Three types of Ganium Nitride (GaN) transistors were studied in this work. The devices were fabricated and exhibited unique characteristics over on-state current and off-state blocking performances. We also compared the performance differences of devices fabricated by multiepitaxial GaN/AlGaN layers on different substrates (Sapphire and Si) and evaluated the correlations among starting substrate, device variation, and manufacturing uniformity. The first device is a normally-on device with Sapphire substrate which shows good drain saturation current (Idsat) and breakdown characteristics, but the gate leakage current is quite large. The second device is a normally-off GaN transistor named metal-insulate-semiconductor (MIS) heterojunction field-effect transistor (MIS-HFET) which exhibits good performance with threshold voltage (Vth) of 3V and breakdown voltage (Vbd) of over 1800V. However the third device is a normally-off GaN metal-oxide-semiconductor field-elect transistor (MOSFET) structure which is rather difficult to exhibit good blocking characteristic due to inadequate doping process control of the reduce-surface-field (RESURF) region.