We report on the evolution of the fabrication and characterization of high-temperature and high-power GaN/SiC n-p-n heterojunction bipolar transistors (HBTs). The HBT structures consists of an n-type GaN emitter and a SiC p-n base/collector. Initially, the HBTs were fabricated using reactive ion etching (RIE) to define both the emitter and base areas. However, the poor etch selectivity between GaN and SiC made it difficult to stop at the thin base layer. Furthermore, the RIE caused damage at the heterojunctions, which resulted in large leakage currents. Selective area growth was therefore employed to form the n-GaN emitters. GaN/SiC HBTs were first demonstrated using the 6H-polytype. These transistors had an extraordinary high de current gain of >10(6) at room temperature and were able to operate at 520 degrees C with a current gain of 100. However, in more recent work, this performance could not easily be reproduced due to the presence of a parasitic deep defect level in the p-type 6H-SiC. The possibility of obtaining higher quality 4H-SiC than 6H-SiC, without this defect level, seemed promising since much of the materials development is focused on 4H-SiC, due to its larger energy band gap and superior electron mobility. GaN/4H-SiC HBTs are demonstrated with a modest de current gain of 15 at room temperature and 3 at 300 degrees C.