Two-dimensional transient simulations of AlGaN/GaN HEMTs are performed in which a deep donor and a deep acceptor are considered in a semi-insulating buffer layer. It is shown that lag phenomena and current slump could be reproduced. Particularly, it is shown that gate lag is correlated with relatively high source access resistance of AlGaN/GaN HEMTs, and that drain lag could be a major cause of current slump. The current slump is more pronounced when the deep-acceptor density in the buffer layer is higher and when an off-state drain voltage is higher, because trapping effects become more significant. It is concluded that an acceptor density in the buffer layer should be made low to minimize current slump, although current cutoff behavior may be degraded when the gate length is short.