Gallium nitride (GaN) represents an excellent material for high-voltage components. The reason for this are its wide bandgap, high saturation velocity, and large critical field. High electron mobility is achieved through the formation of a two-dimensional gas, which is a result of spontaneous and piezoelectric polarization. The spontaneous polarization is a result of the different electronegativities of nitrogen and gallium, while piezoelectric polarization occurs due to strain in the crystal and sublattice displacements. The two-dimensional electron gas is formed a few nanometers away from the AlGaN/GaN interface on the GaN side. Carriers interact with environment during their passage through the structure, leading to scattering, which plays a significant role in the transport properties. There are several mechanisms that determine the overall mobility in GaN HEMTs, but it is predominantly determined by the polar optical phonon (POP) and interface roughness (IFR) scattering mechanisms. In the POP mechanism, emission starting at slightly higher energies is essential. For lower aluminum compositions, in the case of IFR, transitions within the lowest energy band as well as transitions between the lowest and the first higher band are significant. In addition to the dependence of mobility on the aluminum composition, the dependence of energy levels on the aluminum composition and voltage is given, as is the temperature dependence of individual scattering mechanisms, which shows that lattice vibration-related mechanisms become stronger with increasing temperature.