The drain delay in AlGaN/GaN submicron high electron mobility transistors (HEMTs) accounts for almost 25% of the total electron delay. This long delay significantly limits the maximum frequency performance and linearity of these devices. This paper studies the origin of this important delay assuming that it is inversely proportional to α, a parameter related to how injected channel electrons image at different contacts in the HEMT. Through analysis and two-dimensional simulations, we have found that α equals 3 in a standard HEMT. This value has been confirmed experimentally through the coupling of Monte Carlo simulations and drain delay measurements.