Power GaAs FET's of various sizes have been fabricated using MBE material containing a 1 µm-thick semi-insulating buffer layer. These devices, when operated between 6 and 12 GHz, exhibited state-of-the-art microwave performance. For example, the 3 mm devices gave an output power of 1.5 W with 10.9 dB associated gain at 6.4 GHz, a power-added efficiency of 42.6% with 1.4 W output power and 6.4 dB associated gain at 8 GHz. The results confirm the capability of MBE for producing high quality material with a sharp active layer/buffer interface.