Introduction

In Chapter 8 we studied the charge control of the 2DEG (two-dimensional electron gas). In Chapter 9 we studied high-field transport models applicable to horizontal transport in the 2DEG. The motivation for these studies was the application of the 2DEG as the channel of a field-effect transistor (FET). The resulting FET is referred under the various names of MODFET (modulation doped FET (University of Illinois, USA)), HEMT (high electron mobility FET (Japan)), TEGFET (two-dimensional electron gas FET (France)), and SDHT (segregation doping heterojunction transistor (Bell Lab., USA)) depending on the different laboratories which simultaneously developed it. The AlAs–InGaAs–InP lattice-matched MODFET [2] which provides power gain at millimeter frequencies (fmax = 405 GHz) is presently with the HBT one of the fastest semiconductor transistors. The microwave characteristic of the MODFET will be discussed in Chapters 11, 12, 13, 15 and 16.

We compare in Figure 10.1 the layout of an AlGaAs–GaAs MODFET (c) with that of that of a silicon MOSFET (a) and a GaAs MESFET (b). Although the layout of the MODFET is similar to that of the MESFET, its normal principle of operation (control of a 2DEG with a gate voltage) is similar to that of the MOSFET. Other semiconductors can be used to fabricate MODFETs (see [1] for a review). A semiconductor with a bandgap much wider than that of AlGaAs can also be used to simulate an insulator.