4. Heterojunction bipolar transistors

The use of a large-gap-small-gap heterojunction as the emitter-base junction of a bipolar transistor can significantly improve performance. Although the idea goes back a long way, it was only convincingly implemented following the development of modern MBE and MOVPE epitaxy techniques in the early 1980s. This type of component is presented below in the particular case of the In _0.49 Ga _0.51 P/GaAs material pair, the most widely used today.