The values of minority carrier diffusion lengths and lifetimes in GaN and their implications for bipolar devices

Abstract

The wide bandgap semiconductors GaN and AlGaN show promise as the high voltage standoff layers in high power heterostructure bipolar transistors and thyristors due to their electric breakdown characteristics. Material properties which significantly influence the design and performance of these devices are electron and hole diffusion lengths and recombination lifetimes. We report direct measurements of minority carrier diffusion lengths for both holes and electrons by electron beam induced current. For planar Schottky diodes on unintentionally doped n-type and p-type GaN grown by metal organic vapor phase deposition (MOCVD), the diffusion lengths were found to be (0.28±0.03) μm for holes and (0.2±0.05) μm for electrons. Minority carrier lifetimes of approximately 7 ns for holes and 0.1 ns for electrons were estimated from these measured diffusion lengths and mobilities. In the case of GaN grown by halide vapor phase epitaxy (HVPE) diffusion lengths in the 1–2 μm range were found. We attempt to correlate the measured diffusion lengths and lifetimes with the structural properties of GaN and to explain why linear dislocations might act as a recombination centers. We calculate the performance of nitride based bipolar devices, in particular thyristor switches. The forward voltage drop across standoff layer of the nitride based thyristor switch is shown to significantly depend on the minority carrier (hole) lifetime.