This phase I proposal addresses the need for microwave and millimeter wave Low Noise Amplifiers (LNAs) for remote sensing applications of the earth's atmosphere. In this work, IIIAN proposes using group III-nitride materials, specifically AlGaN/GaN HEMT structures, to fabricate LNAs for microwave radiometers operating from 165 GHz up to 270 GHz. The group III-nitrides have excellent physical properties for high frequency transistors with the added benefit that the high breakdown voltage of the material renders them less susceptible to failure due to spurious strong signals, eliminating the need for protective circuitry between the antenna and the LNA. Passive microwave radiometers used in the PATH and GACM missions are used to quantify levels of trace species in the atmosphere such as O3, CO, N2O, HNO3, ClO and SO2, and also to estimate water content, both gas phase and as aerosols. The instruments used for these measurements rely on microwave emission from the relevant species. Key to such low signal level measurements are front-end LNAs used to amplify the weak microwave emission signal from the antenna prior to the RF detection diode.