Towards the Lower Limits of Microwave Noise in HEMTs: Self-Heating and Real-Space Transfer

Cryogenic HEMT amplifiers are an important component of diverse instrumentation including quantum computers based on superconducting qubits. Present noise figures are around five to ten times the standard quantum limit in 1–100GHz, and applications drive ever lower noise figures. However, the physical origin of the microwave noise remains an active topic of study. Here, we report our studies of the limiting mechanisms of noise in modern HEMTs. In particular, we describe the role of self-heating at cryogenic temperatures and real-space transfer of electrons out of the channel.