The de characteristics of pnp GaN/AlGaN heterojunction bipolar transistors are simulated using a quasi-3D-model. The effects of base doping and thickness, contact geometry and device operating temperature on de current gain have been examined. Maximum gains of similar to 50 are expected for high quality materials and an optimized layer design. For similar layer thicknesses, npn devices have higher gains than the pnp structures, but the latter does not suffer from a high base sheet resistance.