett in 2002.
The availability of mobile phone backup applications is growing with the increasing amount of mobile phone data being stored on mobile phones today. With mobile phone manufacturers producing mobile handsets with more and more memory storage capabilities the awareness of the importance in backing up mobile phone data is increasing. Corporate mobile phone users today keep very important company information on their mobiles, information if lost then not easily replaced. Wireless backup applications like SC BackUp offer users the chance to backup mobile phone data using advanced wireless technology. Users can backup, restore or transfer mobile data anytime, anywhere all over the world, to a secured server.

LAPTOPS

An ultraportable IBM X31 with 12" screen on an IBM T43 Thin & Light laptop with a 14" screen
A laptop computer or simply laptop (also notebook computer, notebook and notepad) is a small mobile computer, typically weighing 3 to 12 pounds (1.4 to 5.4 kg), although older laptops may weigh more.
Types

Mainstream
Laptops weighing between 5 and 7 lb (2.3–3.2 kg) with a screen size of 14.1 or 15.4 inches (35 or 39 cm) diagonally.

Desktop replacement

An Apple PowerBook G4 17" often used as a desktop replacement.
Main article: Desktop replacement computer
A desktop replacement computer is a personal computer that provides the full capabilities of a desktop computer while remaining portable. They are often a larger, bulkier laptop. Because of their increased size, this class of computer usually includes more powerful components and a larger display than generally used in smaller portable computers and can have a relatively limited battery capacity (or none at all). Some use a limited range of desktop components to provide better performance per dollar at the expense of battery life. These are sometimes called desknotes, a portmanteau of the words "desktop" and "notebook," though the term is also applied to desktop replacement computers in general.[1]
Powerful laptops meant to be mainly used for fun and infrequently carried out due to their weight and size; the latter provides more space for powerful components and a big screen, usually measuring 17–20 inches (43–51 cm). Desktop replacements tend to have limited battery life, rarely exceeding three hours, because the hardware is not optimized for efficient power usage. Sometimes called a luggable laptop. An example of a desktop replacement computers are gaming notebooks, which are designed to handle 3D graphic-intensive processing for gamers.

Subnotebook

Sony VAIO C1 subnotebook.
Main article: Subnotebook
Laptops weighing typically between 4.6 and 6 lb (1.8–2.7 kg) and a screen of 10.6 to 13.3 inches diagonally. A subnotebook is a small and lightweight portable computer, with most of the features of a standard laptop computer but smaller. The term is often applied to systems that run full versions of desktop operating systems such as Windows or Linux, rather than specialized software such as Windows CE, Palm OS or Internet Tablet OS.
Subnotebooks are smaller than laptops but larger than handheld computers and UMPCs. They often have screens around 10.6" (26.92 cm) (diagonal) and weigh less than 1 to 2 kg, as opposed to full-size laptops with 14.1" (35.81 cm) or 15.4" (39.12 cm) screens that typically weigh 2 kg or more. The savings in size and weight are usually achieved partly by omitting ports or having removable media/optical drives; subnotebooks are often paired with docking stations to compensate.
Subnotebooks have been something of a niche computing product and have rarely sold in large numbers until the 2007 introduction of the Asus Eee PC and the OLPC XO-1.[2]

Parts

2.5" hard disk drive
Most modern laptops feature 12 inch (30 cm) or larger active matrix displays with resolutions of 1024×768 pixels and above, and have a PC Card (formerly PCMCIA) or ExpressCard expansion bay for expansion cards, one or more USB ports, and a external monitor port (VGA or DVI). Most laptops have also an ethernet network port. Some have legacy ports such as a PS/2 keyboard/mouse port or a serial port, parallel port, and S-video or composite video port. Hard disks are physically smaller—2.5 inch (60 mm)—compared to the standard desktop 3.5 inch (90 mm) drive, and usually have lower performance and power consumption. Video and sound chips are usually integrated. This tends to limit the use of laptops for gaming and entertainment, two fields which have constantly escalating hardware demands,[3] however, higher end laptops can come with dedicated graphics processors. These mobile graphics processors tend to have less performance than their desktop counterparts, but this is because they have been optimized for lower power usage. Some subsystems, such as Wi-Fi, come in contemporary laptops on replaceable MiniPCI cards, usually accessible through a door on the bottom. Memory modules (smaller than the usual DIMMs) are often also accessible through the bottom, though some may be on the motherboard under the keyboard and thus not meant to be accessed by the user.
There is a wide range of laptop specific processors available from Intel (Pentium M, Celeron, Intel Core and Intel Core 2), AMD (Athlon, Turion 64, and Sempron) and from VIA (C3 and C7-M). Motorola and IBM developed and manufactured the chips for the former PowerPC-based Apple laptops (iBook and PowerBook). Generally, laptop processors are less powerful than their desktop counterparts, due to the need to save energy and reduce heat dissipation.
Current models of laptops utilize lithium ion batteries with more recent models using the new lithium polymer technology. These technologies have largely replaced the older nickel metal-hydride batteries. Typical battery life for most laptops is two to five hours with light-duty use, but may drop to as little as one hour with intensive use. Batteries gradually deteriorate over time and eventually need to be replaced in one to five years, depending on the charging and discharging pattern.

