The physical mechanisms involved in the trapping and de-trapping processes associated to surface donor traps in GaN transistors are discussed in this work. The paper challenges the conventional transient techniques adopted for extrapolating the trap energy level via experiments and TCAD simulations. Transient TCAD simulations were employed to reproduce the time-dependent electrical behavior of a Metal-on-Insulator Field-Effect-Transistor (MISFET) and explain the influence of the electric field and energy barrier on the transient time associated to the trapping and de-trapping mechanisms of surface traps. The comparison between three test-structures and the relative variation of the trapping and de-trapping times with the bias and trap parameters leads to the suggestion of a proposed test-structure and bias configuration to accurately extrapolate the energy level of surface traps in GaN transistors.