Microwave characterization of very high transconductance MODFET

Submicrometer gate length Al0.3Ga0.7As/GaAs modulation doped FET´s have been fabricated using n⁺GaAs and graded n⁺AlxGa1-xAs capping layers in a deep, narrow recess structure. Intrinsic transconductance steadily and dramatically increased from 240 mS/mm for 0.65 µm gate enhancement mode MODFET´s to 570 mS/mm for 0.33 µm devices, suggesting electron velocity enhancement at short gate lengths. Maximum extrinsic transconductances of 450 mS/mm at 300K and 580 mS/mm at 77K have been measured for the 0.33 µm gate length device. An unusually high unity current gain cutoff frequency, f_{T}, of 55 GHz makes this device potentially superior for high speed logic. Parasitic resistances and high feedback capacitance, however, severely limit f_{max} to 70 GHz.