We derived a simplified analytical expression to estimate the dynamic reduction in the 2DEG density due to the buffer trapping (n_T) in GaN-based high-electronmobility transistors (HEMTs). To compute n_T, gate-lag drain current transient (DCT) measurements are carried out at a fixed temperature for different Vn_GS points. The dynamic decrease in the 2DEG due to the Fe-related trap at Ec-0.5 eV is found to be ∼3.4 × 10$^{11}$ cm$^{-2}$ in AlGaN/GaN HEMT; thus, n_T is an essential quantity to evaluate the current collapse effects in HEMT. The validity of the expression is further demonstrated in the C-doped HEMTs. The calculated n_T for the trap at Ec-0.14 eV is about 9 × 10$^{12}$ cm$^{-2}$ in InAlN/GaN HEMT with C-doped buffer, and the calculated n_T for the trap at Ec-0.33 eV is ∼1.2 × 10$^{13}$ cm$^{-2}$ in AlN/GaN HEMT with C-doped buffer. These values reveal that current collapse effects are more pronounced in the C-doped HEMTs than the Fe-doped HEMTs. Thus, this work is an important first step before further improvements in the model for computing buffer trap concentration.