Transistor

The transistor is a solid state semiconductor device which can be used for amplification, switching, voltage stabilization, signal modulation and many other functions. It acts as a variable valve which, based on its input voltage, controls the current drawn by it from a connected voltage supply.

Operation

Bipolar junction transistor

The bipolar junction transistor (BJT) was the first type of transistor to be commercially mass-produced. Bipolar transistors are so named because the main conduction channel uses both electrons and holes to carry the main electric current. Two p-n junctions exist inside the BJT, colector-base junction and base-emitter junction. When the BJT is not powered, the junctions are in unbiased thermal equilibrium with a depletion region formed at each junction. The arrangement of greatest interest is when the B-E junction is forward biased and the C-B junction is reverse biased because this makes amplification possible. Applying a forward bias voltage to the B-E junction unbalances the thermal equilibrium of the junction. In an NPN type BJT, the p-type base begins to inject surplus holes (holes not required to maintain thermal equilibrium) into the emitter where they quickly recombine in the n-type material at the emitter contact. Similarly, the emitter injects surplus electrons into the base. If not for the close proximity of the reverse-biased C-B junction, the B-E junction would behave like a diode, injected emitter electrons would flow out of the base and transistor action would not occur. In the NPN BJT, the electrons which are injected from the emitter into the base diffuse across the very thin base region before most of them have time to recombine within the base region. The depletion region of the nearby reverse biased C-B junction contains an electric field, which sweeps any electrons close to it into the p-type collector, where the recombine with holes at the collector contact. Through this action, only a small amount of carriers in the base (holes) are needed to produce a large amount of carriers in the emitter (electrons) to flow through to the collector.

Related Topics:
Electron - Hole - P-n junctions - Thermal equilibrium - Depletion region - Biased - Diode - Diffuse

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In analog circuits, the BJT is commonly used with voltage feedback to control the base current. In digital circuits, such as TTL logic the base is driven by large voltages, which draws higher base current and consumes more power. Compared to the FET, the BJT has a lower input impedance when used without voltage feedback.

Related Topics:
Feedback - TTL logic - Impedance

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BJTs can be driven by a current input as well. The ratio of the allowed emitter-collector current to the base-emitter current is called current gain, beta or h_{FE}. An h_{FE} value of 100 is typical for small bipolar transistors. In a typical configuration, a very small signal current flows through the base-emitter junction to control the emitter-collector current. Less commonly, bipolar transistors are operated with emitter and collector reversed, thus a base-collector current can control the emitter-collector current. The gain in this mode is much smaller (i.e., 2 instead of 100), and it is not a value that is controlled by manufacturers so it can vary dramatically among transistors.

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Bipolar transistors can be turned on with light as well as electricity. Devices designed for this purpose are called phototransistors, although these can be standard transistors in a transparent package.

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Field-effect transistor

Field-effect transistors (also called unipolar transistors) use only one of the two types of carrier (either electrons or holes, depending on the subtype of the FET). In FETs the main current appears in a narrow conducting channel with an insulating depletion zone at the side. The width of this insulating zone can be altered by varying the voltage applied to the gate terminal, enlarging or constricting the conducting channel and thereby controlling the main current.

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FETs are divided into two families: junction FET (JFET) and insulated gate FET (IGFET). The IGFET is more commonly known as metal oxide silicon(or semiconductor) FET (MOSFET). Unlike MOSFETs, the JFET gate terminal forms a diode with the channel which forms 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.

Related Topics:
JFET - MOSFET - Diode - Triode - Grid - Cathode

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FETs are further divided into enhancement mode and depletion mode types. Mode refers to the polarity of the gate voltage with respect to the source when the device is conducting. For an N-channel FET: in depletion mode the gate is negative with respect to the source while in enhancement mode the gate is positive. For both modes, if the gate voltage is made more positive the source/drain current will increase. For P-channel devices the polarities are reversed. Most IGFETs are enhancement mode types and nearly all JFETs are depletion mode types.

