Heterojunction bipolar transistors made from GaN emitter and SIC base and collector regions are desired for high power, broad bandwidth microwave amplifiers. Their most critical element is the quality of the base-emitter junction. High efficiency emitter injection requires an interface between these lattice-mismatched materials, which does not produce significant leakage current due to defects, in addition to having desirable band-offsets. An important factor in understanding GaN/SiC heterojunction rectification is accounting for spontaneous polarization and piezoelectric effects. Theory is presented which shows that a strained GaN/SiC junction will form a 2D hole gas in the base, while an unstrained junction will form a 2D-electron gas. Extending the critical thickness of GaN grown on SiC using patterned area growth might permit improved interface properties.