Modeling of the vertical leakage current in AlN/Si heterojunctions for GaN power applications

We present a model for the vertical conduction through an AlN/p-Si junction, which is used as a base for the growth of GaN power devices. First, we recall that for resistive silicon substrates, the I - V curves of the AlN/p-Si show a monotonic increase, interrupted by a plateau region. Then, to quantitatively explain this behavior, we propose a novel two-diode model. More specifically, we demonstrate that the AlN/p-Si structure can be split into the series connection of two substructures: 1) an equivalent AlN/n+-Si junction and 2) an equivalent n+-Si/p-Si diode. The n+ layer models the electron inversion layer in the silicon at the interface with the AlN layer. Technology Computer-Aided Design (TCAD) simulations were used to validate these two diode models. By comparing the leakage current of the AlN/p-Si structure with the current through the diodes, we demonstrate that within the plateau region, all the applied voltage drops on the equivalent n+-Si/p-Si junction, and the current through the diodes is limited by the reverse leakage current of the n+-Si/p-Si diode. The plateau ends as soon as impact ionization occurs in the Si substrate, due to the high electric field in the depletion region. After the plateau, the current through the diodes is again limited by charge injection from the inversion layer into the AlN, which occurs through a phonon-assisted tunneling mechanism (possibly trap-assisted).