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. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify 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.

The essential usefulness of a transistor comes from its ability to use a small signal applied between one pair of its 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; that is, it can act as an amplifier. Alternatively, the transistor can be used to turn current on or 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, which have slight differences in how they are used in a circuit. A bipolar transistor has terminals labeled base, collector, and emitter. A small current at the base terminal (that is, flowing between the base and the emitter) can control or switch a much larger current between the collector and emitter terminals. For a field-effect transistor, the terminals are labeled gate, source, and drain, and a voltage at the gate can control a current between source and drain.

Transistors are commonly used in digital circuits as electronic switches which can be either in an “on” or “off” state, both for high-power applications such as switched-mode power supplies and for low-power applications such as logic gates. Important parameters for this application include the current switched, the voltage handled, and the switching speed, characterised by the rise and fall times.

Transistor circuits can be classified as:

Common base
Common collector
Common gate
Common drain
Common source
Open collector
Two-port network


low power consumption in relation to high power consumption of vacuum tube

very small size and weight, reducing equipment size;

large numbers of extremely small transistors can be manufactured as a single integrated circuit;

low operating voltages compatible with batteries of only a few cells;

responds faster than vacuum tubes

inherent reliability and very long life; tubes always degrade and fail over time.

complementary devices available, providing design flexibility including complementary-symmetry circuits, not possible with vacuum tubes;

very low sensitivity to mechanical shock and vibration, providing physical ruggedness and virtually eliminating shock-induced spurious signals;


Transistors have the following limitations:

silicon transistors can age and fail;

high-power, high-frequency operation, such as that used in over-the-air television broadcasting, is better achieved in vacuum tubes due to improved electron mobility in a vacuum;

solid-state devices are susceptible to damage from very brief electrical and thermal events, including electrostatic discharge in handling; vacuum tubes are electrically much more rugged;

sensitivity to radiation and cosmic rays (special radiation-hardened chips are used for spacecraft devices);

vacuum tubes in audio applications create significant lower-harmonic distortion, the so-called tube sound, which some people prefer.

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