Laptops typically use SODIMMs, as shown here.
Docking stations became another common laptop accessories in the early 1990s. They typically were quite large and offered 3.5" and 5.25" storage bays, one to three expansion slots (typically AT style), and a host of connectors. The mating between the laptop and docking station was typically through a large, high-speed, proprietary connector. The most common use was in a corporate computing environment where the company had standardized on a common network card and this same card was placed into the docking station. These stations were very large and quite expensive. As the need to additional storage and expansion slots became less critical because of the high integration inside the laptop itself, the emergence of the Port Replicator as a major accessory commenced. The Port Replicator was often a passive device that simply mated to the connectors on the back of the notebook and allowed the user to quickly connect their laptop so VGA, PS/2, RS-232, etc. devices were instantly attached. As higher speed ports like USB and Firewire became commonplace, the Port Replication was accomplished by a small cable connected to one of the USB 2.0 or FireWire ports on the notebooks. Wireless Port Replicators followed.
Virtually all laptops can be powered from an external AC converter. This device typically adds half a kilogram (1 lb) to the overall "transport weight" of the equipment.
A pointing stick or touchpad is used to control the position of the cursor on the screen. The pointing stick is usually a rubber dot that is located between the G, H and B keys on the laptop keyboard. To navigate the cursor, pressure is applied in the direction intended to move. The touchpad is touch-sensitive and the cursor can be navigated by moving the finger on the pad.
Intel, Asus, Compal, Quanta and other laptop manufacturers have created Common Building Block standard for laptop parts

CAPACITOR

A capacitor is an electrical/electronic device that can store energy in the electric field between a pair of conductors (called "plates"). The process of storing energy in the capacitor is known as "charging", and involves electric charges of equal magnitude, but opposite polarity, building up on each plate.

Electric circuits
When a capacitor is connected to a current source, charge is transfered between its plates at a rate i(t) = dq(t) / dt. As the voltage between the plates is proportional to the charge, it follows that
.
Conversely, if a capacitor is connected to a voltage source, the resulting displacement current is given by
.
For example, if one were to connect a 1000 µF capacitor to a voltage source, then increase the sourced voltage at a rate of 2.5 Volts per second, the current flowing through the capacitor would be
.

DC sources

A simple resistor-capacitor circuit demonstrates charging of a capacitor.
A circuit containing only a resistor, a capacitor, a switch and a constant (DC) voltage source vsrc(t) = V0 in series is known as a charging circuit. From Kirchhoff's voltage law it follows that
,
where vr(t) and vc(t) are the voltages across the resistor and capacitor respectively. This reduces to a first order differential equation

Assuming that the capacitor is initially uncharged, there is no internal electric field, and the initial current is I0 = V0 / R. This initial condition allows solution of the differential equation as
.
The corresponding voltage drop across the capacitor is
.
Therefore, as charge increases on the capacitor plates, the voltage across the capacitor increases, until it reaches a steady-state value of V0, and the current drops to zero. Both the current, and the difference between the source and capacitor voltage decay exponentially with respect to time. The time constant of the decay is given by τ = RC.

AC sources
When connected to an alternating current (AC) voltage source, the plates on a capacitor repeatedly charge and discharge relative to each other. The current varies sinusoidally, with a nonzero amplitude. For this reason, capacitors effectively conduct AC although charge ideally never passes directly through the dielectric. Since the current is proportional to the time derivative of the voltage, a sinusoidal current leads the voltage by a 90 degree phase shift, or equivalently a quarter cycle. The amplitude of the voltage depends on the amplitude of the current divided by the product of the frequency of the current with the capacitance, C.

