This work investigates the effect of non-ideal material and contact conditions on the characterization of Al0.17Ga0.83N/GaN heterojunction bipolar transistors (HBTs) by comparing numerical simulations with measured device characteristics. Al0.17Ga0.83N/GaN HBTs are grown by metalorganic chemical vapor deposition and fabricated by direct mesa dry etching. From measured Gummel plot, anomalously high current gain is observed at low current level and then decreases to 2459 at collector current of 1 mA. The common-emitter output characteristics demonstrate dc current gain of 1.22. The inclusion of base-collector and collector-emitter leakage currents with Schottky-like Ohmic contacts in simulation enables good agreement with measured Gummel plots and common-emitter output currents, and shows that leakage current at the base-collector junction is the major source of the non-ideality in device characteristics. Finally, the characteristic of the intrinsic Al0.17Ga0.83N/GaN HBT is presented.