High-power GaAs FET's have been developed by using ion implantation to form channel layers and n+ohmic contact regions. The burn-out characteristics have been improved by introducing n+regions with high surface carrier concentration. The source-drain burnout voltage has been found to be more than 40 V. The distributions of saturated source-drain current (Idss) and RF output power of the devices have been found much more uniform than those of power GaAs FET's prepared by metalorganic chemical vapor deposition (MOCVD). Multichip operation of the FET's has demonstrated an excellent power combining efficiency due to the good uniformity among the chips. The two-chip device (total gate width WG= 14.4 mm) has delivered 5 W at 10 GHz with 4-dB gain and 23-percent power added efficiency (ηadd). The four-chip device (WG= 28.8 mm) has given 10 W at 8 GHz (gain = 4.5 dB, ηadd= 23 percent). The four-chip device (WG= 48 mm) has developed 15 W at 5 GHz (gain = 8 dB, ηadd= 30 percent).