Transistors with backside field plate structures

Disclosed herein are IC structures that implement field plates for III-N transistors in a form of electrically conductive structures provided in a III-N semiconductor material below the polarization layer (i.e., at the "backside" of an IC structure). In some embodiments, such a field plate may be implemented as a through-silicon via (TSV) extending from the back/bottom face of the substrate towards the III-N semiconductor material. Implementing field plates at the backside may provide a viable approach to changing the distribution of electric field at a transistor drain and increasing the breakdown voltage of an III-N transistor without incurring the large parasitic capacitances associated with the use of metal field plates provided above the polarization material. In addition, backside field plates may serve as a back barrier for advantageously reducing drain-induced barrier lowering.