Microwave characterization of very high transconductance MODFET

Abstract

Submicrometer gate length Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.3</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.7</inf> As/GaAs modulation doped FET's have been fabricated using n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> GaAs and graded n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> Al <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</inf> Ga <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1-x</inf> As 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.