In this work, we demonstrate the technology of wafer-scale transistor-level heterogeneous integration of GaAs pseudomorphic high electron mobility transistors (pHEMTs) and Si complementary metal-oxide semiconductor (CMOS) on the same Silicon substrate. GaAs pHEMTs are vertical stacked at the top of the Si CMOS wafer using a wafer bonding technique, and the best alignment accuracy of 5 mu m is obtained. As a circuit example, a wide band GaAs digital controlled switch is fabricated, which features the technologies of a digital control circuit in Si CMOS and a switch circuit in GaAs pHEMT, 15% smaller than the area of normal GaAs and Si CMOS circuits.