Degradation characterization of AlGaInP LEDs using I-V and low frequency noise measurements

The influence of thermal stress on I-V characteristics and low frequency noise (LFN) measurements of AlGaIn ultra high brightness (UHB) light emitting diodes (LEDs) has been investigated. The LEDs were stressed at a temperature of 670 K in combination with an applied current of 20 mA simultaneously. After each stress period, the I-V characteristics and the LFN were measured at room temperature. The results reveal a strong dependence of the degradation mechanisms on the operating current. The unstressed diode could be modelled by a simple series combination of an ideal light emitting diode with an ideality factor of 1.8, a leakage current of about 5×10^-19 A, and a series resistance of about 4 Ω. Within the first four hours of applied stress, the series resistance increased up to 8 Ω and the leakage current and ideality factor of the diode remained constant, while the LFN increases by more than one decade. All measurements showed a knee voltage of about 1.7 V. After seven hours of stress, the I-V characteristics of the LED changed and the diode started emitting light only above 10 mA. Increasing the stress time to 9:15 hours resulted in a completely different I-V curve with a knee voltage of about 0.8 V. Moreover, the diode does not emit light up to a current limit of 100 mA and the noise increased rapidly. The experimental results indicated the importance of LFN measurements for detection of the LED degradation processes.