Transistor in General

Transistor A transistor is a semiconductor device used to amplify and switch electronic signals. It is made of a solid piece of semiconductor material, with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, the transistor provides amplification of a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

The transistor is the fundamental building block of modern electronic devices, and is ubiquitous in modern electronic systems. Following its release in the early 1950s the transistor revolutionized the field of electronics, and paved the way for smaller and cheaper radios, calculators, and computers, amongst other things. Advantages The key advantages that have allowed transistors to replace their vacuum tube predecessors in most applications are * Small size and minimal weight, allowing the development of miniaturized electronic devices. Highly automated manufacturing processes, resulting in low per-unit cost. * Lower possible operating voltages, making transistors suitable for small, battery-powered applications. * No warm-up period for cathode heaters required after power application. * Lower power dissipation and generally greater energy efficiency. * Higher reliability and greater physical ruggedness. * Extremely long life. Some transistorized devices have been in service for more than 50 years. * Complementary devices available, facilitating the design of complementary-symmetry circuits, something not possible with vacuum tubes. Insensitivity to mechanical shock and vibration, thus avoiding the problem of microphonics in audio applications. Limitations * Silicon transistors do not operate at voltages higher than about 1,000 volts (SiC devices can be operated as high as 3,000 volts). In contrast, electron tubes have been developed that can be operated at tens of thousands of volts. * High power, high frequency operation, such as that used in over-the-air television broadcasting, is better achieved in electron tubes due to improved electron mobility in a vacuum. Silicon transistors are much more vulnerable than electron tubes to an electromagnetic pulse generated by a high-altitude nuclear explosion. Classification Transistors are categorized by * Semiconductor material: germanium, silicon, gallium arsenide, silicon carbide, etc. * Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types" * Polarity: 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 fT, an abbreviation for "frequency of transition".

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 * Amplification factor hfe (transistor beta) Thus, a particular transistor may be described as silicon, surface mount, BJT, 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 (BJT) 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. " In an NPN transistor operating in the 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 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 (Vgs) is increased, the drain–source current (Ids) increases exponentially for Vgs below threshold, and then at a roughly quadratic rate () (where VT 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 (JFET) 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.

Metal–semiconductor FETs (MESFETs) are JFETs in which the reverse biased PN junction is replaced by a metal–semiconductor Schottky-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. Single-electron transistors Single Electron Transistor (SET) consists of a gate island between two tunnelling junctions. The tunnelling current is controlled by a voltage applied to the gate through a capacitor. In physics, a Coulomb blockade (abbreviated QB), named after Charles-Augustin de Coulomb's electrical force, is the increased resistance at small bias voltages of an electronic device comprising at least one low-capacitance tunnel junction.

Because of the QB, the resistances of devices are not constant at low bias voltages, but increase to infinity for zero bias (i. e. no current flows). Single electron transistor Schematic of a single electron transistor Energy levels of source, island and drain (from left to right) in a single electron transistor for both the blocking state (upper part) and the transmitting state (lower part). Single electron transistor with niobium leads and aluminium island The simplest device in which the effect of Coulomb blockade can be observed is the so-called single electron transistor.

It consists of two tunnel junctions sharing one common electrode with a low self-capacitance, known as the island. The electrical potential of the island can be tuned by a third electrode (the gate), capacitively coupled to the island. In the blocking state no accessible energy levels are within tunneling range of the electron (red) on the source contact. All energy levels on the island electrode with lower energies are occupied. When a positive voltage is applied to the gate electrode the energy levels of the island electrode are lowered.

The electron (green 1. ) can tunnel onto the island (2. ), occupying a previously vacant energy level. From there it can tunnel onto the drain electrode (3. ) where it inelastically scatters and reaches the drain electrode Fermi level (4. ). The energy levels of the island electrode are evenly spaced with a separation of ? E. ?E is the energy needed to each subsequent electron to the island, which acts as a self-capacitance C. The lower C the bigger ? E gets.

To achieve the Coulomb blockade, three criteria have to be met: * the bias voltage can't exceed the charging energy divided by the capacitance Vbias =  ; * the thermal energy kBT must be below the charging energy EC = , or else the electron will be able to pass the QB via thermal excitation; and * the tunneling resistance (Rt) should be greater than , which is derived from Heisenberg's Uncertainty principle. Coulomb Blockade Thermometer A typical Coulomb Blockade Thermometer (CBT) is made from an array of metallic islands, connected to each other through a thin insulating layer.

A tunnel junction forms between the islands, and as voltage is applied, electrons may tunnel across this junction. The tunneling rates and hence the conductance vary according to the charging energy of the islands as well as the thermal energy of the system. Coulomb Blockade Thermometer is a primary thermometer based on electric conductance characteristics of tunnel junction arrays. The parameter V? =5. 439NkBT/e, the full width at half minimum of the measured differential conductance dip over an array of N junctions together with the physical constants provide the absolute temperature.

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