Two-dimensional electron and hole gases in In_xGa _1-xN/Al_yGa_{1 -y}N/GaN heterostructure for enhancement mode operation

In this paper, a numerical study of In_xGa_{1 -x}N/Al_yGa_{1- y}N/GaN heterostructure is presented. The dependence of two-dimensional electron gas (2DEG) and two-dimensional hole gas (2DHG) sheet densities on variables, such as In_xGa_{1- x}N layer thickness and In content, and Al_yGa _1-yN barrier layer thickness and Al content, are systematically investigated. The effect of P-type doping in In _xGa_1-xN on 2DEG and 2DHG sheet densities in this heterostructure is also studied. It is shown that the strong reverse electric field in In_xGa_1-xN cap layer contributes to the depletion of 2DEG at the Al_yGa _1-yN/GaN interface. When In_xGa_{1 -x}N layer thickness and In content increases, 2DEG sheet density decreases significantly. P-type doping shows less influence on 2DEG compared to the polarization electric field in In_xGa_{1 -x}N layer. In addition, there exist critical values for all the variables beyond which 2DHG appears at the interface of In _xGa_1-xN/Al_yGa _1-yN. Once 2DHG appears, it will prevent 2DEG from being further depleted. With proper design of Al_yGa _1-yN layer, the coexistence of 2DEG and 2DHG in In_xGa_1-xN/Al_yGa _1-yN/GaN structure can be avoided, showing that this structure has great potential in the fabrication of enhancement mode (E-mode) high electron mobility transistors.