Two-dimensional transient analyses of AlGaN/GaN HEMTs and GaN MESFETs are performed in which a deep donor and a deep acceptor are considered in a semi-insulating buffer layer. Quasi-pulsed I-V curves are derived from the transient characteristics. It is studied how the existence of field plate affects buffer-related lag phenomena and current collapse. It is shown that in both FETs, the lag phenomena and current collapse could be reduced by introducing a field plate, because electron injection into the buffer layer is weakened by it, and trapping effects are reduced. The dependence on insulator-thickness under the field plate is also studied, suggesting that there is an optimum thickness of insulator to minimize the current collapse and drain lag.