This is a theoretical study of the distribution of two-dimensional electron gas (2DEG) in an AlGaN/AIN/GaN heterostructure. It is shown that the addition of an AIN interfacial layer leads to a slight increase of 2DEG density. The role of quantum-confinement effect in the change of 2DEG distribution with an inserted AIN interfacial layer is revealed by comparing calculated 2DEG distributions in an AlGaN/(AIN)/GaN heterostructure with and without considering the quantum-confinement effect. In the classic model, insertion of an AIN interfacial layer has a negligible effect on the 2DEG distribution. While taking the quantum effect into account, we can clearly see that incorporation of an AIN interfacial layer results in a substantial influence of 2DEG distribution, with the electron spillover into barrier layer reduced from 1.35 x 10(12) to 0.23 x 10(12) cm(-2), which are 9.8% and 1.5%, respectively, of the total 2DEG sheet density. (c) 2007 American Vacuum Society.