Modulation-doped (Al,Ga)As/GaAs FETs with high transconductance and electron velocity

High-performance normally-off and normally-on field-effect transistors have been fabricated from modulation-doped (Al,Ga)As/GaAs heterostructures grown by molecular-beam epitaxy. At 300 K, transconductances of 210 and 240 mS/mm were obtained, respectively, for normally-off and normally-on transistors with a 1 ¿m gate length. The mode of operation was determined by the depth to which the gate was recessed into the doped (Al,Ga)As. Although the total (Al,Ga)As thickness between the gate and the conduction path in the GaAs is greater for normally-on devices, larger transconductances were obtained. This is a direct result of the smaller source resistance in the normally-on device. Using a model developed particularly for modulation-doped transistors, electron saturation velocities of about 1.7×107 cm/s are inferred. These velocities and the transconductance values are the largest reported to date.