A GSM900/DCS1800 dual-band AlGaAs/GaAs HBT (heterojunction bipolar transistor) MMIC (monolithic microwave integrated circuit) power amplifier has been developed. It includes power amplifiers for GSM900 and DCS1800, constant voltage bias circuits and a d. c. switch. In order to achieve high efficiency, the outside-base/center-via-hole layout is applied to the final-stage HBT of the MMIC amplifier. The layout can realize uniform output load impedance and thermal distribution of each HBT finger. The developed MMIC amplifier could provided output power of 34.5 dBm and power-added efficiency of 53.4% for GSM900, and output power of 32.0 dBm and power-added efficiency of 41.8% for DCS1800.