Effect of current crowding on the ideality factor in MQW InGaN/GaN LEDs on sapphire substrates

To date, the reason for high ideality factor, β, in GaN-based LEDs grown on sapphire substrate is not fully understood and explained. It is believed that β-factor exceeding 2.0 originates from the trap-assisted tunneling and charge carrier leakage inside the active MQW LED region or is due to additional junctions available in the LED circuit. In this research, we demonstrate that β values higher than those predicted by the classical theory may be related to the current crowding effect that is difficult to avoid in LEDs grown on the insulating substrates. By analyzing theoretical model and testing commercial lateral blue LEDs with two different p-electrode pattern, we show that β -factor could increase from 2.0 (current spreading geometry) up to 3.5 (current crowding geometry). This modification of β-factor occurs in the intermediate range of current (10 μA - 10 mA, the space charge region dominates in LED performance) and therefore could be erroneously treated as the change of carrier transport mechanism and charge carrier recombination nature. At higher current (series resistance dominates) even insignificant increase of β-factor makes the current value of efficiency rollover to decrease (from 35 mA to 15 mA) and the efficiency droop to increase by 10%.