The design, fabrication and characterization of GaN based varactor diodes are presented. MOCVD was used for layer growth and the DC characteristic of 4µm diameter diodes showed a turn-on voltage of 0.5V, a breakdown voltage of 21V and a modulation ratio of 1.63. High frequency characterization allowed obtaining the diode equivalent circuit and observed the bias dependence of the series resistance. The diode cutoff frequency was 900GHz. A large-signal model was developed for the diode and the device power performance was evaluated. A power of 7.2dBm with an efficiency of 16.6% was predicted for 47GHz to 94GHz doubling.