We demonstrate that the performance of silicon carbide MESFETs is largely determined by short-channel effects. Parasitic bipolar transistor turn-on limits the operation voltage to a small fraction of the theoretically expected value for an ideal device. Tradeoffs are shown to exist between optimum gate length and on-state current on one hand, and the maximum blocking voltage on the other hand. Composite p-buffers with an elevated doping in the vicinity of the active layer considerably increase the operation voltage. Silicon carbide MESFETs utilizing composite buffers are reported.