There is a large difference between the intervalley energy difference of GaN predicted by first-principles calculations and that measured by experiments. The results of full-band MonteCarlo simulation prove that intervalley transitions can occur at a high electric field when the intervalley energy difference measured by experiments is used. When the electric field is lower than $1\times 10^7$ V/m and the intervalley energy difference is higher than 1.2 eV, intervalley transitions hardly ever happen. Because there are almost no electrons in the high-energy valleys inequilibrium states, the electrons in the $\Gamma_2$ valley should be regarded as excess carriers. The occupation probability of electrons in the $\Gamma_2$ valley rapidly increases with the decreased intervalley energy difference between the $\Gamma_1$ and the $\Gamma_2$ valleys. Similarly, it rapidly decreases with the increasing effective electron mass in $\Gamma_1$ valley. When electrons in GaN get more kinetic energy from light or surface electric field, such an increase in the electron energy can be equivalent to a reduction in the intervalley energy difference. By using the concept of effective intervalley energy difference, we can explain why the increase in the source–drain current depends on the gate voltage, light, the sweep mode of source–drain voltage and different types of traps can be explained. In all calculations, there areno electrons in the L-M valleys of GaN.