Two-dimensional electron gas (2DEG) property is crucial for the performance of GaN-based high electron mobility transistors (HEMTs). The 2DEGrelated concentration and mobility can be improved as device's performance booster. Electrical characteristics of AlGaN/AlN/GaN HEMT are numerically simulated and compared with conventional AlGaN/GaN HEMT. The main findings of this study indicate that 2DEG's concentration level is increased when a spacer layer of AlN in the interface of AlGaN/AlN/GaN is inserted owing to large conduction band off set, high polarization field, and high barrier. Notably, when a thin spacer layer of AlN is introduced, the 2DEG's distribution virtually shifts away from the interface which reduces the interface scattering. The scattering appearing in conventional AlGaN/GaN HEMT includes alloy and interface roughness scatterings. They are reduced in AlGaN/AlN/GaN HEMT due to binary nature of AlN material. A critical thickness of spacer layer for mobility is 0.5nm and the maximum drain current and transconductance (Gm) are at 1.5 and 1.2nm thickness of AlN spacer layer. Increasing thickness of AlN spacer layer deteriorates the ohmic resistance of source/drain contact and hence degrades the performance of device beyond 1.5-nm-thick AlN spacer layer.