Impedance
The ratio of the phasor voltage across a circuit element to the phasor current through that element is called the impedance Z. For a capacitor, the impedance is given by
where is the capacitive reactance, is the angular frequency, f is the frequency), C is the capacitance in farads, and j is the imaginary unit.
While this relation (between the frequency domain voltage and current associated with a capacitor) is always true, the ratio of the time domain voltage and current amplitudes is equal to XC only for sinusoidal (AC) circuits in steady state.
See derivation Deriving capacitor impedance.
Hence, capacitive reactance is the negative imaginary component of impedance. The negative sign indicates that the current leads the voltage by 90° for a sinusoidal signal, as opposed to the inductor, where the current lags the voltage by 90°.
The impedance is analogous to the resistance of a resistor. The impedance of a capacitor is inversely proportional to the frequency - that is, for very high-frequency alternating currents the reactance approaches zero - so that a capacitor is nearly a short circuit to a very high frequency AC source. Conversely, for very low frequency alternating currents, the reactance increases without bound so that a capacitor is nearly an open circuit to a very low frequency AC source. This frequency dependent behaviour accounts for most uses of the capacitor (see "Applications", below).
Reactance is so called because the capacitor does not dissipate power, but merely stores energy. In electrical circuits, as in mechanics, there are two types of load, resistive and reactive. Resistive loads (analogous to an object sliding on a rough surface) dissipate the energy delivered by the circuit as heat, while reactive loads (analogous to a spring or frictionless moving object) store this energy, ultimately delivering the energy back to the circuit.
Also significant is that the impedance is inversely proportional to the capacitance, unlike resistors and inductors for which impedances are linearly proportional to resistance and inductance respectively. This is why the series and shunt impedance formulae (given below) are the inverse of the resistive case. In series, impedances sum. In parallel, conductances sum.

Laplace equivalent (s-domain)
When using the Laplace transform in circuit analysis, the capacitive impedance is represented in the s domain by:
where C is the capacitance, and s (= σ+jω) is the complex frequency.
Capacitor types
Main article: capacitor (component)
Practical capacitors are available commercially in many different forms. The type of internal dielectric, the structure of the plates and the device packaging all strongly affect the characteristics of the capacitor, and its applications.

A 12 pF 20 kV fixed vacuum capacitor

Dielectric materials
Most types of capacitor include a dielectric spacer, which increases their capacitance. However, low capacitance devices are available with a vacuum between their plates, which allows extremely high voltage operation and low losses. Air filled variable capacitors are also commonly used in radio tuning circuits.
Several solid dielectrics are available, including paper, plastic, glass, mica and ceramic materials. Paper was used extensively in older devices and offers relatively high voltage performance. However, it is susceptible to water absorption, and has been largely replaced by plastic film capacitors. Plastics offer better stability and aging performance, which makes them useful in timer circuits, although they may be limited to low operating temperatures and frequencies. Ceramic capacitors are generally small, cheap and useful for high frequency applications, although their capacitance varies strongly with voltage, and they age poorly. They are broadly categorized as Class 1 dielectrics, which have predictable variation of capacitance with temperature or Class 2 dielectrics, which can operate at higher voltage. Glass and mica capacitors are extremely reliable, stable and tolerant to high temperatures and voltages, but are too expensive for most mainstream applications.

Capacitor materials. From left: multilayer ceramic, ceramic disc, multilayer polyester film, tubular ceramic, polystyrene, metalized polyester film, aluminum electrolytic. Major scale divisions are in centimetres.
Electrolytic capacitors use an aluminum or tantalum plate with an oxide dielectric layer. The second electrode is a liquid electrolyte. Electrolytic capacitors offer very high capacitance but suffer from poor tolerances, high instability, gradual loss of capacitance especially when subjected to heat, and high leakage current. The conductivity of the electrolyte drops at low temperatures, which increases equivalent series resistance. While widely used for power-supply conditioning, poor high-frequency characteristics make them unsuitable for many applications. Tantalum capacitors offer better frequency and temperature characteristics than aluminum, but higher dielectric absorption and leakage.[3] OS-CON (or OC-CON) capacitors are a polymerized organic semiconductor solid-electrolyte type that offer longer life at higher cost than standard electrolytic capacitors.
Several other types of capacitor are available for specialist applications. Supercapacitors made from carbon aerogel, carbon nanotubes, or highly porous electrode materials offer extremely high capacity and can be used in some applications instead of rechargeable batteries. Alternating current capacitors are specifically designed to work on line (mains) voltage AC power circuits. They are commonly used in electric motor circuits and are often designed to handle large currents, so they tend to be physically large. They are usually ruggedly packaged, often in metal cases that can be easily grounded/earthed. They also tend to have rather high direct current breakdown voltages.

Capacitor packages: SMD ceramic at top left; SMD tantalum at bottom left; through-hole tantalum at top right; through-hole electrolytic at bottom right. Major scale divisions are cm.

Structure
Capacitors may have their plates arranged in many configurations, for example axially or radially. Small, cheap discoidal ceramic capacitors have existed since the 1930s, and remain in widespread use. Since the 1980s, surface mount packages for capacitors have been widely used. These packages are extremely small and lack connecting leads, allowing them to be soldered directly onto the surface of printed circuit boards. Surface mount components avoid undesirable high-frequency effects due to the leads and simplify automated assembly, although manual handling is made difficult due to their small size.
Variable capacitors are available in various forms. Mechanically controlled variable capacitors allow the plate spacing to be adjusted, for example by rotating or sliding a set of movable plates into alignment with a set of stationary plates. Very cheap variable capacitors squeeze together alternating layers of aluminum and plastic with a screw, but the resulting capacitance is unstable, and unreproducible. Electrical control of capacitance is achievable with varactors (or varicaps), which are reverse-biased semiconductor diodes whose depletion region width varies with applied voltage. They are used in phase-locked loops, amongst other applications.

LIGHT EMITTING DIODE

A light-emitting diode, usually called an LED (pronounced /ˌɛliːˈdiː/),[1] is a semiconductor diode that emits incoherent narrow-spectrum light when electrically biased in the forward direction of the p-n junction, as in the common LED circuit. This effect is a form of electroluminescence.
Discovery and development
The first known report of a light-emitting solid-state diode was made in 1907 by the British experimenter H. J. Round of Marconi Labs. Russian Oleg Vladimirovich Losev independently created the first LED in the mid 1920s; his research, though distributed in Russian, German and British scientific journals, was ignored, [5] [6] and no practical use was made of the discovery for several decades. Rubin Braunstein of the Radio Corporation of America reported on infrared emission from gallium arsenide (GaAs) and other semiconductor alloys in 1955. [7] . Braunstein observed infrared emission generated by simple diode structures using GaSb, GaAs, InP, and Ge-Si alloys cooled by liquid nitrogen to 77 K. Experimenters at Texas Instruments, Bob Biard [8] and Gary Pittman, found in 1961 that gallium arsenide gave off infrared radiation when electric current was applied. Biard and Pittman were able to establish the priority of their work and received the patent for the infrared light-emitting diode.

Practical use
The first commercial LEDs were commonly used as replacements for incandescent indicators, and in seven-segment displays, first in expensive equipment such as laboratory and electronics test equipment, then later in such appliances as TVs, radios, telephones, calculators, and even watches. These red LEDs were bright enough only for use as indicators, as the light output was not enough to illuminate an area. Later, other colors became widely available and also appeared in appliances and equipment. As the LED materials technology became more advanced, the light output was increased, while maintaining the efficiency and the reliability to an acceptable level, causing LEDs to become bright enough to be used for illumination.
Physical principles

I-V diagram for a diode an LED will begin to emit light when the on-voltage is exceeded. Typical on voltages are 2-3 Volt
Like a normal diode, the LED consists of a chip of semiconducting material impregnated, or doped, with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon.
The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to near-infrared, visible or near-ultraviolet light.
LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors.
LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate.
Ultraviolet and blue LEDs

Ultraviolet GaN LEDs.
Blue LEDs are based on the wide band gap semiconductors GaN (gallium nitride) and InGaN (indium gallium nitride). They can be added to existing red and green LEDs to produce the impression of white light, though white LEDs today rarely use this principle.
The first blue LEDs were made in 1971 by Jacques Pankove (inventor of the gallium nitride LED) at RCA Laboratories.[17] However, these devices had too little light output to be of much practical use. In the late 1980s, key breakthroughs in GaN epitaxial growth and p-type doping by Isamu Akasaki and Hiroshi Amano (Nagoya, Japan)[18] ushered in the modern era of GaN-based optoelectronic devices. Building upon this foundation, in 1993 high brightness blue LEDs were demonstrated through the work of Shuji Nakamura at Nichia Corporation.[19]
By the late 1990s, blue LEDs had become widely available. They have an active region consisting of one or more InGaN quantum wells sandwiched between thicker layers of GaN, called cladding layers. By varying the relative InN-GaN fraction in the InGaN quantum wells, the light emission can be varied from violet to amber. AlGaN aluminium gallium nitride of varying AlN fraction can be used to manufacture the cladding and quantum well layers for ultraviolet LEDs, but these devices have not yet reached the level of efficiency and technological maturity of the InGaN-GaN blue/green devices. If the active quantum well layers are GaN, as opposed to alloyed InGaN or AlGaN, the device will emit near-ultraviolet light with wavelengths around 350–370 nm. Green LEDs manufactured from the InGaN-GaN system are far more efficient and brighter than green LEDs produced with non-nitride material systems.
Considerations in use

Close-up of a typical LED in its case, showing the internal structure.


