III-N transistors with epitaxial semiconductor heterostructures having steep subthreshold slope are described. In embodiments, a III-N HFET employs a gate stack with balanced and opposing III-N polarization materials. Overall effective polarization of the opposing III-N polarization materials may be modulated by an external field, for example associated with an applied gate electrode voltage. In embodiments, polarization strength differences between the III-N materials within the gate stack are tuned by composition and/or film thickness to achieve a desired transistor threshold voltage (Vt). With polarization strengths within the gate stack balanced and opposing each other, both forward and reverse gate voltage sweeps may generate a steep sub-threshold swing in drain current as charge carriers are transferred to and from the III-N polarization layers and the III-N channel semiconductor.