Prof. Dr. Thorsten Jungmann (last updated 2026-10-11)
1 What does it look like?
A transistor (German: Transistor) is a semiconductor component with three terminals; a MOSFET has a fourth, the bulk (section 3). As a single component, it sits in a small plastic or metal package with three legs or contact pads. In an integrated circuit, a single transistor cannot be seen with the naked eye: modern integrated circuits contain billions of them [5, p. 62].
2 What can it do?
A transistor controls a large current with a small current or a small voltage. It can switch – then it works as an electrically controlled switch, as in every digital circuit – or it can amplify, as in analogue amplifiers.
3 How does it work?
There are two families of transistors:
- The bipolar junction transistor (BJT, German: Bipolartransistor) has the terminals base, collector and emitter. It is controlled by a current: a small current into the base switches a larger current between collector and emitter, and the base current has to keep flowing to keep the transistor on [5, p. 71].
- The metal oxide semiconductor field-effect transistor (MOSFET, German: MOS-Feldeffekttransistor) has the terminals gate, source and drain, and the bulk, the connection of the substrate, which lies at a constant potential in CMOS circuits [12, p. 39]. It is controlled by a voltage: the voltage between gate and source creates an electric field that opens or closes a conducting channel between source and drain [5, pp. 62–63]. Its gate is insulated, so that it needs practically no power to stay on or off.
An n-channel MOSFET (NMOS) switches on when the voltage between gate and source is greater than its threshold voltage, typically 0.2 V to 1 V; a p-channel MOSFET (PMOS) switches on when its gate is lower than its source by more than the threshold voltage [5, p. 63].
4 What does it look like inside?
Inside, a transistor consists of differently doped regions of a semiconductor. A bipolar junction transistor has two pn junctions, as an npn or a pnp transistor. In a MOSFET, a thin insulating oxide layer separates the gate electrode from the channel between source and drain [5, p. 63]; DET-02, 1.2 shows its cross-section. What happens in the semiconductor is explained on the page semiconductor.
5 What role does it play in teaching?
In Digital Electronics, the transistor appears as a switch. Two complementary MOSFETs form the inverter of complementary metal oxide semiconductor (CMOS) technology (DET-02, 1.3), four of them a NAND gate (DET-02, 4.1). The resistance of the switched-on transistor and the capacitance that it has to charge or discharge explain the propagation delay of a logic gate.
6 What role does it play in practice?
Transistors are found in every electronic device: in processors, memories, microcontrollers, power supplies and amplifiers. Digital circuits use almost only MOSFETs in CMOS technology, because they need hardly any power at rest and can be packed densely [5, pp. 64, 71]. Bipolar transistors are found in analogue amplifiers and in the older logic family transistor–transistor logic (TTL) [5, p. 71].
Symbols
- Schematic symbols for the n-channel and the p-channel MOSFET and for the npn and the pnp bipolar junction transistor according to IEC 60617.
Calculations
- Switching time of a gate: , with the resistance of the switched-on transistor and the capacitance at the output (DET-02, 3.5)