12.1: Introduction

The MOSFET shares many similarities with the JFET including very low gate current and being modeled as a voltage-controlled current source. It is also available in N- and P-channel varieties. Unlike the JFET, it has two variations: the depletion-enhancement mode variant, or DE-MOSFET; and the enhancement-only mode variant, or E-MOSFET. All of the bias types discussed for JFETs will work for DE-MOSFETs, plus a few others. EMOSFETs, on the other hand, require new biasing prototypes.

For AC analysis, both common source and common drain amplifier topologies may be realized with DE- and EMOSFETs. The equations for input impedance, voltage gain and the like are generally unchanged from the JFET. E-MOSFETs are also available as power devices. They have certain advantages over power BJTs, including higher speed and a negative temperature coefficient of transconductance which means they are less likely to suffer from thermal instabilities such as current hogging.

One item of practical importance is that MOSFETs are very susceptible to ESD (electrostatic discharge) and special precautions must be taken to prevent accidental damage to the device. Unlike both the JFET and the BJT, the MOSFET does not rely on a PN junction for its operation. Instead, it uses a charge-based system not unlike a capacitor. The gate is, in fact, insulated from the channel. For this reason it is sometimes referred to as an IGFET, which stands for Insulated Gate FET. This insulation layer will lead to very, very high input resistance due to extremely low gate current but also leads to the issue of ESD susceptibility.