![]() Thus, we can use the following simplified symbols: 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. 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. MOSFET-symbol aficionados will be glad to know that there is yet another way to represent these components. In Version 1, an arrow pointing toward the gate indicates a PMOS in Version 3, an arrow pointing toward the gate indicates an NMOS. ![]() If you switch back and forth between Version 1 and Version 3, be careful with the arrows. 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. In the Version 3 symbols, the source is the terminal that has a connection to the body terminal. In this case, you don’t have an arrow that distinguishes source from drain. If for some reason you don’t like the Version 1 symbols, you’re in luck: The body terminal is included between the source and drain. ![]() However, in cases where the body connection is important, we have these symbols: The Version 1 symbols above reflect the fact that the body terminal is usually not relevant to circuit operation. In most situations, the body terminal can be ignored, because its effect is negligible. The physical structure of a MOSFET results in a fourth terminal called the body. The terminal on the left is the gate, the arrow identifies the source, and the remaining terminal is the drain. A convenient way to discuss MOSFETs is to call an N-channel device an NMOS and a P-channel device a PMOS. Like BJTs, MOSFETs fit into two broad categories: N-channel or P-channel. In my experience, though, this name is almost never used. There is a more accurate term for these devices, however: IGFET, which stands for insulated-gate field-effect transistor. 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. 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). Symbols for MOSFETsīJTs are still used, but the transistor scene is currently dominated by MOSFETs. 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. 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: Symbols for logic gates, latches, and flip-flops in the AAC textbookīipolar junction transistors (BJTs) consist of three layers of semiconductor material.In this article, we'll take a look at the symbols for various transistors. 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. 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. In the previous article, we covered the symbols for passive components and diodes, which are semiconductor devices that don’t provide amplification. This two-article series presents an overview of circuit symbols and also provides some information on the components themselves.
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