HIGH-ELECTRON-MOBILITY TRANSISTORS (HEMTs)

A high-electron-mobility transistor (HEMT) is a GaAs transistor designed for IC integration. As shown in Fig. 2-13, it is fabricated on a layer of aluminum gallium arsenide (AlGaAs) grown on a GaAs substrate. This heterojunction design improves transistor performance and permits even higher levels of integration than are possible with the MESFET.

 High-electron-mobility transistor (HEMT).

HETEROJUNCTION BIPOLAR TRANSISTORS (HBTs)

A heterojunction bipolar transistor (HBT) is a bipolar GaAs transistor grown on a heterojunction. Heterojunction E/D technology was developed to achieve cost-effective GaAs digital large-scale integrated (LSI) and very large scale integrated (VLSI) devices. The structure, shown in Fig. 2-14, permits high levels of integration. Both HEMTs and HBTs require special processing to achieve precise, sharp heterojunctions.