The design of Gallium Nitride based Metal-Semiconductor-Metal Ultra-Violet detector is discussed. We introduce a simulation model using Medici to describe the performances of such detectors. Structure parameters, such as the inter-digitated electrode dimension and the GaN layer thickness, are optimized for response current and time using this model. The simulation results can be explained by the variation of depletion region. We introduce the “effective electric field intensity” to describe the depletion region. The relationship between the “effective electric field intensity” and structure parameters are simulated and discussed.