Abstract
A GaAs MESFET fabrication process based on self-aligned WSi gates with gatelengths down to 0.5 μm is described. A buried p-layer is employed to suppress short-channel effects. The shallow FET channels are connected to the deeper implants of the ohmic source and drain contacts by means of an intermediate-dose “lightly-doped drain” (LDD) implant. Owing to a planarization concept which allows the use of a low-resistance Au overlayer on top of the high resistivity WSi gates, the process yields FETs with excellent high-frequency performance. This is proved by transit frequencies ƒ t exceeding 30 GHz, maximum frequencies of oscillation ƒ max of typically 100 GHz, and a noise figure NF min of 1.0 dB at 12 GHz (nominal gatelength 0.5 μm). FETs with three different threshold voltages are fabricated. All three types of transistors have transconductances of 300 mS/mm or above. This combination of properties makes the technology suitable for the realization of digital as well as microwave circuits.
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