At high electron mobility transistors (HEMT), increase of 2DEG carrier density is an important issue to achieve high performance. Inserting a thin undoped AlN interfacial layer between undoped AlGaN barrier and undoped GaN channel layer is one of the methods to achieve high performance AlGaN/GaN HEMT structures. In this work, 1-band Schrodinger simulations on pseudomorphically grown undoped AlGaN/GaN HEMT structures with and without AlN interfacial layer have been done. The effects of AlN interfacial layer thicknesses on band structures, carrier densities and 2DEG eigenstates have been investigated. In addition, the effects of compositional AlGaN barrier layer have been showed for different layer thicknesses and different compositions.