An active microwave frequency tripler using an AlGaN/GaN HEMT device is developed. This is the first reported frequency tripler implemented in GaN technology. Design of the frequency tripler is performed using a high-accuracy, multi-harmonic, wideband model which predicts the effects of self-heating and charge-trapping. A method of determining the optimal load and source networks using harmonic load-/source-pull simulations and synthesizing using harmonic reflectors is described. The tripler upconverts fo=3.33GHz to 10GHz to achieve a maximum power of +30.0dBm (1.0W). As such, it provides multiplied powers which are approximately 50 times greater than those previously reported.