This paper presents an AlGaN/GaN High Electron Mobility Transistor (HEMT) structure with SiNx surface passivation layer. A T-shaped gate is formed at the top of GaN cap layer. The structure is simulated with Sapphire substrate. Output characteristics curve (Id-Ids), threshold voltage (FT), subthreshold slope and unity current gain cut off frequency (fr) of the device are observed. The proposed device exhibits a threshold voltage Vt= −4 V, peak current is about 500 mA, subthreshold slope of 185 mV/dec and unity current gain cutoff frequency fT= 73.6 GHz. All the simulations are performed using Silvaco ATLASTM.