The goal of the project proposed here is to investigate heterojunction materials at liquid helium temperatures in a magnetic field. These materials have been shown to detect millimeter waves in the 100-300 GHz range. Very high electron mobility samples (>500,00 cm2/Vs) are particular interest. Two such samples have been received from IBM, and other samples are expected through cooperation with scientists in the United Kingdom. The project will explore how to maximize the responsivity of detectors using these materials, as well as attempt to show mixing in such devices for the first time.