When designing transistors for microwave power applications a good understanding of their large signal behaviour in a real circuit context is essential. We have used the device simulator Medici in a novel way to simulate the large signal high frequency time domain properties of different SiC MESFET structures. The simulations show that even for transistors with good DC properties the exact design of the channel-buffer and the buffer-substrate regions is important when a good high power RF performance is required. Our simulations indicate that output power densities above 6W/mm are possible if heating problems are solved.