Small-signal characteristics of n/sup +/-Ge gate AlGaAs/GaAs MISFET´s

The small-signal characteristics have been clarified by S-parameter measurements and equivalent circuit modeling. A large intrinsic transconductance of 630 mS/mm and a maximum cutoff frequence f/sub T/ of 70 GHz have been achieved for a MISFET with a gate length of 0.4 mu m. The average electron drift velocity in the channel, evaluated from the f/sub T/, was as high as 1.7*10/sup 7/ cm/s. In obtaining an equivalent-circuit model, a gate conductance parallel to the gate-source capacitance is introduced to take into account the gate forward current of normally-off FETs The gate conductance does not cause the f/sub T/ of the MISFET to deteriorate due to a small gate forward current at a large gate bias, in contrast to GaAs MESFETs.<>