Types of Transistors - Wiki(Description)

http://en.wikipedia.org/wiki/Transistor

Types of Transistors

	PNP		P-channel
	NPN		N-channel
BJT		JFET

BJT and JFET symbols

				P-channel
				N-channel
JFET	MOSFET enh		MOSFET dep

JFET and IGFET symbols

Transistors are categorized by

• Semiconductor material (date first used): the metalloids germanium (1947) and silicon (1954)— in amorphous, polycrystalline and monocrystallineform; the compounds gallium arsenide (1966) and silicon carbide (1997), the alloy silicon-germanium (1989), the allotrope of carbon graphene(research ongoing since 2004), etc.—see Semiconductor material
• Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types"
• Electrical polarity (positive and negative) : NPN, PNP (BJTs); N-channel, P-channel (FETs)
• Maximum power rating: low, medium, high
• Maximum operating frequency: low, medium, high, radio frequency (RF),microwave (The maximum effective frequency of a transistor is denoted by the term , an abbreviation for transition frequency—the frequency of transition is the frequency at which the transistor yields unity gain)
• Application: switch, general purpose, audio, high voltage, super-beta, matched pair
• Physical packaging: through-hole metal, through-hole plastic, surface mount, ball grid array, power modules—see Packaging
• Amplification factor h_fe or β_F (transistor beta)^[23]

Thus, a particular transistor may be described assilicon, surface mount, BJT, NPN, low power, high frequency switch.

Bipolar junction transistor (BJT)

Main article: Bipolar junction transistor

Bipolar transistors are so named because they conduct by using both majority and minoritycarriers. The bipolar junction transistor, the first type of transistor to be mass-produced, is a combination of two junction diodes, and is formed of either a thin layer of p-type semiconductor sandwiched between two n-type semiconductors (an n-p-n transistor), or a thin layer of n-type semiconductor sandwiched between two p-type semiconductors (a p-n-p transistor). This construction produces two p-n junctions: a base–emitter junction and a base–collector junction, separated by a thin region of semiconductor known as the base region (two junction diodes wired together without sharing an intervening semiconducting region will not make a transistor).

The BJT has three terminals, corresponding to the three layers of semiconductor – an emitter, a base, and a collector. It is useful in amplifiers because the currents at the emitter and collector are controllable by a relatively small base current."^[24] In an NPN transistor operating in the active region, the emitter-base junction is forward biased (electrons and electron holes recombine at the junction), and electrons are injected into the base region. Because the base is narrow, most of these electrons will diffuse into the reverse-biased (electrons and holes are formed at, and move away from the junction) base-collector junction and be swept into the collector; perhaps one-hundredth of the electrons will recombine in the base, which is the dominant mechanism in the base current. By controlling the number of electrons that can leave the base, the number of electrons entering the collector can be controlled.^[24] Collector current is approximately β (common-emitter current gain) times the base current. It is typically greater than 100 for small-signal transistors but can be smaller in transistors designed for high-power applications.

Unlike the field-effect transistor (see below), the BJT is a low–input-impedance device. Also, as the base–emitter voltage (V_be) is increased the base–emitter current and hence the collector–emitter current (I_ce) increase exponentially according to the Shockley diode model and the Ebers-Moll model. Because of this exponential relationship, the BJT has a higher transconductance than the FET.

Bipolar transistors can be made to conduct by exposure to light, since absorption of photons in the base region generates a photocurrent that acts as a base current; the collector current is approximately β times the photocurrent. Devices designed for this purpose have a transparent window in the package and are called phototransistors.

Field-effect transistor (FET)

Main articles: Field-effect transistor, MOSFET, and JFET

The field-effect transistor, sometimes called a unipolar transistor, uses either electrons (in N-channel FET) or holes (in P-channel FET) for conduction. The four terminals of the FET are named source, gate, drain, and body (substrate). On most FETs, the body is connected to the source inside the package, and this will be assumed for the following description.

In a FET, the drain-to-source current flows via a conducting channel that connects the source region to the drain region. The conductivity is varied by the electric field that is produced when a voltage is applied between the gate and source terminals; hence the current flowing between the drain and source is controlled by the voltage applied between the gate and source. As the gate–source voltage (V_gs) is increased, the drain–source current (I_ds) increases exponentially for V_gs below threshold, and then at a roughly quadratic rate () (where V_T is the threshold voltage at which drain current begins)^[25] in the "space-charge-limited" region above threshold. A quadratic behavior is not observed in modern devices, for example, at the 65 nm technology node.^[26]

For low noise at narrow bandwidth the higher input resistance of the FET is advantageous.

FETs are divided into two families: junction FET (JFET) and insulated gate FET (IGFET). The IGFET is more commonly known as ametal–oxide–semiconductor FET (MOSFET), reflecting its original construction from layers of metal (the gate), oxide (the insulation), and semiconductor. Unlike IGFETs, the JFET gate forms a p-n diode with the channel which lies between the source and drain. Functionally, this makes the N-channel JFET the solid-state equivalent of the vacuum tube triode which, similarly, forms a diode between its grid and cathode. Also, both devices operate in the depletion mode, they both have a high input impedance, and they both conduct current under the control of an input voltage.

Metal–semiconductor FETs (MESFETs) are JFETs in which the reverse biased p-n junction is replaced by a metal–semiconductor junction. These, and the HEMTs (high electron mobility transistors, or HFETs), in which a two-dimensional electron gas with very high carrier mobility is used for charge transport, are especially suitable for use at very high frequencies (microwave frequencies; several GHz).

Unlike bipolar transistors, FETs do not inherently amplify a photocurrent. Nevertheless, there are ways to use them, especially JFETs, as light-sensitive devices, by exploiting the photocurrents in channel–gate or channel–body junctions.

