The two-dimensional device simulator, MediciTM, was used to simulate 4H silicon carbide (4H-SiC) n-channel power metal semiconductor field effect transistors (MESFETs) with 0.5 µm gate length with and without p-type buffer layer between the n-channel and the semi-insulating (SI) substrate. The devices, which have previously been fabricated and characterized experimentally, have ion-implanted n+ source and drain ohmic contact regions. The simulations were performed with transient 30 V amplitude symmetrical triangular pulse with 30 s pulse width. Simulations show that hysteresis in drain I-V curves of MESFETs is due to substrate traps and source/drain implant damage traps. The hysteresis is caused by trapping and emission of channel electrons by the traps as VDS rises from 0 V to VDS(max) and as VDS falls from VDS(max) back to 0 V. This leads to difference in trap occupation, and hence difference in channel electron concentration as VDS rises and falls. This finally leads to difference in drain-source current (IDS) at a given VDS for a given VGS as VDS rises and falls, giving rise to the hysteresis in the I-V curves.