Electrical properties of In_x Al_(1-x)N/AlN/GaN structure are investigated by solving coupled Schr(o|¨)dinger and Poisson equations self-consistently.The variations in internal polarizations in In_xAl_(1-x)N with indium contents are studied and the total polarization is zero when the indium content is 0.41.Our calculations show that the twodimensional electron gas(2DEG) sheet density will decrease with increasing indium content.There is a critical thickness for AIN.The 2DEG sheet density will increase with In_xAl_(1-x)N thickness when the AIN thickness is less than the critical value.However,once the AIN thickness becomes greater than the critical value,the 2DEG sheet density will decrease with increasing barrier thickness.The critical value of AIN is 2.8 nm for the lattice-matched In_(0.18)Al_(0.82)N/AlN/GaN structure.Our calculations also show that the critical value decreases with increasing indium content. Electrical properties of In_x Al_ (1-x) N / AlN / GaN structures are investigated by solving coupled Schr (o | ¨) dinger and Poisson equations self-consistently. These variations in internal polarizations in In_xAl_ (1-x) N with indium contents are studied and the total polarization is zero when the indium content is 0.41.Our calculations show that the twodimensional electron gas (2DEG) sheet density will decrease with increasing indium content. Here is a critical thickness for AIN.The 2DEG sheet density will increase with In_xAl_ (1-x) N thickness when the AIN thickness is less than the critical value. When, once the AIN thickness becomes greater than the critical value, the 2DEG sheet density will decrease with increasing barrier thickness. The critical value of AIN is 2.8 nm for the lattice-matched In_ (0.18) Al_ (0.82) N / AlN / GaN structure. Our calculations also show that the critical value decreases with increasing indium content.