Origin of the High Forward Voltage and Low Voltage Efficiency of GaN-Based Light-Emitting Diodes at Cryogenic Temperatures

It is known that the forward voltage of the GaN-based light-emitting diode (LED) increases significantly at cryogenic temperatures. In this work, the origin of the high forward voltage is investigated by utilizing photoexcitation measurements. Using the characteristics of short-circuit current versus open-circuit voltage, which reveals the ideal diode behavior, the cause of the additional potential drop outside the active region is analyzed. The results suggest that the abnormally high forward voltage, thus the low voltage efficiency (VE), at cryogenic temperatures below 100 K is induced by the space-charge-limited current (SCLC) due to the insufficient activation of p-type dopants. The present work clarifies the location of the SCLC and its cause in the GaN-based LEDs at cryogenic temperatures, thus enabling further improvement of the forward voltage and the VE.