A highly efficient power MOSFET has been developed for 3.6 V RF power amplifiers for use in GSM (Global Systems for Mobile communications) cellular telephone systems (880-915 MHz). It was fabricated using a 0.45-µm CMOS LSI process with an Al-shorted metal-silicide/Si gate structure instead of a conventional 0.8-µm Mo-gate. The resulting power MOSFET has an on-state resistance of 6.9 Ωmm, a breakdown voltage of 13 V, and a cut-off frequency of 11 GHz. The RF performance of the device achieved an output power (Pout) of 1.8 W, a power gain of 10 dB, and a power-added efficiency (ηadd) of 60%. In addition, an RF power module using our proposed power MOSFET achieved an output power of 4 W and a total efficiency of 50%.