The MOSFET

The acronym MOSFET (pronounced “MOSS-fet”) stands for metal-oxidesemiconductor field-effect transistor. A simplified cross-sectional drawing of an n-channel MOSFET and its schematic symbol are shown in Fig. 1a and b. The p-channel device is shown at Fig. 1 c and d.
When the MOSFET was first developed, it was called an insulated-gate FET or IGFET. This is perhaps more descriptive of the device than the currently accepted name. The gate electrode is actually insulated, by a thin

Fig. 1. Pictorial diagram of an n-channel MOSFET (a), schematic symbol for an n-channel MOSFET (b), pictorial diagram of a p-channel MOSFET (c), and schematic symbol for a p-channel MOSFET (d).

layer of dielectric, from the channel. As a result of this, the input resistance (and hence the impedance) is extremely high. The MOSFET draws essentially no current from the input signal source. This is an asset in weak-signal amplifiers.