Chapter 14 - Field-Effect Devices

Summary

One of the most important physical mechanisms of importance to semiconductor devices is the field effect. Several important devices exploit this effect in their operation, such as metal-oxide–semiconductor field-effect transistors (MOSFETs), metal–semiconductor field-effect transistors (MESFETs), and junction field-effect transistors (JFETs). In fact, the field effect transistor (FET) is arguably the most important innovation that has fueled the computer and information revolution. In this chapter, the fundamentals of the FET operation are presented; the reader is directed to the references for a more comprehensive study.

The Field Effect

The field effect can be simply defined as the modulation of the conductivity of an underlying semiconductor layer by the application of an electric field to a gate electrode on the surface. As we learned in Chapter 11, the application of a bias to a MIS structure results in a modulation in the carrier concentration within the underlying semiconductor layer. If the semiconductor is naturally n type and a positive gate bias is applied, electrons accumulate at the semiconductor-insulator interface. Conversely, if a negative gate bias is applied to the same structure, the electrons are repelled from the interface and, depending on the magnitude of the bias, the underlying semiconductor layer is either depleted or inverted. If the semiconductor becomes inverted, the carrier type changes.