Importance of source and drain resistance to the maximum f/sub T/ of millimeter-wave MODFETs

Abstract

The usual approximate expression for measured f/sub T/=(g/sub m//2 pi (C/sub gs/+C/sub gd/)) is inadequate. At low drain voltages just beyond the knee of the DC I-V curves, where intrinsic f/sub t/ is a maximum for millimeter-wave MODFETs, the high values of C/sub gd/ and G/sub ds/ combine with the high g/sub m/ to make terms involving the source and drain resistance significant. It is shown that these resistances can degrade the measured f/sub T/ of a 0.30- mu m GaAs-AlGaAs MODFET from an intrinsic maximum f/sub T/ value of 73 GHz to a measured maximum value of 59 GHz. The correct extraction of maximum f/sub T/ is essential for determining electron velocity and optimizing low-noise performance.