In the previous article, we covered the symbols for passive components and diodes, which are semiconductor devices that don’t provide amplification. We also looked at two devices—the SCR and the TRIAC—that are more like amplifiers because they allow a small-amplitude signal to control a larger-amplitude signal.
Semiconductor devices that provide both switching action and signal amplification are called transistors, but nowadays this term is rather uninformative because there are so many different types of transistors.
In this article, we'll take a look at the symbols for various transistors.
- Symbols for logic gates, latches, and flip-flops in the AAC textbook
Symbols for Bipolar Junction Transistors
Bipolar junction transistors (BJTs) consist of three layers of semiconductor material. These can be arranged as either 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 the drain).
The “MOS” stands for “metal oxide semiconductor” but unfortunately this is now inaccurate since the gate of a typical MOSFET is made from polysilicon instead of metal.
There is a more accurate term for these devices, however: IGFET, which stands for insulated-gate field-effect transistor. In my experience, though, this name is almost never used.
NMOS vs PMOS
Like BJTs, MOSFETs fit into two broad categories: N-channel or P-channel. A convenient way to discuss MOSFETs is to call an N-channel device an NMOS and a P-channel device a PMOS.
MOSFETs, 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 terminal can be ignored, because its effect is negligible.
The Version 1 symbols above reflect the fact that the body terminal is usually not relevant to circuit operation. However, in cases where the body connection is important, we have these symbols:
MOSFETs, Version 2. The body terminal is included between the source and drain.
If for some reason you don’t like the Version 1 symbols, you’re in luck:
MOSFETs, Version 3.
In this case, you don’t have an arrow that distinguishes source from drain. In the Version 3 symbols, the source is the terminal that has a connection to the body terminal. This is easy to remember if you happen to know that in real circuits the body of a FET is often shorted to the source.
If you switch back and forth between Version 1 and Version 3, be careful with the arrows. In Version 1, an arrow pointing toward the gate indicates a PMOS; in Version 3, an arrow pointing toward the gate indicates an NMOS.
MOSFET-symbol aficionados will be glad to know that there is yet another way to represent these components. When we’re analyzing or designing 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 by a logic-low voltage and the NMOS is activated by a logic-high voltage.
Thus, we can use the following simplified symbols:
MOSFETs, Version 4. The circle, which indicates “active-low” input behavior, distinguishes PMOS from NMOS.
Other Types of Transistors
The IGBT (insulated-gate bipolar transistor) incorporates MOSFET properties and BJT properties into a single device. It is used primarily in switching applications.
The symbol shown below seems to be the more common version; if you want to see the alternative representation, refer to the AAC textbook page on IGBTs.
N-channel IGBT. Note how the vertical empty space indicates that the gate terminal is insulated from the rest of the device.
A JFET (junction field-effect transistor) is like a MOSFET, but the gate is not insulated. Nowadays these devices are rare. If you know of a modern application in which JFETs are the preferred type of transistor, let us know in the comments.
A Darlington pair combines two BJTs such that the emitter current of the first becomes the base current of the second. The result is 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 them all. However, I’ve tried to present the symbols for some of the most important and common devices. I hope that this information and the accompanying diagrams will help to make your schematics more accurate and maybe even a little nicer to look at.