Effect of the Critical and Operational Temperatures on the Sensitivity of ${\hbox{MgB}}_{2}$ HEB Mixers

In this paper, we present a study of the noise and the gain of MgB
₂
hot-electron bolometer mixers with different critical temperatures (T
_c
) and at various operation temperatures. At a local oscillator (LO) frequency of 1.63 THz the minimum input receiver noise temperature (T
_r
) was 700 K with a gain of -18 dB for a device with a T
_c
of 8.5 K. For a device with a T
_c
of 22.5 K the corresponding values were 1700 K and -19 dB. For the latter device the T
_r
was 2150 K at a bath temperature of 12 K, which is not achievable with Nb-compound based HEB mixers. We present and compare different methods for measurements of the HEB mixer gain and the output noise.