III-N transistor including a vertically-oriented lightly-doped III-N drift region between an overlying III-N 2DEG channel and an underlying heavily-doped III-N drain. In some embodiments, the III-N transistors are disposed over a silicon substrate. In some embodiments, lateral epitaxial overgrowth is employed to form III-N islands self-aligned with the vertically-oriented drift region. A gate electrode disposed over a portion of a III-N island may modulate a 2DEG within a channel region of the III-N island disposed above the III-N drift region. Charge carriers in the 2DEG channel may be swept into the drift region toward the drain. Topside contacts to each of the gate, source, and drain may be pitch scaled independently of a length of the drift region.