A semiconductor structure can include a substrate, a high-voltage blocking layer overlying the substrate, a doped buffer layer overlying the high-voltage layer, and a channel layer overlying the doped buffer layer, wherein the doped buffer layer and the channel layer include a same compound semiconductor material, and the doped buffer layer has a carrier impurity type at a first carrier impurity concentration, the channel buffer layer has the carrier impurity type at a second carrier impurity concentration that is less than the first carrier impurity concentration. In an embodiment, the channel layer has a thickness of at least 650 nm. In another embodiment, the high-voltage blocking includes a proximal region that is 1000 nm thick and adjacent to the doped buffer layer, and each of the proximal region, the doped buffer layer, and the channel layer has an Fe impurity concentration less than 5×1015 atoms/cm3.