14 - Analytical DC analysis of short-gate MODFETs

Summary

The most creative theories are often imaginative visions imposed on facts.

Stephen J. Gould

Introduction

Of the various three-terminal devices proposed or demonstrated over the last couple of decades, the modulation doped field-effect transistor (MODFET) (or high-electronmobility transistor (HEMT)) and the heterojunction bipolar transistor (HBT) (Chapters 2, 18 and 19) are the most successful. The two-terminal resonant tunneling diode (RTD) (Chapters 4 and 6) is also of great interest because of the extremely compact low-power high-speed digital circuitry it makes possible when integrated, for instance, with HEMTs (see e.g. [1]). Table 14.1 shows, for several transistor technologies, representative values (1999) for circuit frequency range, device cut-off frequencies, off-state breakdown voltage, maximum output power, power-added efficiency and noise figures with associated gain. The SiGe HBT is very attractive because of the potentially low cost of manufacturing and cut-off frequencies high enough for most wireless applications. For applications in a similar frequency range that require larger microwave output power, the GaAs-based HBT is an even better candidate. This device also shows very low 1/ƒ noise, which translates into low oscillator phase noise. In addition to its speed, one advantage of the InP-based HBT is its surface properties which allow smaller devices. When special processing techniques are employed such down-scaling can result in very impressive power-gain cut-off frequency and circuit performance [2, 3].

When high frequencies and low noise are required the device of choice is typically a III–V Schottky-barrier-gate field-effect transistor FET (SBGFET), several examples of which appear in Table 14.1.