MOSFET's in the 0°K approximation: Static characteristics of MOSFET's in the 0°K approximation

Abstract

The static electrical characteristics below current saturation of MOSFET's with degenerate source and drain regions are calculated for operation at 0°K. The expression for current takes the same form as at room temperature although the flat-band voltage and the voltage across the depletion region at threshold are altered slightly. Potential hills occur in the channel if the gate does not overlap source and drain or if the oxide thickness is increased in the overlap regions. Although these barriers do not affect operation appreciably at room temperature, at 0°K a finite drain voltage (source-drain threshold voltage) is required to initiate conduction. This threshold voltage is included in the theory and the theory is compared with experimental results on p-channel enhancement mode MOSFET's at 4·2°K using hole mobility in the channel as a matching parameter. The channel hole mobility (assumed constant along the channel) is found to be relatively independent of gate voltage but to increase with increasing (negative) drain voltage. Values ranging between 500 and 1000 cm 2/V-sec are deduced for drain voltages ranging from −1·2 V to −7 V. This compares to channel hole mobility values of 200–300 cm 2/V-sec at room temperature. It is found that the channel width is on the order of 30–50 Å—appreciably less than that at room temperature.