Influence of lattice constants of GaN and InGaN on npn-Type GaN/InGaN heterojunction bipolar transistors

We investigated the a- and c-axis lattice constants of GaN buffer and 180-nm-thick p-InGaN layers grown on SiC and sapphire substrates using reciprocal space mapping of the X-ray diffraction intensity. It was found that the a-axis lattice constant of the GaN buffer layer on a SiC substrate is larger than those of unstrained GaN and a GaN buffer layer on a sapphire substrate. As a result, the p-InGaN layer on GaN/SiC is fully strained even at the In mole fraction of 9.0% where that on GaN/sapphire is relaxed. This result means that fewer defects are generated in p-InGaN on GaN/SiC at higher In mole fractions. This is another advantage of SiC substrate for npn-type GaN/InGaN heterojunction bipolar transistors, in addition to its high thermal conductivity. The collector current density dependence of current gain shows the ideality factor of 2 for GaN/InGaN HBTs on both SiC and sapphire substrates. This is ascribed to the recombination current at the emitter-base interface, which arises from the threading dislocations generated at the interface between the substrate and nitride buffer layer.