This chapter givens an introduction to the theory of small signal GaAs MESFETs with particular emphasis on the models used to predict the FET gain and noise figure performance. Such calculations have led to better FET structures and geometries being introduced as a means to exploit the intrinsically superior properties of GaAs over Si. The properties of the dual-gate FET have also been covered in some depth as this particular type of device has proved itself to be especially useful in microwave circuit applications.

This chapter has attempted to give a review of material preparation techniques and FET fabrication processes related specifically to GaAs.With the advent of ion implantation and more complicated device structures 'wet' etching techniques are giving way to 'dry' methods such as ion beam milling to define gate metallization and plasma etching for dielectric definition. Also, considerable effort has been applied to improving the reliability of both low-noise and power FETs by careful choice of metallizations and device design.

This chapter has dealt with the subject of single and dual gate FETs used in frequency conversion applications. It has been shown (Loriou et al, 1976) that both types of MESFET suffer from 1/f noise contributions for IFs much below a few hundred megahertz. The general device requirements for FETs to be used in mixer and other conversion circuits are very similar to those requirements for low noise amplifiers. Recently, considerable theoretical work has been achieved to explain observed FET mixer noise figure performances with the frequency of the IF and the signal LO and IF loads. Recent studies into the signal, LO, IF, image and sum frequency terminations have given an insight into the optimum conditions for conversion gain especially with single gate FET mixers. Since the matching of the second gate of a dual gate FET can be difficult over relatively broad bandwidths, as it is in the amplifier case discussed in Chapter 2, the use of dual-gate FETs in mixers, although producing simple injection of LO and signal frequencies, has led to other FET based circuits being realized particularly at the lower frequencies (Van Tuyl, 1980; Suffolk et al, 1980; Ablassmeier, et al, 1980). Dual-gate FETs are now receiving considerable attention as efficient up-converters and multipliers as well as in self-oscillating mixer circuits (Stahlmann et al, 1979).

Small signal GaAs FETs are, at present, available in hermetic packages up to 20 GHz or so with power FET packages containing some internal matching being available up to 18 GHz. Advances in packaging techniques are being seen in all areas perhaps most noticeably in the area of low-cost sub-systems using analogue and digital ICs. Such packaged devices offer considerable advantages to the user. The components can be pretested and screened to the quality levels demanded by commercial, military and space applications much more easily than bare-chip devices. The user is provided with a much easier to handle component especially if die handling and wire bonding facilities are not available.

It has been seen in this chapter that the GaAs field effect transistor is playing an important role in the development of integrated circuits. Analogue microwave circuits fabricated on gallium arsenide have a performance which is unattainable using silicon. Digital logic circuits on GaAs have a five times speed advantage over silicon at the present time but the massive investment in the very high speed IC (VHSIC) program in the USA will undoubtedly narrow the gap. However, there are many promising logic architectures based on GaAs MESFETs, JFETs and MOSFETs which will progress rapidly as the technology matures. The bibliography below is intended as a guide to further reading and papers not referred to in the text are also included. The complexity of both analogue and digital GaAs Ics is increasing rapidly. Markets for GaAs Ics are becoming well defined and a $1500 million market in the USA alone is anticipated by 1990, which assuming modest requirements in the early 1980's infers a growth in the market in excess of 100% per annum over the next decade.