A 10 GHz operational amplifier in GaAs MESFET technology

Previous implementations of high-performance op amps in GaAs technology have been hindered by low transistor g/sub m/r/sub ds/, light sensitivity, and excessive backgating, which have limited gain and bandwidth. These limitations are overcome in the circuit reported here by the use of improved processing technologies and circuit design approaches. The GaAs MESFET depletion-mode n-channel technology employs 0.2- mu m e-beam defined gates, air-bridge interconnects for low capacitance, and molecular beam epitaxy (MBE) to grow the channel layers. The average threshold voltage of the resulting FETs is -0.6 V, and the extrinsic transconductance is approximately 500 mS/mm. H/sub 21/ measurements on individual devices yield an extrapolated f/sub T/ of approximately 8 GHz. The small-signal equivalent circuit model of a 50- mu m-wide device, derived from the measured S-parameters, is shown, and the resulting element values are presented.<
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