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Large periphery AlGaN/GaN HEMT's gave normalized power > 2 W/mm, and > 4 W/mm on sapphire and SiC substrates, respectively. A new processing method, using photolithography for all steps except for the gate, is being developed to reduce cost and raise throughput and reproducibility. The properties of the two-dimensional electron gas in undoped HEMT structures are compared with theory and explained by a combination of spontaneous and piezoelectric polarization, and by interface roughness. Static induction transistor processing has been developed. Circuits for broad band amplifiers are simulated, and a dual-gate cascade power stage has experimentally yielded 8 db higher gain. A non-linear model for HEMT power devices has been successfully developed. Theoretical models for electron transport in high electric fields have been compared with measured frequency response. Initial measurements of 1/f noise are being made. OMVPE growth has been developed to reach approx. 1,600 sq cm/V-s electron mobility with +/- 7% thickness variation. MBE growth has reached the same electron mobility with +/- 3% thickness variation and is able to deposit a 25 A GaN cap layer for chemical protection. Growth by OMVPE on SiC substrates is underway but requires further optimization to reduce deep donors in the AlGaN barrier.