This article presents how the emission time constant of deep-level traps is responsible for the achievable linearity (linearizability) degradation of gallium nitride high electron mobility transistor (GaN HEMT)-based power amplifiers. It is shown that the worst case scenario happens when the emission rate is on the order of the signal bandwidth, with substantial improvement when they begin to differ. Moreover, because of the strong temperature dependence of the emission time constant, self-heating is shown to play a major role in that linearizability degradation. To demonstrate the importance of the coupling of these two effects (electrothermal and trapping), two GaN HEMT dies with the same structure but different trap activation energies (which dictate the impact of temperature on trapping) were used. Digital predistortion tests using long-term evolution-like signals showed that these devices present different adjacent channel power ratios and normalized mean squared errors, after linearization with the same procedure, which varied with the signal bandwidth as theoretically predicted.