Threshold voltage characteristics of superconductor gate nMOSFET at 4.2 K

We propose a high mobility superconductor gate nMOSFET using undoped Si as the substrate. We have studied superconducting NbN gate nMOSFETs fabricated on p-type Si, undoped Si and n-type Si substrate, respectively. The nMOSFETs on undoped Si and n-type Si have operated properly at 4.2 K. The transconductance of nMOSFETs on undoped Si substrate is higher than that of nMOSFET on p-type Si. The difference in transconductance is attributed to the difference of impurity density in channels. The observed threshold voltages of NbN gate nMOSFETs on p-type Si, undoped Si and n-type Si have little difference at 4.2 K. (orig.)