Microwave power double-heterojunction HEMT's

The RF and dc characteristics of microwave power double-heterojunction HEMt's (DH-HEMT's) with low doping density have been studied. Small-signal RF measurements indicated that the cutoff frequency and the maximum frequency of oscillation in DH-HEMT's with 0.8-1 µm gate length and 1.2 mm gate periphery are typically 11- 16 GHz and 36-41 GHz, respectively. However, the cutoff frequency in DH-HEMT's degrades strongly with increasing drain bias voltage. This may be caused by both effects of increasing effective transit length of electrons and decreasing average electron velocity, due to Gunn domain formation. In large-signal microwave measurement, the DH-HEMT (2.4 mm gate periphery) delivered a maximum output power of 1.05 W with 2.8 dB gain and 0.58 W with 1.6 dB gain at 20 and 30 GHz, respectively. These are the highest output powers yet reported for HEMT devices. For the dc characteristics, the onset of two-terminal gate breakdown voltage is found to correlate with the drain current I dss and recessed length, and three-terminal source-drain breakdown characteristics near pinchoff are limited by the gate-drain breakdown. A simple model on gate breakdown voltage in HEMT is also presented.