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Transistor in general

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The frequency of transition is the purpose, audio, high voltage, super-beta, matched pair * Physical packaging: through hole metal, through hole plastic, surface mount, ball grid array, power odules * Amplification factor hfe (transistor beta) Thus, a particular transistor may be described as silicon, surface mount, BIT, NPN, low power, high frequency switch. Bipolar Junction transistor Bipolar transistors are so named because they conduct by using both majority and minority carriers.

The bipolar Junction transistor (BIT) 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 ransistor). 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. ” In an NPN transistor operating in he active region, the emitter-base Junction is forward biased (electrons and 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. Collector current is approximately (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. The BJT is a low-input-impedance device. Also, as the base-emitter voltage (Vbe) is increased the base-emitter current and hence the collector-emitter current (Ice) increase exponentially according to the Shockley diode model and the Ebers-Moll model. Because of this exponential relationship, the BJT has a good transconductance. Simple circuit to show the labels of a BJT As an electronic switch

Amplifier circuit 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 times the photocurrent. Devices designed for this purpose have a transparent window in the package and are called phototransistors. Field-effect transistor The field-effect transistor (FET), 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 FETs, the drain-to-source current flows via a conducting channel that connects the source region to the drain region. The 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 (Vgs) is increased, the drain-source current (‘ds) increases exponentially for Vgs below threshold, and then at a roughly quadratic rate 0 (where W is the threshold voltage at which drain current begins) 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. For low noise at narrow bandwidth the higher input resistance of the FET is advantageous. FETs are divided into two families: Junction FET OFET) and insulated gate FET (IGFET).

The IGFET is more commonly known as a metal-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 PN 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.

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