  To achieve the low noise and wide bandwidth required for millimeter
wavelength astronomy applications, superconductor-insulator-superconductor
(SIS) mixer based receiver systems have typically been used. This paper
investigates the performance of high electron mobility transistor (HEMT) based
low noise amplifiers (LNAs) as an alternative approach for systems operating in
the 125 - 211 GHz frequency range. A four-stage, common-source, unconditionally
stable monolithic microwave integrated circuit (MMIC) design is presented using
the state-of-the-art 35 nm indium phosphide HEMT process from Northrop Grumman
Corporation. The simulated MMIC achieves noise temperature (Te) lower than 58 K
across the operational bandwidth, with average Te of 38.8 K (corresponding to
less than 5 times the quantum limit (hf/k) at 170 GHz) and forward transmission
of 20.5 +/- 0.85 dB. Input and output reflection coefficients are better than
-6 and -12 dB, respectively, across the desired bandwidth. To the authors
knowledge, no LNA currently operates across the entirety of this frequency
range. Successful fabrication and implementation of this LNA would challenge
the dominance SIS mixers have on sub-THz receivers.
