GaN high-electron-mobility transistors with superconducting Nb gates for low-noise cryogenic applications

The successful integration of superconducting niobium (Nb) gate electrodes into cryogenic gallium nitride (GaN)-based high-electron-mobility transistors (HEMTs) is reported. This is achieved through a specifically developed microfabrication process. The device's DC, microwave, and noise performances at cryogenic temperatures, down to 4 K, are studied and presented. The superconductivity of the gate is tested using DC end-to-end measurements. A clear superconducting state transition at a critical temperature, T${}_c$, of ≈9.2 K is shown. This is further verified with GaN HEMTs with two gate fingers and a gate length of 0.2 µm, through the extraction and validation of a small-signal model at T < T${}_c$. Additionally, the superconductivity of the gate is verified for several gate widths and lengths, showing a significant reduction of the gate resistance independently of its dimensions. Finally, a comparative study of the cryogenic microwave noise performances of the GaN HEMTs with gold (Au) and Nb gates is presented. The Au-gated device presents a competitive optimum noise temperature, T${}_{min-opt}$, of ≈8 K at 5 GHz, demonstrating the potential of this technology for cryogenic low-noise applications. The Nb-gated device presents a 5 K higher T${}_{min-opt}$, which is found to be related to the suppression of the superconductivity of the Nb gate at the optimum-noise bias. (© 2022 The Authors. physica status solidi (a) applications and materials science published by Wiley‐VCH GmbH)