Design, Characterization and Modeling of GaN-based HEMTs for Low-Noise and Cryogenic Applications

Doctoral thesis, 2025

Radio astronomy relies on detecting extremely weak signals and requires robust and rugged technologies,capable of preventing and withstand radio frequency interference (RFI). Low-noise amplifiers (LNAs)operating at cryogenic temperatures are key components in radio astronomy instrumentation. WhileLNAs based on advanced semiconductor technologies with limited power-handling capabilities have beenwidely used, gallium nitride (GaN)-based high-electron-mobility transistors (HEMTs) offer a promisingalternative due to their high robusteness and excellent low-noise performance at room temperature.However, their low-noise behavior at cryogenic temperatures has remained largely unexplored.This thesis investigates the potential of GaN-based HEMTs for cryogenic low-noise operation. Theminimum noise temperature of GaN-HEMTs at 10 K was found to be in the range of 4–5 K (0.06–0.07 dBnoise figure), which is comparable to other advanced technologies in the field. This was achieved throughwell-established experimental and modeling techniques, allowing for the characterization of noisecontributions in GaN-HEMTs as a function of operating frequency, dissipated power, and total deviceperiphery. The findings provide a foundation for designing future GaN-based LNAs that meet therequirements of cryogenic applications.For the first time, GaN-HEMTs with superconducting niobium (Nb) gates were demonstrated. Acomparative study with conventional gold (Au)-gated GaN-HEMTs revealed that superconducting Nbgates suppress the gate resistance dependence on gate width and length below Nb critical temperature(Tc < 9.2 K). However, self-heating effects were found to prevent the maintenance of Nb superconductivityat optimal noise-bias conditions, highlighting the need for further optimization of the device's heatdissipation capabilities.GaN-based metal-insulator-semiconductor (MIS)-HEMTs with a silicon nitride (SiNx) gate dielectric werealso examined at 4 K, demonstrating a minimum noise temperature of 8 K—comparable to theirconventional HEMT counterparts under the same conditions. These results highlight the impact of thegate dielectric on the cryogenic small-signal and noise parameters of the device, suggesting that furtherreduction of gate leakage current through improved gate insulation could enable additional noisereduction.The incorporation of gate field plates (FPs) was shown to improve device reliability by mitigating high-fieldand trapping effects, which become more pronounced at cryogenic temperatures. However, noiseanalysis of devices with and without FPs at 4 K revealed an overall detrimental impact of FPs, leading toat least a 35% noise degradation. This was attributed to increased parasitic capacitances, which reducedthe cutoff frequency. Nonetheless, devices with FPs exhibited improved drain-source conductance,offering advantages for low-noise impedance matching.