A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor’s terminals controls the current through another pair of terminals. Since the output power can be higher than the input power, a transistor can be used to amplify a signal. Transistors can be manufactured individually or embedded with integrated circuits. The most widely used transistor is the MOSFET ( metal – oxide – semiconductor field effect transistor). The MOSFET first invented in 1959 was the first truly compact transistor that could be miniaturized and mass produced for a wide variety of used. Transistors revolutionized the field of electronics and and paved the way for smaller and cheaper radios, computers, phones, calculators etc. Most transistors are made from pure silicon or germanium or other semiconductor materials. A transistor may have only one kind of charge carrier, in a field effect transistor or may have two kinds of charge carriers in bipolar junction transistor devices. Compared to vacuum tubes, transistors are generally smaller and require less power to operate.
Materials for production of transistors include germanium, silicon and other compounds such as gallium arsenide and semiconductor alloy silicon germanium. Parameters used to manufacture transistors vary with temperature, electric field, impurity level, strain and other factors. The junction- forward voltage is the voltage applied to the emitter- base junction of a biplar junction transistor to make the base conduct a specified current. The current increases exponentially as the junction forward voltage is increased. The lower the junction forward voltage the better as this means less power is required to drive the transistor. The junction forward voltage for a given current decreases with increase in temperature. Germanium while suitable for construction of transistors has its drawbacks when compared to silicon and gallium arsenide. These are; it’s maximum temperature is limited, it has relatively high leakage current, it cannot withstand high voltages and is less suitable for fabricating integrated circuits. Because the electron mobility is higher than the hole mobility for all semiconductor materials, a given bipolar n- p- n transistor tends to be swifter than the p-n- p equivalent. Gallium arsenide has the highest electron mobility of the three semiconductors hence it is used in high frequency applications. Another transistor known as high electron mobility transistor or HEMT composed of aluminum, gallium arsenide has twice the electron mobility of a gallium arsenide metal barrier junction. Because of their high speed and low noise, HEMTs are used in satellite recievers working at frequencies around 12GHz.
A transistor can use a small signal applied between one pair of it’s terminals to control a much larger signal at another pair of terminals. This property is called gain. It can produce a stronger output signal, a voltage or current which is proportional to a weaker input signal and thus acts as an amplifier. Alternatively the transistor can be used to turn current on and off in a circuit as an electrically controlled switch, where the amount of current is determined by other circuit elements. There are two types of transistors. These are; the Bipolar Junction transistor and the Field Effect transistor. The Bipolar Junction transistor consist of three terminals known as emitter, base and collector, while the Field Effect transistor has three terminals known as gate, source and drain.
Transistors are classed into different types;
- Structure: these include; MOSFET, BJT, insulated gate bipolar transistor,( IGBT).
- Semiconductor material, the metalloids germanium and silicon, in amorphous, polycrystalline or monocrystalline form, the compounds gallium arsenide and silicon carbide, the alloy silicon- germanium, the allotrope of carbon graphene.
- Electrical polarity ( positive and negative) n- p-n, p-n-p.
- Maximum power rating; low, medium, high.
- Maximum operating frequency: low, medium, high, radio ( RF), microwave frequency,
- 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.
- Temperature: Extreme temperature transistors and traditional temperature transistors.
FIELD EFFECT TRANSISTORS
The field effect transistors also called unipolar transistors use either electrons in n- channel FET or holes in p- channel FET for conduction. The four terminals of the FET are; the source, gate, drain and body. On most FETs the body is connected to the source inside the package. 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.