A highly efficient 94-GHz push-push frequency doubler is presented. The push-push frequency doubler is implemented with a commercial 65-nm CMOS process and can be operated in high-power mode (HPM) and high-efficiency mode (HEM). In HPM, it shows peak conversion gain of 1.94 dB, operating in the frequency range from 83.6 to 102.9 GHz (20.7%) with the input power of -1 dBm within 3-dB gain variation. Also at 94 GHz, it operates in the input power range of −11.9 to 8 dBm within 3-dB gain variation. In HEM, the peak total efficiency (P OUT /(P DC+ P IN )) is 16.28%, which is the highest among those of the reported W-Band frequency doublers.