Breakdown Voltage and Leakage Current of the Nonuniformly Activated Lightly Doped p-GaN

Abstract

Many emerging GaN electronic and optoelectronic devices comprise p-type gallium nitride (p-GaN) layers buried below n-type layers, which often show nonuniform (i.e., laterally graded) acceptor concentration (NA) after activation. In power devices, such buried p-GaN layer could be thick and lightly doped, yet its voltage blocking characteristics remain unclear. This work fills this gap by studying the breakdown voltage (VBD) and leakage current of vertical GaN p-n+ diodes with the uniform and nonuniform NA profiles in the 3.8-μ m-thick, lightly doped p-GaN drift region. The nonuniform NA is produced by burying the p-GaN under an n-GaN cap, followed by sidewall activation. The diodes show VBD up to over 400 V, which is the highest reported in vertical GaN devices with a p-type drift region. For the sidewall-activated diodes, their VBD is determined either by the peak electric field at the device edge (with the highest activation ratio) or the punchthrough in the center (with the lowest activation ratio). An additional p++-GaN cap is essential to suppress the latter breakdown mechanism. For devices with either uniform or nonuniform NA, their leakage current can be explained by variable range hopping (VRH), while in the sidewall-activated devices, the edge region instead of the total device area dominates the leakage current. These results provide key guidance for the design and processing of future high-voltage GaN devices with buried p-GaN, such as the GaN superjunction.