Study On In-situ Measurement And Reliability Of Light Emitting Diodes Using An Integrated Sensor

Light emitting diodes (LEDs) as next generation of solid state lighting sources, have superior characteristics to the traditional light sources in terms of smaller exterior outline dimensions, higher response characteristic, higher performance, guaranteed longer lifetime and energy conservation, which have been widely used in various lighting fields such as the backlight, display, traffic lights, and interior lighting. In situ measurement of junction temperature (Tj) and luminance becomes important recently for LED’s reliability issue and smart lighting. Real-time Tj and luminance are important parameters for accurate understanding of LED’s multi-stage degradation behavior and complicated degradation mechanisms; enable protecting LED against being overheated, or monitoring the LED status remotely, which makes LED lighting smarter; also contribute to collection of field data automatically, which is important for LED’s lifetime prediction.However, the existing methods to measure Tj are difficult to meet the requirement for in situ accurate monitoring of Tj, and luminance measurement is also relied mainly on the peripheral equipment. On account of the importance of in-situ measurement of Tj and luminance and the insufficiency of the method to online measurement, we designed a novel LED chip with a built-in sensor in this thesis, which realized the real-time measurement. Two electrically isolated units are integrated on a single chip:the LED unit and the sensor unit. The epi-layer structure and the process steps of the two unit are identical, while the area of the sensor unit is much smaller than that of the LED unit, which means small added st. The main results in this thesis are highlighted as below:1.Tj can be determined by measuring forward voltage (VF) of the sensor unit. The luminance of the LED unit can be precisely extrapolated from photocurrent of the sensor unit under zero bias condition. According to the theoretical analysis and experimental results, the relationship between VF and Tj behaves a good linearity. The photocurrent increases with the increase of light output power (LOP) and Tj, which is associated with the improvement of the quantum efficiency of the diode detector due to the current injection and the increase of Tj. LED’s LOP can be determined very well as a quadratic polynominal empirical function of photocurrent and Tj so that we can realize the online measurement, and the absolute value of the difference is less than 2 mW for all experimental samples after calibration, which shows a high accuracy. Normalized empirical expression of LOP can be applied to the same batch samples with appropriate lumen coefficient, greatly reducing the workload of calibration and suitable for mass production.2. By the comparation between the cooling curve derived from VF of the LED unit with that derived from VF of the sensor unit, the results showed that temperature offset between two units is small, the value is less than 0.1℃. Therefore, the sensor unit can be used to measure Tj accurately. As for the received light converted into photocurrent, about 88% comes from inside the chip by zigzag reflections, and the other 12% is from the surface of the LED unit by encapsulation reflection. Moreover, the influence of illumination on VF of the sensor unit is minimal, in other words, junction temperature measurement is not affected by illumination.3. Utilizing the integrated sensor LED, life prediction have been preliminarily researched with dual-stress accelerated life model of current and temperature. Lumens depreciations under difference accelerated conditions all show approximately exponential decay. Nevertheless,Tj behaves distinct trends for different operation currents:Tj gradually increased for larger current, but remained stable for lower current along with the aging time. This is mainly due to the increase of series resistance and forward voltage induced by high junction temperature under larger current, which resulted in the drop in the LED quantum efficiency and a larger portion of electric power is converted to heat so that Tj rise. The relationship between VF and Tj shows a good stability during the aging time, which is critical for accurate measurement of Tj. Based on the online data of Tj and luminance, combined with dual-stress accelerated life model, we predicted that the actual lifetime is about 35841 hours, preliminarily achieving the fast life prediction.