This paper deduces the expression of the Schottky contact capacitance of AlGaN/AlN/GaN high electron mobility transistors (HEMTs), which will help to understand the electron depleting process. Some material parameters related with capacitance-voltage profiling are given in the expression. Detailed analysis of the forward-biased capacitance has been carried on. The gate capacitance of undoped AlGaN/AlN/GaN HEMT will fall under forward bias. If a rising profile is obviously observed, the donor-like impurity or trap is possibly introduced in the barrier. This paper deduces the expression of the Schottky contact capacitance of AlGaN / AlN / GaN high electron mobility transistors (HEMTs), which will help to understand the electron depletion transistors analysis of the forward-biased capacitance has been carried on. The gate capacitance of undoped AlGaN / AlN / GaN HEMT will obviously be observed, the donor-like impurity or trap may have introduced in the barrier .