InAs-inserted-channel InAlAs/InGaAs inverted HEMTs with superconducting electrodes

We investigate the device characteristics of InAs-inserted-channel In/sub 0.52/Al/sub 0.48/As/In/sub 0.53/Ga/sub 0.47/As inverted high electron mobility transistors (HEMTs) with superconducting Nb electrodes. In these transistors, the ohmic contact between Nb and the two-dimensional electron gas formed in the InAs layer is obtained by contact with the Nb-InAs direct, instead of with an alloyed normal metal and semiconductor as in a conventional HEMT. The contact resistance of 0.15 /spl Omega/mm between the Nb ohmic electrodes and the channel decreased by a factor of 3 compared with that when a conventional AuGe/Ni alloyed ohmic contact is used. For a 0.5-/spl mu/m-gate device, the maximum extrinsic transconductance at 4.2 K was 1 S/mm, even at a very low drain voltage of 0.2 V. These results indicate that this ohmic contact formation will allow us to utilize in this device both the improved HEMT characteristics and the superior performance of superconducting electrodes.