The present disclosure relates to a vertical gallium-nitride (GaN) power field-effect transistor (FET) with a field plate structure. The vertical GaN power FET includes a conductive substrate, a drift region, a field plate structure, a channel region with tapered side walls, a gate dielectric region, a gate contact, a drain contact and source contacts. The field plate structure includes a lower layer formed of pi p-type graded AlGaN and a upper layer formed of p-type GaN. The field plate structure utilizes the charge separation at the interface between the lower layer and the upper layer to achieve high breakdown voltage.