Maria Montero

Schematic Symbols for Electronic Components: Transistors

This series of two articles presents an overview of circuit symbols and also provides information on components.

In the previous article, we covered the symbols for diodes and passive components, which are semiconductor devices that do not provide amplification. We also looked at two devices, the SCR and the TRIAC, which are more like amplifiers in that they allow a low-amplitude signal to drive a higher-amplitude signal.

Semiconductor devices that provide both the switching action and the amplification of the signal are called transistors, but today this term is quite uninformative because there are many different types of transistors.

In this article, we will take a look at the symbols of various transistors.

Related information

Symbols for Bipolar Junction Transistors

Bipolar junction transistors (BJTs) consist of three layers of semiconductor material. These can be arranged as an NPN or PNP transistor and the circuit symbol (as well as the functionality) changes according to the layer arrangement:

Bipolar Junction Transistors

The horizontal terminal is called the base, the diagonal terminal without an arrow is the collector, and the diagonal terminal with an arrow is the emitter.

Symbols for MOSFETs

BJTs are still used, but the transistor scene is currently dominated by MOSFETs. These are field effect transistors (FETs) that have an insulating layer between a conductive control terminal (called the gate) and the semiconductor structure that connects the other two terminals (called the source and drain).

The “MOS” stands for “metal oxide semiconductor”, but unfortunately this is now inaccurate since the gate of a typical MOSFET is made of polysilicon rather than metal.

However, there is a more precise term for these devices: IGFET, which stands for insulated gate field effect transistor. In my experience, however, this name is hardly ever used.

NMOS vs PMOS

Like BJTs, MOSFETs fall into two broad categories: the N-channel or the P-channel. A convenient way to discuss MOSFETs is to call an N-channel device a NMOS and a P-channel device a PMOS.

MOSFET version 1. The terminal on the left is the gate, the arrow identifies the source, and the remaining terminal is the drain.

The physical structure of a MOSFET results in a fourth terminal called the body. In most situations, the body’s terminal can be ignored, because its effect is negligible.

The Version 1 symbols above reflect the fact that the body terminal is generally not relevant to the operation of the circuit. However, in cases where the body connection is important, we have these symbols:

MOSFET version 2. The body terminal is included between the source and drain.

If for some reason you don’t like the symbols in version 1, you’re in luck: