Two-port S-parameter characterization of high electron mobility transistors at millimeter wave and microwave frequencies

The two-port S-parameters of 0.1- mu m gate length AlGaAs/GaAs high-electron-mobility transistors (HEMTs) at W-band (75 to 115-GHz) frequencies were found using a specially constructed six-port network analyzer. In conjunction with the millimeter-wave measurements, cryogenic microwave characterizations were also performed on 0.25- mu m-gate-length HEMTs (made with similar materials and fabrication technology as the 0.1- mu m HEMTs) to determine the extent to which the frequency range could be increased by cooling. A gas helium refrigerator was used to cool the transistors down to 50 K, and the S-parameters were measured from 2 to 18 GHz with an automatic network analyzer. The maximum available gains of a typical HEMT at room temperature and 50 K are shown. Small-signal transistor models were derived from the measured S-parameters to determine the projected maximum unity-gain frequency, f/sub max/. For this transistor, f/sub max/ increased from 33 to 45 GHz on cooling. >