This work presents dc characteristics (gain and leakage current) of a GaN/InGaN/ZnO npn collector-up heterojunction bipolar transistor (HBT) measured at 300, 200, and 100 K. The GaN-based epilayers of HBT were grown by metallorganic chemical vapor deposition, and the n-ZnO film was then deposited on top of p-InGaN by sputtering. X-ray diffraction analysis demonstrates that annealing at 600 degrees C for 60 s in ambient N(2) improved the quality of the ZnO film. Moreover, this study shows that the current gains (beta) from the Gummel plot increased as the temperature decreased because the leakage current of the collector current (I(C)) is not a strong function of temperature. However, gains (G(CE)) from the measured common-emitter current-voltage characteristics increased slightly as the temperature decreased due to a reduction in the recombination current coupled with lower injection efficiency. These preliminary results show the potential of fabricating GaN/InGaN/ZnO HBTs without dry etching to reach the p-GaN layer, as required in the emitter-up geometry.