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

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 (
${N}_{\text {A}}\text {)}$
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 (
${V}_{\text {BD}}\text {)}$
and leakage current of vertical GaN p-n+ diodes with the uniform and nonuniform
${N}_{\text {A}}$
profiles in the 3.8-
$\mu $
m-thick, lightly doped p-GaN drift region. The nonuniform
${N}_{\text {A}}$
is produced by burying the p-GaN under an n-GaN cap, followed by sidewall activation. The diodes show
${V}_{\text {BD}}$
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
${V}_{\text {BD}}$
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
${N}_{\text {A}}$
, 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.