The investigating of the GaN-based high electron mobility transistors (HEMTs) is focused to achieve two goals: increase of carrier density and mobility. Increasing the number of the carriers in the 2-dimensional-electron-gas (2DEG) and localize the carriers in the 2DEG properly are required to achieve high performance. Inserting a thin undoped AlN interlayer between undoped AlxGa1-xN barrier and undoped GaN channel layer is one of the methods to achieve high performance AlxGa1-xN/GaN HEMT structures. In this work, self-consistent 1-band 1-dimension Schrodinger-Poisson equations are solved for pseudomorphically grown undoped AlxGa1-xN/GaN HEMT structures with and without AlN interlayer. The effects of AlN interlayer and AlxGa1-xN barrier layer thicknesses and different Al-mole fractions in AlxGa1-xN barrier layer on band structures, carrier densities and 2DEG wave functions are investigated.