A cryogenic GaAs HBT microwave amplifier and its application to a superconductor digital IC

The first microwave GaAs HBT (heterojunction bipolar transistor) amplifier results at 4.2 K are benchmarked. The amplifier nominal gain is 6 dB and is measured from 130 MHz to 10 GHz at fixture temperatures of 295 K, 77 K, and 4.2 K. The maximum gain variation over temperature was found to be about 2 dB. Maximum gain occurred at temperatures around 50-85 K, whereas at 4.2 K the gain seemed to drop slightly from that at room temperature. Only slight RF evidence of carrier freeze-out was observed at a fixture temperature of 4.2 K, although HBT junction temperatures are estimated to be around 25-30 K. This chip was integrated as a buffer amplifier with a high-temperature-superconductor digital logic gate and has shown functionality up to 320 MHz.<>