We have optimized the In-0.5(AlxGa1-x)(0.5)P/In0.2Ga0.8As high electron mobility transistor＇s (HEMT＇s) design by considering the material aspects of conduction band discontinuity, indirect-to-direct band electron transfer, donor-related deep levels, and the Schottky barrier height of this material system. We found that the In-0.5(AlxGa1-x)(0.5)P/In0.2Ga0.8As PIEMTs with 0.2 less than or equal to x less than or equal to 0.3 have the best device performance. For 1 mu m x 5 mm In-0.5(AlxGa1-x)(0.5)P/In0.2Ga0.8As double heterostructure HEMTs, power-added efficiency (PAE) of 65.2% and 68.2% at 850 MHz was demonstrated at 1.2 and 2 V drain biases, respectively. These are the highest PAEs ever reported for 1 mu m gate-length HEMTs. At 1.9 GHz and 2 V drain bias, the maximum output power was 25 dBm with a maximum PAE of 53% and linear gain of 8.3 dB. These results demonstrate that the new In-0.5(AlxGa1-x)(0.5)P/In0.2Ga0.8As HEMTs have great potential for one-battery-cell operated wireless communication applications.