GaN High‐Electron‐Mobility Transistors with Superconducting Nb Gates for Low‐Noise Cryogenic Applications

Abstract

Listen

Translate article

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, Tc, 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 < Tc. 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, Tmin‐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 Tmin‐opt, which is found to be related to the suppression of the superconductivity of the Nb gate at the optimum‐noise bias.