GAAs High-Speed Devices: Physics, Technology, and Circuit Applications

8

GaAs METAL-SEMICONDUCTOR FIELD-EFFECT TRANSISTOR

8.1 INTRODUCTION TO MESFET

The gallium arsenide transistor with a diffusion-gate structure, which was first reported by Turner [Tur67], yields useful gains in the lower megahertz frequency bands. In 1969 Middelhoek obtained, by projection masking, a silicon metal-semiconductor field-effect transistor (MESFET) with a 1-μm gate length and a high maximum frequency of oscillation, F_max. In 1971 a significant step was made by Turner et al. [Tur71], when 1-μm gate length FETs on GaAs were made with f_max equal to 50GHz and useful gains up to 18GHz.

The substantial improvement in FET performance over silicon bipolar transistors is due mainly to the material's properties [Pen86]: First, in gallium arsenide the conduction electrons have a six times larger mobility and twice the peak drift velocity of those in silicon [Ruc70]. Second, the active layer is grown on a semi-insulating GaAs substrate with resistivity larger than 10⁷ Ω-cm. This compares with a typical value of 30 Ω-cm for intrinsic silicon. The first property results in lower parasitic resistances, larger transconductances, and shorter electron transit time. The second property results in lower parasitic capacitance when the gate pad is on the semi-insulating (SI) substrate.

Oxide growth has been tried on GaAs surfaces for more than 20 years. The quality of the oxide grown on GaAs has been poor, and a high density of surface states results at the GaAs-insulator interface. These ...