The small-signal characteristics of n+-Ge gate AlGaAs/ GaAs MISFET's have been clarified by S-parameter measurements and equivalent circuit modeling. A large intrinsic transconductance of 630 mS/mm and a maximum cutoff frequency fT of 70 GHz have been achieved for a MISFET with a gate length of 0.4 μm. The average electron drift velocity in the channel, evaluated from the fT was as high as 1.7 x 107 cm/s. In obtaining an equivalent circuit model, a gate conductance is introduced parallel to the gate-source capacitance in order to take into account the gate forward current of normally-off FET's. The gate conductance does not cause the fT of the MISFET to deteriorate due to a small gate forward current at a large gate bias, in contrast to GaAs MESFET's.
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