Forward-Capacitance Measurement on Wide-Bandgap Light-Emitting Diodes

Capacitance-voltage characteristics of a GaN-based light-emitting diode under high forward bias is investigated. We examine the validity of measurement results from different measurement systems and then consider the equivalent circuits used to extract the device capacitance. By analyzing the magnitude and phase of the device impedance, we show that the commonly used equivalent circuit with parallel resistor and capacitor is inadequate to extract the capacitance values under high forward bias. By using a more appropriate equivalent circuit with an added series resistance, we demonstrate that more precise extraction of the forward capacitance is possible, free of any abnormal behavior of decreasing capacitance under high forward bias. We conclude that the so-called negative capacitance of the p-n diode reported in the literature is not the actual device capacitance: it is rather an artifact caused by an inadequate equivalent circuit used by the measurement system in combination with the system limitation.