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.
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:
MOSFETs, Version 3.
In this case, it does not have an arrow distinguishing the source from the drain. In Version 3 symbols, the source is the terminal that has a connection to the main terminal. This is easy to remember if you know that in real circuits, the body of a FET is often shorted to the source.
If you switch between version 1 and version 3, watch out for the arrows. In Version 1, an arrow pointing towards the door indicates a PMOS; in Version 3, an arrow pointing towards the gate indicates an NMOS.
Fans of MOSFET symbols will be glad to know that there is another way to represent these components. When we analyze or design CMOS circuits, we often think of MOSFETs as voltage-controlled on / off switches, without any specific reference to source and drain terminals. In this context, the only difference between an NMOS and a PMOS is that the PMOS is activated with a low logic voltage and the NMOS is activated with a high logic voltage.
Therefore, we can use the following simplified symbols:
MOSFET version 4. The circle, indicating “active-low” input behavior, distinguishes PMOS from NMOS.
Other types of transistors
The IGBT (Insulated Gate Bipolar Transistor) incorporates MOSFET properties and BJT properties in a single device. It is mainly used in switching applications.
The symbol shown below appears to be the most common version; For the alternate representation, see the AAC textbook page on IGBT.
Channel N IGBT. Notice how the vertical void space indicates that the gate terminal is isolated from the rest of the device.
A JFET (junction field effect transistor) is like a MOSFET, but the gate is not isolated. Today these devices are rare. If you know of a modern application where JFETs are the preferred transistor type, please let us know in the comments.
A Darlington pair combines two BJTs so that the emitter current of the first becomes the base current of the second. The result is a very high current gain.
NPN Darlington Pair
A Darlington pair is considered an extension of the concept of a common collector amplifier.
There are numerous electronic components, and we certainly haven’t covered all of them. However, I have tried to present the symbols of some of the most important and common devices. I hope this information and the accompanying diagrams help make your schematics more accurate and perhaps a little more pleasant to look at.