Reliability Study of High-Power White LED 1

Since high-power LED has been widely used, semiconductor lighting materials display its outstanding advantages: energy saving environmental protection, high efficiency, long life, low cost of maintenance. It has quite attractive characteristics comparing to the traditional light source. However, in the actual application process, the performance of LED doesn’t display as people’s expectation, it has a lot of problems. For example, the long life is estimated to be ten thousand hours in theory. However, it only has several thousand hours in the actual service conditions. Therefore, it is necessary and urgent to study the reliability of LED for the further development of high-power white LED and the final replacement for the traditional light source. Usually, the reliability study of LED can be divided into two directions: the failure mechanism analysis based on semiconductor physics and the creditability forecast based on engineering reliability.

The failure mechanism analysis based on semiconductor physics

The reliability analysis of semiconductor is characterized by its life service; therefore most LED life test is carried out together with the failure mechanism analysis. Because the life of LED is as long as ten thousand hours in theory, usually, the general life tests needs a very long time, when the test is finished, the testing product will fall into disuse. For LED device with long life, we can choose the accelerated life experiment according to the condition.

Accelerated life experiment can be classified into constant stress accelerated aging test, stepping stress accelerated life tests and sequence stress accelerated life tests. It is relatively easy to operate and control the constant stress accelerated life tests due to the mature technology and the accurate extrapolating data. Therefore, the test result of life is reliable, but the disadvantage is time consuming. Stepping stress accelerated life tests have been popular in the present research because it can shorten the testing time, reduce the number of testing samples, and it has higher accelerated efficiency than the constant stress accelerated life tests. However, at present, it is usually used in the stress range of the earlier stage. For example the accelerated life test of GaAs infrared emitting diode is to use the current stepping test to find out the highest current stress that the testing device can bear. The stress rises over time during the sequence stress test, which can stimulate faster device failure and improve the efficiency of the accelerated life test. However, since it is hard to accurately control the applied stress, it is easy to cause the change of the mechanism failure. Therefore, it is not used commonly. LED device is usually driven by the current and influenced by temperature; the current and temperature are usually chosen as the applied stress. Accelerated test is the effective means to carry out the reliability study unless the mechanism failure of the device is guaranteed not to change during the whole test. The reliability study is not only based on the life calculation and the data measurement, but also relies on the mechanism failure analysis by using the effective data. The device failure is divided into three stages such as early failure, random failure and wearout failure shown as picture 1:

Picture1 Device failure distribution curve (bathtub curve)