Overview

One of the main differences between Si VLSI and RF electronics is the choice of semiconductor materials and transistor types. Until recently, CMOS has been the standard device and Si is the only semiconductor used in VLSI. In RF electronics, on the other hand, a wide variety of different semiconductor materials (i.e., Si, SiGe, GaAs, InP and wide bandgap materials) and various transistor types, such as bipolar junction transistor, heterojunction bipolar transistor, metal-semiconductor field-effect transistor (MESFET), high electron mobility transistor (HEMT) and metal-oxide-semiconductor field-effect transistor (MOSFET) have found their applications.

Bipolar junction transistors (BJTs) have the benefits of the silicon technology, for example, very high integration and low-cost production, but are limited to lower frequencies. Important steps forward to faster silicon-based devices were the invention of the polySi emitter transistor and now the SiGe HBTs, which are competitive in terms of speed to the III V devices. HBTs and HEMTs are among the most advanced semiconductor devices. They both benefit from the use of heterojunctions formed by different materials. The HBTs make use of wide bandgap emitter and narrow bandgap base. In an npn transistor this favours the electron injection from the emitter to the base, and restricts hole injection from the base the emitter. This advantage can be maintained even if the base is highly doped to get a low base resistance and the emitter is lightly doped. Microwave, millimetre-wave and high-speed digital HBT ICs are used for microwave power and low power wireless communications applications between 0.9 and...