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Other transistor types

• Unijunction transistors can be used as simple pulse generators. They comprise a main body of either P- 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).
• Dual gate FETs have a single channel with two gates in cascode; a configuration that is optimized for high frequency amplifiers, mixers, and oscillators.
• Transistor arrays are used for general purpose applications, function generation and low-level, low-noise amplifiers. They include two or more transistors on a common substrate to ensure close parameter matching and thermal tracking, characteristics that are especially important for long tailed pair amplifiers.
• Darlington transistors comprise a medium power BJT connected to a power BJT. This provides a high current gain equal to the product of the current gains of the two transistors. Power diodes are often connected between certain terminals depending on specific use.
• 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 http://library.abb.com/GLOBAL/SCOT/scot256.nsf/VerityDisplay/71B8625C035676C2C1256F9000471D3C/$File/5SNA%202400E170100_5SYA%201555-02Aug%2004.pdf illustrates just how far power semiconductor technology has advanced. Intended for three-phase power supplies, this awesome device houses three NPN IGBTs in a case measuring 38 by 140 by 190 mm. Each IGBT is rated at 1,700 volts and can handle 2,400 amperes.

Semiconductor material

The first BJTs were made from germanium (Ge) and some high power types still are. Silicon (Si) types currently predominate but certain advanced microwave and high performance versions now employ the compound semiconductor material gallium arsenide (GaAs). Germanium was largely replaced by silicon because silicon semiconductor behavior is stable at higher relative temperatures. Single element semiconductor material (Ge and Si) is described as elemental.

Related Topics:
Germanium - Silicon - Si - Gallium arsenide - GaAs

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Characteristics of the most common semiconductor materials used to make transistors are given in the table below:

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Semiconductor material characteristics

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Semiconductor material Junction forward voltage V @ 25°C Electron mobility m/s @ 25°C Hole mobility m/s @ 25°C Max. junction temp. °C

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Ge 0.27 0.39 0.19 70 to 100

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Si 0.71 0.14 0.05 150 to 200

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GaAs 1.03 0.85 0.05 150 to 200

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Al?Si junction 0.3 - - 150 to 200

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The junction forward voltage is the voltage applied to the emitter-base junction of a BJT in order to make the base conduct a specified current. The values given in the table are typical for a current of 1 mA (the same values apply to semiconductor diodes). The lower the junction forward voltage the better, as this means that less power is required to "drive" the transistor. The junction forward voltage for a given current decreases with temperature. For a typical silicon junction the change is approximately -2.1 mV/°C.

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The electron mobility and hole mobility columns show the average speed that electrons and holes diffuse through the semiconductor material with an electric field of 1 Volt per meter applied across the material. In general, the higher the electron mobility the faster the transistor. The table indicates that Ge is a better material than Si in this respect. However, Ge has four major shortcomings compared to silicon and gallium arsenide: its maximum temperature is limited, it has relatively high leakage current, it cannot withstand high voltages and it is less suitable for fabricating integrated circuits. Because the electron mobility is higher than the hole mobility for all semiconductor materials, a given bipolar NPN transistor tends to be faster than an equivalent PNP transistor type. GaAs has the fastest electron mobility of the three semiconductors. It is for this reason that GaAs is used in high frequency applications. A relatively recent FET development, the high electron mobility transistor (HEMT), has a heterostructure (junction between different semiconductor materials) of aluminium gallium arsenide (AlGaAs)?gallium arsenide (GaAs) which has double the electron mobility of a GaAs?metal barrier junction. Because of their high speed and low noise, HEMTs are used in satellite receivers working at a frequency around 12 GHz.

Related Topics:
Electron mobility - Hole mobility - Electric field - Leakage current - NPN - PNP - HEMT - Heterostructure

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Max. junction temperature values represent a cross section taken from various manufacturers' data sheets. This temperature should not be exceeded or the transistor may be destroyed.

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Al?Si junction refers to the high-speed (aluminum?silicon) semiconductor?metal barrier diode, commonly known as a Schottky diode. This is included in the table because most silicon power IGFETs have a parasitic reverse Schottky diode formed between the source and drain as part of the fabrication process.

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