In this paper the influence of doping level in AlGaN and GaN on the carriers surface density in a channel of modulation doped field effect transistors (MODFETs) has been investigated. The carriers concentration profile in the channel (including the position of its famous peak) as well as the surface density have been found. Our calculations have been based on the Wigner Distribution Function (WDF) formalism. Thermal emission at the hetero-interface as well as the penetration of carriers into undoped spacer layer has been taken into account. The results are in good agreement with the available experimental data.