Electrical Characteristics of GaN-Based Light-Emitting Diodes on Patterned Sapphire Substrates

Abstract

Patterned sapphire substrate light-emitting diodes display obvious negative capacitance (NC) at large forward biases. This is measured using a method based on a small signal alternating current together with direct I–V plots. The NC in patterned sapphire substrate LEDs grows exponentially with the forward applied voltage. This observation is unexpected and in contrast with Shockley’s p–n junction theory, which only includes an increasing diffusion capacitance and not a NC. However, this result is in good agreement with conventional sapphire substrate LEDs. Furthermore, the negative terminal capacitance confirmed the prediction of Laux and Hess’ theory. The ideal factor of a patterned sapphire substrate LED is about 5, greatly exceeding the traditional theoretical value. The capacitance increased to a maximum and then gradually decreased, which was similar to the results for a p–n junction. Patterned sapphire substrate LEDs can withstand higher voltages than conventional sapphire substrate LEDs. This work could further confirm the existence of NC.