A fully integrated S-band high efficiency power amplifier using the TSMC 0.35 um SiGe BiCMOS technology is presented. The two-stage power amplifier has been optimized for the whole S-band covering 2 GHz to 4 GHz frequency band for higher 1-dB compression point and efficiency. The input and output matching networks are designed on chip. From the simulate result, the two-stage power amplifier achieves high PAE of 28% and saturation output power of 23.3 dBm at 3 GHz, with the small signal gain of 18.7 dB. Besides, this PA realizes the PAE within the band of 2.4GHz to 4GHz exceed 25%, and the highest PAE of 30.6% at 3.4 GHz.