A large-signal HEMT (high electron mobility transistor) model and a time-domain nonlinear circuit analysis program have been developed. A systematic method to simulate HEMT mixers and design them for maximum conversion gain is presented. The transconductance-compression effect reduced the mixer's conversion gain at high frequencies. Simulation results from mixers designed to operate at 10, 20, and 40 GHz show that a reduction in parasitic conduction in the AlGaAs layer significantly increases the conversion gain.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>