Overview

The first transistor invented used elemental germanium as the semiconducting material. Silicon because of its inherent advantages, such as thermal stability, abundance and availability of a good oxide, soon replaced Ge as the substrate material. The transistors in the early phase of semiconductor device development were bipolar junction transistors (BJTs). Junction field effect transistors (FETs) were fabricated soon after, followed by metal oxide semiconductor field effect transistors (MOSFETs) in the late 1950s. Alloys of silicon and germanium were occasionally studied principally for optical applications. Silicon carbide has been used as a resistive element and sometimes utilized in transistors. A very different course is evident in electron devices made of III-V semiconductors. To realize a semiconductor out of metals and semimetals, one must use a compound. The first binary III-V semiconductor developed for device use was GaAs. GaAs diodes found applications as varactors, Gunn diodes and impact ionization avalanche transit time diodes (IMPATTs). The GaAs metal semiconductor field effect transistor (MESFET) was the first III-V compound semiconductor transistor. Meanwhile ternary compounds were developed and heterostructure diodes were fabricated which found a phenomenal success in diode lasers.

The concept of heterojunction devices is as old as the transistor itself which appeared in the late 1940s. William Shockley suggested in the patent granted to him in 1951 that improved unidirectional charge carrier injection could be obtained in bipolar devices by using a wide bandgap emitter [1]. The basic theory of operation of heterojunction transistors was later described in a classic paper by...