In this paper a surface density analytical model of Two-Dimensional Electron Gas (2DEG) in High Electron Mobility Transistor (HEMT) under gate voltage control is presented. In the model, an approximation of triangular potential well is used. The results obtained by this model are in good agreement with the experimentally obtained results, and the results of comprehensive numerical simulations of HEMT devices, which are made of different materials, including HEMT devices with strained crystal lattice. The model also gives relatively good results for surface density 2DEG in AlGaN/GaN structures with piezo-electric effect, which has a great influence on the density. On the basis of the model that is presented, the simulations were performed and obtained results are given graphically.