We discuss room temperature CW operation of AlGaAs/GaAs SQW lasers and GaAs MESFETs with good pinch-off characteristic on SiO2 -back coated Si. The all-MOCVD-grown,SQW laser on Si with thermal-cycle annealing, which has the EPD of 1.5 × 107 cm−2, has the threshold current as low as 55 mA (1.41 kA/cm2) under CW operation at room temperature. The pinch-off and the sidegating effect characteristics of GaAs MESFETs on Si have been improved by using SiO2-back coating of Si and higher growth temperature. The maximum transconductance of 160 mS/mm has been obtained for the MESFET with a 2.5 × 15 µm gate. The main origin of sidegating effect associates with the channel layer-undoped GaAs layer beneath it interface. The SiO2 -back coating is found effective to obtain a lower background electron concentration in undoped GaAs layer and to control doping of the active layer in GaAs/Si.