Superconducting gates for InP HEMTs

Abstract: The thesis examines the prospects of using the superconductor NbN as the gatemetal for an InP HEMT. A HEMT or High Electron Mobility Transistor is aheterostructure transistor engineered to reach very high electron mobility. InPHEMTs are used as cryogenic Low Noise Amplifiers (LNAs), which have increasedin demand as quantum computing is scaling up. A superconducting NbN gate isof interest as it has the potential to decrease the amount of noise generated by theHEMT LNAs.A gate width dependence for both the transconductance (gm) and the large-signal HEMT channel resistance (RON ) of the NbN HEMTs at room temperaturehas been observed, and the first goal pf the thesis is to determine the originof the dependence. Moreover, the measured RF characteristics of the NbNdevices tend to deviate from the norm of a standard HEMT, and the secondgoal is to understand why. The third goal is to determine if the NbN gate stayssuperconducting at cryogenic temperatures or if self-heating from the channelduring DC operations will break superconductivity.In the thesis, it was possible to recreate the observed gate width dependence withnew devices, and additionally, a gate width dependence in the threshold voltageis observed. The origin of width dependence is most likely related to the straincreated by the NbN gate. At DC, extremely high peaks in the transconductanceare observed, which is most likely related to impact ionization and a subsequentincrease in hole trapping caused by the introduction of the NbN gate.Using simulations, it was possible to accurately recreate the observed deviantbehaviour, likely associated with the NbN gate’s high capacitance, inductance andresistance at room temperature. The high capacitance is likely partly related tosome NbN gates of the HEMTs being broken. Finally, the HEMT can operatein DC at 2 K with VG = 0.3 V and a maximum VD = 0.1 V before self-heatingfrom the channel will break the NbN superconductivity of the gate. This is oneof the critical conclusions of the work because it shows that a superconductinggate electrode can be implemented and functional in a high-performance HEMTdevice structure and under realistic operating bias conditions. As long as it can bedemonstrated that the superconductivity does not break when operating in RF, aNbN gate is a promising avenue to increase the noise performance of the cryogenicHEMT.