Effect of NbN-Au contact pads on the performance of a superconducting NbN hot electron bolometer mixer

In this paper we study the effect of NbN-Au contact pads on the performance of a superconducting NbN hot electron bolometer (HEB) mixer. The receiver noise temperature (Trec) and IF noise bandwidth (BN) at a LO frequency of 1.3 THz are measured at different bath temperatures between 3.3 K and 8.5 K. The measured Trec and BN of the superconducting HEB mixer are nearly constant for Tbath/Tc less than 0.7, and increase quickly with the further increase of Tbath. Measurement results demonstrate that the NbN-Au contact pads in superconducting state nearly have no influence on Trec and Bn, but the contact pads in resistive state make Trec and BN increase quickly with Tbath, indicated by the slope change of current-voltage curves and IF output power peak.