This brief presents a new design technique for high-efficiency CMOS millimeter-wave power amplifiers (PAs) and the implementations of a two-stage moderate-power PA, a three-stage high-power PA, and a transmitter all working over 68-78 GHz. The proposed PAs adopt nMOS capacitors connected at the gates of the transistors of the last one or two amplifying stages to compensate for the gate capacitance variation over a large signal swing, thus improving the linearity and the power efficiency. Implemented in a 65-nm CMOS process, the two-stage PA achieves a peak power-added efficiency (PAE) of 24.2%, a maximum gain of 17 dB, and a 3-dB bandwidth from 68 to 78 GHz. The three-stage PA achieves a saturated power (Psat) of 17.3 dBm, a peak PAE of 18.9%, and a maximum gain of 21.4 dB. The transmitter consisting of the three-stage PA and a passive double-balanced mixer with local oscillator shaping technique achieves a Psat of 14.6 dBm, a peak efficiency of 13.9%, and a conversion gain of 15.6 dB.