Improved performance in vertical GaN Schottky diode assisted by AlGaN tunneling barrier

In a vertical GaN Schottky barrier diode, the free electron concentration n in the 6-μm-thick drift layer was found to greatly impact the diode reverse leakage current, which increased from 2.1 × 10^-7 A to 3.9 × 10^-4 A as n increased from 7.5 × 10¹⁴cm^-3 to 6.3 × 10¹⁵cm^-3 at a reverse bias of 100 V. By capping the drift layer with an ultrathin 5-nm graded AlGaN layer, reverse leakage was reduced by more than three orders of magnitude with the same n in the drift layer. We attribute this to the increased Schottky barrier height with the AlGaN at the surface. Meanwhile, the polarization field within the graded AlGaN effectively shortened the depletion depth, which led to the formation of tunneling current at a relatively small forward bias. The turn-on voltage in the vertical Schottky diodes was reduced from 0.77 V to 0.67 V - an advantage in reducing conduction loss in power switching applications.