The behavior of gallium nitride transistor under the electric–optical phonon scattering is studied and the variation of current density with the concentration charge density is explained. Due to polarization in GaN, two-dimensional electron gas causes the electrons drift velocity to change when some voltage applied to device. The mobility of GaN MOSFET is been studied and plotted with respect to the temperature. The current density which is related to the drift velocity and carrier concentration is analyzed and plotted. © 2019, Springer Nature Singapore Pte Ltd.