A GaN high electron mobility transistor (HEMT) with a P-GaN gate coupled with a hybrid doped cap layer structure is presented to further improve the threshold voltage and breakdown voltage of P-GaN gate HEMT devices. The new device controlled the overall polarization effect by using a hybrid doped cap layer structure, which further depleted the two-dimensional electron gas in the channel region below the hybrid cap layer and raised the threshold voltage. The device also controlled the electric field distribution on the right side of the gate in the reverse blocking state, which enhanced the electric field concentration phenomenon at the gate edge, and raised the breakdown voltage of the device. Sentaurus TCAD simulation results reveal that the novel structure device's breakdown voltage is boosted from 593 V to 733 V, a 24% increment, and the threshold voltage is boosted from 0.509 V to 1.323V, a 33% increment when compared with that of the common P-GaN gate enhanced device.