THERMAL MANAGEMENT AND CHARACTERIZATION OF LIGHT-EMITTING DIODES Susu Yan Submitted to Mount Holyoke College in partial fulfillment of the requirements for the degree of Bachelor of Arts with Honor

Light-emitting diodes (LEDs) are promising electronic devices used in various lighting system, such as traffic signals, large screen displays with high brightnesses, sources of general illumination. Currently, LEDs are increasingly used for everyday purposes due to their multiple advantages, which include low energy consumption, long life time and solid state high power lighting. However, temperature plays an important role in the function of LEDs and affects their performance and reliability. As the heat in LEDs escalates, several key characteristics may alter; the forward voltage may decrease, the optical wavelength may shift, and LEDs’ output light may change color. Therefore, thermal management and full characterization of LEDs have been investigated to examine the effect of heat on LEDs. In this work, we measure the thermal distribution inside the operating LED, both laterally and vertically by using thermoreflectance microscopy and two traditional measurements, in order to better understand the thermal performance of the LED. Thermoreflectance microscopy is used to provide the first high resolution 2-Dimensional temperature distribution across the surface of the operating LED. This technique uses a CCD camera to measure the change in reflectivity (∆R) of the test device in response to a change in temperature (∆T): ∆R R = 1 R ∂R ∂T ∆T = к∆T If the temperature calibration coefficient к is known, ∆T can then be extracted from the measured ∆R. Thermoreflectance microscopy is a promising new means of mapping the thermal distribution of operating LEDs because it is a non-contact optical technique with very high spatial resolution (250nm) that is suitable for use on light emitting surfaces. We have compared our thermoreflectance microscopy results to two more traditional means of estimating LEDs’ temperature, based on temperature-induced variations in the emission wavelength and forward voltage of operating LEDs. Both of these techniques are bulk measurements that estimate a single temperature for the entire device, so they lack the ability to resolve thermal distributions across the LED surface. In addition, we compare thermal performance of encapsulated and de-encapsulated LEDs. Our initial results from the wavelength shift measurement and forward bias voltage measurement show that the temperature of the encapsulated LED is lower than that of the de-encapsulated LED. This result suggests that silicone encapsulant and the lens act as a heat spreader and help conduct the heat from the active region of the LED device to the ambient. Compared these results with the thermoreflectance microscopy result, we find that the surface temperature is slightly higher than the p-n junction temperature, which is slightly higher than the average operating temperature. Furthermore, the thermoreflectance microscopy provides the high resolution 2D temperature distribution across the LED surface which suggests that the temperature of the LED chip edge is higher than that of the center. Moreover, we obtain a 3-Dimensional temperature profile of the LED based on the three temperature measurement techniques.