The Analysis on LED chips by the thermal-electrical coupling method

• Main Authors: Farn-Shiun Hwu, 胡凡勳
• Other Authors: Jyh-Chen Chen
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/12524780942242841428

博士 === 國立中央大學 === 機械工程研究所 === 97 === The usages of Light-emitting diodes (LED) are widely in the modern life. A novel design for LED chip to operate under the alternating current named alternating current light-emitting diode (AC LED). Recently, the manufacturing technology for AC LED is progressive, and the commercial products of AC LED are in the market. The situation of heat generation in AC LED is different from that in conventional LED. But few literatures have mentioned about the thermal issue of AC LED or the determining method of the junction temperature of AC LED. A numerical simulation is used to simulate the temperature distribution during AC and DC operations of an alternating current light-emitting diode (AC LED). The relationship between the junction temperature and the temperature at the center of the bottom surface of the submount of an AC LED is measured under DC operation. This relationship is confirmed by numerical simulation. The numerical results are consistent with the experimental observations in that the temperature at the center of the bottom surface of the submount is insensitive to the current variations that occur in an AC LED, probably due to the large mass of the submount. However, it is difficult to measure the temperature oscillation at the junctions in an AC LED, although this oscillation can be clearly seen in the numerical results. Therefore, we propose a formula for predicting the range of the oscillating junction temperature for an AC LED. Since the input power is increased for high power LED, the heat generating form the chip increases. The electrical and optical characteristics of LED are significantly influenced by the temperature of LED chip. In order to analyze the thermal effect in the LED chip, a three-dimentional numerical simulation model with the coupling of the thermal and electrical characteristics is developed. The influence of the size of the n-electrode and current blocking layer (CBL) on the thermal and electrical characteristics of a vertical-injection GaN-based light-emitting diode (LED) chip is investigated by a numerical simulation. The predicted forward voltages are quite consistent with previous experimental data. The coupled thermal and electrical effects affect the performance of an LED chip. For cases without CBL, the variation of current density and temperature distributions in the active layer, and the forward voltage and Joule heating percentage of the LED chip increase as the n-electrode width (L) decreases. The insertion of a CBL into a 600×600 μm2 chip leads to greater uniformity in the distribution of the current density in the effective light-emitting area in the case when L = 500 μm. A more uniform temperature distribution in the active layer occurs when L = 200 μm while the case when L = 300 μm has the maximum Wall-plug Efficiency (WPE). Some parameters of LED chip, such as conductivity and thickness of n-GaN layer will influence the electrical and thermal characteristics of the chip. Especially the current density is significantly influenced by the conductivity of n-GaN layer. When the conductivity is lower, the current crowding in the active layer under the n-electrode. But when conductivity is enlarged to the value of 5×104 S/m, the distributing of current density is rather uniform. The influence on current density in the active layer by increment of the n-GaN layer thickness is not obviously. And the increment of n-GaN thickness will also decrease the light output, increase the time and cost of manufacturing process. It is unusual to carry on the improvement of the uniformity of current density by increasing the thickness of n-GaN layer. Nevertheless, if the product of conductivity and thickness of n-GaN layer is a constant, the current density distribution in active layer will be the same one. The current density in the active layer of LED chip may be more uniform when the product of conductivity and thickness of n-GaN layer is higher than one particular value.