Two-dimensional transient analyses of AlGaN/GaN HEMTs 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 shown that the deep levels affect the results essentially in a similar way as for GaN MESFETs. It is shown that so-called 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 suggested that to minimize current slump in GaN-based FETs, an acceptor density in a semi-insulating GaN layer should be made low.