Fully integrated, on-chip DC-DC power converters are provided. In one aspect, a DC-DC power converter includes: a SOI wafer having a SOI layer separated from a substrate by a buried insulator, wherein the SOI layer and the buried insulator are selectively removed from at least one first portion of the SOI wafer, and wherein the SOI layer and the buried insulator remain present in at least one second portion of the SOI wafer; at least one GaN transistor formed on the substrate in the first portion of the SOI wafer; at least one CMOS transistor formed on the SOI layer in the second portion of the SOI wafer; a dielectric covering the GaN and CMOS transistors; and at least one magnetic inductor formed on the dielectric. A method of forming a fully integrated DC-DC power converter is also provided.