This work analyzes the effect of various gate structures on the DC and radio frequency (RF) performance of AlGaN/GaN high-electron mobility transistors (HEMTs). AlGaN/GaN HEMT devices with a 3 mu m drain-to-source spacing, 125 mu m gate width and 0.3 mu m gate length in various gate structures were fabricated to achieve the desired frequency response with a robust, high yield, and repeatable process. The maximum drain current (I-DS,I-max), maximum DC transconductance (g(m) ), pinch-off voltage (V-th ), current-gain cutoff frequency (f(T) ), maximum oscillation frequency (f(max) ), and RF characteristics of the devices in terms of the small-signal gain and RF output power (P-out ) at 8 GHz were investigated. The results showed that the output power is increased by 1 dB when the gate structure is changed from field plate to gamma gate. The V-th, g(m) , f(T) and f(max) , values are maximized when the thickness of the passivation layer between the gate foot and the gate head is minimized It is shown that the I-DS,I-max , is decreased and P-out is increased when the gate recess etching process is performed.