Unlike incandescent light bulbs, which light up regardless of the electrical polarity, LEDs will only light with correct electrical polarity. When the voltage across the p-n junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. LEDs can be operated on an alternating current voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.
While the only definitive way to determine the polarity of the LED is to examine its datasheet, these methods are usually reliable:
sign:
+
-
terminal:
anode (A)
cathode (K)
leads:
long
short
exterior:
round
flat
interior:
small
large
wiring:
red
black
Less reliable methods of determining polarity are:
sign:
+
-
marking:
none
stripe
pin:
1
2
PCB:
round
square
While it is not an officially reliable method, it is almost universally true that the cup that holds the LED die corresponds to the cathode. It is strongly recommended to apply a safe voltage and observe the illumination as a test regardless of what method is used to determine the polarity.
Because the voltage versus current characteristics of the LED are much like any diode (that is, current approximately an exponential function of voltage), a small voltage change results in a huge change in current. Added to deviations in the process this means that a voltage source may barely make one LED light while taking another of the same type beyond its maximum ratings and potentially destroying it.
Since the voltage is logarithmically related to the current it can be considered to remain largely constant over the LED's operating range. Thus the power can be considered to be essentially proportional to the current. In order to keep power nearly constant with variations in supply and LED characteristics, the power supply should be a “current source”, that is, it should supply an almost constant current. If high efficiency is not required (e.g., in most indicator applications), an approximation to a current source is made by connecting the LED in series with a current limiting resistor to a regulated voltage source.
Most LEDs have low reverse breakdown voltage ratings, so they will also be damaged by an applied reverse voltage of more than a few volts. Since some manufacturers don't follow the indicator standards above, if possible the data sheet should be consulted before hooking up the LED, or the LED may be tested in series with a resistor on a sufficiently low voltage supply to avoid the reverse breakdown. If it is desired to drive the LED directly from an AC supply of more than the reverse breakdown voltage then it may be protected by placing a diode (or another LED) in inverse parallel.
LEDs can be purchased with built in series resistors. These can save PCB space and are especially useful when building prototypes or populating a PCB in a way other than its designers intended. However, the resistor value is set at the time of manufacture, removing one of the key methods of setting the LED's intensity. To increase efficiency (or to allow intensity control without the complexity of a DAC), the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear to be continuously lit.
Multiple LEDs can be connected in series with a single current limiting resistor provided the source voltage is greater than the sum of the individual LED threshold voltages. Parallel operation is also possible but can be more problematic. Parallel LEDs must have closely matched forward voltages (Vf) in order to have equal branch currents and, therefore, equal light output. Variations in the manufacturing process can make it difficult to obtain satisfactory operation when connecting some types of LEDs in parallel.[27]
Bicolor LED units contain two diodes, one in each direction (that is, two diodes in inverse parallel) and each a different color (typically red and green), allowing two-color operation or a range of apparent colors to be created by altering the percentage of time the voltage is in each polarity. Other LED units contain two or more diodes (of different colors) arranged in either a common anode or common cathode configuration. These can be driven to different colors without reversing the polarity, however, more than two electrodes (leads) are required.
LEDs are usually constantly illuminated when a current passes through them, but flashing LEDs are also available. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit inside which causes the LED to flash with a typical period of one second. This type of LED comes most commonly as red, yellow, or green. Most flashing LEDs emit light of a single wavelength, but multicolored flashing LEDs are available too.
Generally, for newer common standard LEDs in 3 mm or 5 mm packages, the following forward DC potential differences are typically measured. The forward potential difference depending on the LED's chemistry, temperature, and on the current (values here are for approx. 20 mA, a commonly-found maximum value).
Color
Potential Difference (Vf)
Infrared
1.6 V
Red
1.8–2.1 V
Orange
2.2 V
Yellow
2.4 V
Green
2.6 V
Blue
3.0–3.5 V
White
3.0–3.5 V
Ultraviolet
3.5 V
Many LEDs are rated at 3 V maximum reverse potential.