Originally, the most significant difference between a MOSFET and a BJT was the high input impedance afforded by the gate insulation of the FET. The high quality of the SiO2/Si system in Si MOSFETs has not proved possible to replicate in other semiconductor systems, particularly those involving III-V compound semiconductors. Therefore, to capitalize on the advantages that the latter semiconductors may have over silicon, such as mobility (see Fig. 11.1), and still realize a FET device, some other way of implementing the field-effect is necessary. This has been done by using a metal/semiconductor junction, rather than a metal/insulator/semiconductor combination, to create a vertical field to control the charge in the channel. The two main devices that exploit this are shown in Fig. 11.2: the MESFET (metal-semiconductor FET), and the HEMT (high-electron-mobility transistor). The latter device is also sometimes called a MODFET, where MOD refers to modulation doping, and relates to the fact that the doping in the top barrier layer plays a key role in controlling (modulating) the channel charge. All these transistors are HJFETs (heterojunction FETs) on account of the presence of a metal/semiconductor heterojunction in their structures.

The use of high-mobility semiconductors in MESFETs and HEMTs enables attainment of a high transconductance, which is a prerequisite for good high-frequency perfomance (see Section 14.4).