Enhancement of electron velocity in modulation-doped (Al,Ga)As/GaAs FETs at cryogenic temperatures

High-performance normally-off modulation-doped (Al,Ga)As/GaAs field-effect transistors with a 1 ¿m gate length were fabricated and characterised. The transconductance obtained was 225 mS/mm at 300 K and 400 mS/mm at 77 K, leading to intrinsic transconductances (zero source resistance) of 305 and 565 mS/mm at 300 and 77 K, respectively. Since the device performance in short-gate transistors is limited by the electron saturation velocity, the increasing transconductance observed as the device is cooled is due to an increase in the electron velocity from about 2×107 cm/s to 3×107 cm/s. These velocities are inferred from a model developed for modulation-doped transistors and are predicted by pulse measurements in similar structures.