Investigation of self-heating phenomena in high-electron mobility transistors (HEMTs) using millisecond-resolution Y-factor method

Low noise microwave amplifiers are critical components in measurement systems with applications ranging from qubit readout in quantum computing to the detection of fast radio bursts in RF astronomy. Recent work has suggested that the noise floor in high electron mobility transistors (HEMTs), presently around a factor of 5 above the quantum limit, is in part set by the thermal noise from the gate metal corresponding to a temperature exceeding the cryostat temperature. This temperature differential originates from high thermal resistance associated with phonon blackbody radiation at cryogenic temperatures. In this talk, we report on experiments designed to examine self-heating phenomena in HEMTs at cryogenic temperatures using liquid helium cooling and a custom noise figure analyzer that measures noise temperature with millisecond resolution. Our results confirm that self-heating at cryogenic temperatures is key mechanism in setting the noise floor of transistor amplifiers. These findings help to identify a path for transistor microwave amplifiers operating closer to the standard quantum limit of noise.