[en] Electrical properties of In_xAl_1-xN/AlN/GaN structure are investigated by solving coupled Schroedinger and Poisson equations self-consistently. The variations in internal polarizations in In_xAl_1-xN with indium contents are studied and the total polarization is zero when the indium content is 0.41. Our calculations show that the two-dimensional electron gas (2DEG) sheet density will decrease with increasing indium content. There is a critical thickness for AlN. The 2DEG sheet density will increase with In_xAl_1-xN thickness when the AlN thickness is less than the critical value. However, once the AlN thickness becomes greater than the critical value, the 2DEG sheet density will decrease with increasing barrier thickness. The critical value of AlN is 2.8 nm for the lattice-matched In_0.18Al_0.82N/AlN/GaN structure. Our calculations also show that the critical value decreases with increasing indium content.