The impact of bulk traps in GaN buffer on the transient characteristics of AlGaN/GaN HEMTs is studied based on two-dimensional (2-D) numerical simulations. Starting from the calibration of simulation model compared with the experimentally measured data, we carried out both the pulse-on and pulse-off gate-lag transient simulations. The simulation reproduced the experimental result, suggesting that the rising trend of the turn-on pulsed gate-lag transient current in the experiment can be explained with bulk traps. It is proved that the bulk acceptor trap in GaN buffer can play an important role in the gate-lag transient response of GaN-based HEMTs besides the surface and interface traps. Bulk traps which cannot be eliminated by passivation should be taken into account for modeling. The mechanism of bulk trapping effect in buffer is analyzed in this paper, which may benefit the future work for design and model development of these devices. Furthermore, the influence of trap filling time on the drain current collapse is investigated. (C) 2014 Elsevier Ltd. All rights reserved.