Transmitters having increased efficiency, such as may be useful in millimeter-wave devices. A semiconductor device, comprising a transmitter, comprising a modulator configured to receive a differential input signal having a first frequency and provide a differential modulated signal having the first frequency and a first clock phase; a series comprising one or more frequency multipliers, wherein the series of frequency multipliers is configured to receive the differential modulated signal and provide a differential second signal having a second frequency greater than the first frequency and having a second clock phase; and an output transformer configured to receive the differential second signal and transform the differential second signal to a single-ended output signal. Methods of using such transmitters. Systems for manufacturing devices comprising such transmitters.