Design and Fabrication of InP High Electron Mobility Transistors for Cryogenic Low-Noise Amplifiers

Licentiate thesis, 2022

High electron mobility transistor (InP HEMT) cryogenic low noise amplifiers (LNAs) have made significant improvements in noise and gain following decades of development. Applications are found from radio astronomy to quantum computing. The noise figure for the best InP HEMT cryogenic LNA, however, is still almost one order of magnitude higher than for a quantum-noise limited amplifier. This motivates further studies to understand the physical mechanisms limiting noise reduction in the InP HEMT. In this thesis, 100-nm gate-length InP HEMTs were developed for probing the intrinsic channel noise in the transistor. Electrochemical etching was found to strongly deteriorate the gate recess etch. This was mitigated by modifying the InP HEMT fabrication scheme to a recess-first process. A comparison of two different device passivation methods, atomic layer deposition of Al2O3 and plasma enhanced chemical vapor deposition of SixNy, did not reveal any significant difference in neither gain nor noise for the InP HEMT cryogenic LNA. Channel noise of the InP HEMT was investigated by varying the spacer thickness from 1 to 7 nm in the InAlAs-InGaAs heterostructure. It was found that the optimum spacer thickness was 5 nm for lowest noise temperature in a 4-8 GHz three-stage hybrid LNA at 5 K. This was 2 nm thicker than previously reported spacer thickness used for a similar state-of-the-art InP HEMT cryogenic LNA. The 5 nm spacer InP HEMT LNA minimum average noise temperature was determined to 1.4 K. The channel noise dependence on spacer thickness for the cryogenic InP HEMT was explained by a real-space transfer mechanism associated with the injection of a minor fraction of hot electrons from channel to barrier. Finally, the subthreshold swing of the InP HEMT at 5 K was observed to exhibit a correlation with the noise temperature in the cryogenic LNA. This suggests that the subthreshold swing serves as an indicator of the amount of carrier fluctuations in the InP HEMT channel giving rise to noise.