FETs are further divided into depletion-mode and enhancement-mode types, depending on whether the channel is turned on or off with zero gate-to-source voltage. For enhancement mode, the channel is off at zero bias, and a gate potential can "enhance" the conduction. For depletion mode, the channel is on at zero bias, and a gate potential (of the opposite polarity) can "deplete" the channel, reducing conduction. For either mode, a more positive gate voltage corresponds to a higher current for N-channel devices and a lower current for P-channel devices. Nearly all JFETs are depletion-mode as the diode junctions would forward bias and conduct if they were enhancement mode devices; most IGFETs are enhancement-mode types.

Usage of bipolar and field effect transistors

The bipolar junction transistor (BJT) was the most commonly used transistor in the 1960s and 70s. Even after MOSFETs became widely available, the BJT remained the transistor of choice for many analog circuits such as amplifiers because of their greater linearity and ease of manufacture. In integrated circuits, the desirable properties of MOSFETs allowed them to capture nearly all market share for digital circuits. Discrete MOSFETs can be applied in transistor applications, including analog circuits, voltage regulators, amplifiers, power transmitters and motor drivers.

Other transistor types

Transistor symbol drawn on Portuguese pavementin the University of Aveiro.

This article contains embedded lists that may be poorly defined, unverified or indiscriminate.Please help to clean it up to meet Wikipedia's quality standards. Where appropriate, incorporate items into the main body of the article. (September 2009)

For early bipolar transistors, see Bipolar junction transistor#Bipolar transistors.

• Bipolar junction transistor
  • Heterojunction bipolar transistor, up to several hundred GHz, common in modern ultrafast and RF circuits
  • Schottky transistor
  • Avalanche transistor
  • Darlington transistors are two BJTs connected together to provide a high current gain equal to the product of the current gains of the two transistors.
  • Insulated gate bipolar transistors (IGBTs) use a medium power IGFET, similarly connected to a power BJT, to give a high input impedance. Power diodes are often connected between certain terminals depending on specific use. IGBTs are particularly suitable for heavy-duty industrial applications. The Asea Brown Boveri (ABB) 5SNA2400E170100 illustrates just how far power semiconductor technology has advanced.^[27] Intended for three-phase power supplies, this device houses three NPN IGBTs in a case measuring 38 by 140 by 190 mm and weighing 1.5 kg. Each IGBT is rated at 1,700 volts and can handle 2,400 amperes.
  • Photo transistor
  • Multiple-emitter transistor, used in transistor–transistor logic
  • Multiple-base transistor, used to amplify very low level signals in noisy environments such as the pickup of a record player orradio front ends. Effectively, it is a very large number of transistors in parallel where, at the output, the signal is added constructively, but random noise is added only stochastically.^[28]
• Field-effect transistor
  • Carbon nanotube field-effect transistor (CNFET)
  • JFET, where the gate is insulated by a reverse-biased p-n junction
  • MESFET, similar to JFET with a Schottky junction instead of a p-n junction
    • High Electron Mobility Transistor (HEMT, HFET, MODFET)
  • MOSFET, where the gate is insulated by a shallow layer of insulator
  • Inverted-T field effect transistor (ITFET)
  • FinFET, source/drain region shapes fins on the silicon surface.
  • FREDFET, fast-reverse epitaxial diode field-effect transistor
  • Thin film transistor, in LCDs.
  • OFET Organic Field-Effect Transistor, in which the semiconductor is an organic compound
  • Ballistic transistor
  • Floating-gate transistor, for non-volatile storage.
  • FETs used to sense environment
    • Ion-sensitive field effect transistor, to measure ion concentrations in solution.
    • EOSFET, electrolyte-oxide-semiconductor field effect transistor (Neurochip)
    • DNAFET, deoxyribonucleic acid field-effect transistor
• Diffusion transistor, formed by diffusing dopants into semiconductor substrate; can be both BJT and FET
• Unijunction transistors can be used as simple pulse generators. They comprise a main body of either P-type or N-type semiconductor with ohmic contacts at each end (terminals Base1 and Base2). A junction with the opposite semiconductor type is formed at a point along the length of the body for the third terminal (Emitter).
• Single-electron transistors (SET) consist of a gate island between two tunneling junctions. The tunneling current is controlled by a voltage applied to the gate through a capacitor.^[29]
• Nanofluidic transistor, controls the movement of ions through sub-microscopic, water-filled channels.^[30]
• Multigate devices
  • Tetrode transistor
  • Pentode transistor
  • Trigate transistors (Prototype by Intel)
  • Dual gate FETs have a single channel with two gates in cascode; a configuration optimized for high frequency amplifiers, mixers, and oscillators.
• Junctionless Nanowire Transistor (JNT), developed at Tyndall National Institute in Ireland, was the first transistor successfully fabricated without junctions. (Even MOSFETs have junctions, although its gate is electrically insulated from the region the gate controls.) Junctions are difficult and expensive to fabricate, and, because they are a significant source of current leakage, they waste significant power and generate significant waste heat. Eliminating them held the promise of cheaper and denser microchips. The JNT uses a simple nanowire of silicon surrounded by an electrically isolated "wedding ring" that acts to gate the flow of electrons through the wire. This method has been described as akin to squeezing a garden hose to gate the flow of water through the hose. The nanowire is heavily n-doped, making it an excellent conductor. Crucially the gate, comprising silicon, is heavily p-doped; and its presence depletes the underlying silicon nanowire thereby preventing carrier flow past the gate.
• Vacuum channel transistor: In 2012, NASA and the National Nanofab Center in South Korea were reported to have built a prototype vacuum channel transistor in only 150 nanometers in size, can be manufactured cheaply using standard silicon semiconductor processing, can operate at high speeds even in hostile environments, and could consume just as much power as a standard transistor.^[31]