The effect of the GaN cap layer on the performance of AlInN/GaN-based HEMTs has been studied. Detailed analysis of the DC characteristics reveals that the addition of the GaN cap layer reduces the gate leakage current and increases the saturation drain current of HEMT. We have observed a slight increase in the two-dimensional electron gas density on the addition of the GaN cap layer. The addition of the GaN cap layer shifts the threshold voltage towards the negative direction, reduces the off-state drain leakage current, improves subthreshold swing, and enhances the I_ON/I_OFF ratio of HEMT. The TCAD simulation shows that the addition of the GaN cap layer reduces the peak electric field at the drain side edge of the gate and improves the breakdown voltage. From the double pulsed I_D-V_DS measurement, we have found that the presence of the GaN cap layer reduces the current collapse due to gate-lag and drain-lag by passivating the surface traps of the access regions. Although there is a slight reduction in transconductance, the current gain cut-off frequency (f_T) of the HEMT with GaN cap is higher than the HEMT without GaN cap.