There are disclosed herein various implementations of a group III-V transistor with a semiconductor field plate. Such a group III-V transistor includes a group III-V heterostructure situated over a substrate and configured to produce a two-dimensional electron gas (2DEG). In addition, the group III-V transistor includes a source electrode, a drain electrode, and a gate situated over the group heterostructure. The group III-V transistor also includes an insulator layer over the group III-V heterostructure and situated between the gate and the drain electrode, and a semiconductor field plate situated between the gate and the drain electrode, over the insulator layer.