The geometries of silicon BJTs and MOSFETs have been implemented in gallium arsenide (GaAs) to take advantage of the higher speed and operating frequencies made possible by the substitution of GaAs silicon. Because GaAs is a compound semiconductor material, it does not form natural oxides as silicon does, so this made it necessary to alter the silicon device geometries to devise different manufacturing methods.
Metal semiconductor field-effect transistor (MESFET).
Three different gallium arsenide transistor designs have been developed: (1) metal semiconductor field-effect transistor (MESFET), high-electron-mobility transistor (HEMT), (3) heterojunction bipolar junction transistor (HBT).
METAL SEMICONDUCTOR FIELD-EFFECT TRANSISTORS (MESFETs)
The metal semiconductor field-effect transistor (MESFET) is a widely used discrete and integrated-circuit GaAs transistor geometry. Its structure is similar to that of a MOSFET, but its metal gate is deposited directly on the doped GaAs substrate, as shown in Fig. 2-12, to form a Schottky barrier diode. However, silicon oxides are deposited on the substrate for isolation and insulation. The length of the metallized gate (positioned between the source and drain) is critical in both discrete GaAs transistors and ICs.
Typically 0.5 to 1.0 μm in most discrete transistors, it could be as small as 0.2 μm in ICs. But the gate structure is usually much wider with respect to its length—typically 900 to 1200 μm. MESFETS can have interdigitated structures with multiple gates formed as comblike structures. Ion implantation is favored for doping active regions of MESFETs. A 0.1- to 0.2-μm-thick N-doped region is made for the most common depletion-mode MESFETs (D-MESFETs). The enhancement-mode MESFET (E-MESFET) and the enhancement-mode JFET (E-JFET) are other GaAs transistors that have been developed. Both E-MESFETs and D-MESFETs can be combined in ICs to form enhancement/ depletion-mode (E/D) logic.