高電壓發光二極體晶片之熱電耦合模擬研究

The research on high voltage LED chips by the thermal-electrical coupling method

李皓鈞(Hao-chun Lee)

指導教授 ： 陳志臣

國立中央大學/工學院/機械工程學系/碩士(2011年)

摘要

高電壓發光二極體(HV LED)為一種以串聯的方式將多顆微晶粒連結而成的新式LED陣列結構，HV LED可透過全波整流器將交流電壓轉換成全波電壓或直接由直流電壓驅動點亮。且透過該技術可使LED在高的工作電壓下驅動並降低流過每顆微晶粒的電流。也因為HV LED小電流、多微晶粒的設計，因此提高電流分布的均勻性。
本研究以有限元素法建立一套可應用於直流及全波電壓輸入之數值模擬模型，該模型可用以計算及分析HV LED的電流密度與溫度分布型態。此外，亦透過接面溫度及光學特性之量測實驗與數值模擬之結果相驗證，且活化層之電流密度模擬結果也與實驗測得之光強度有一致性的分布。由數值模擬之結果可知，隨著HV LED的功率提升將使晶片的電流往電極周圍聚集，使得該區域產生較高的溫度。在固定直流電壓與全波電壓的平均輸入電功率條件下，全波電壓驅動時所產生的熱堆積較直流電壓驅動時少，擁有較低的接面溫度，故較能提升光輸出功率。透過積分球量測實驗，於280 mW的輸入條件下，直流及全波輸入之光功率分別為45.63 mW及54.48 mW，可證實全波電壓驅動之光功率較直流驅動為佳。
最後，利用我們提出之數值模擬模型可針對電極設計方式進行分析。在p電極設計方面，我們發現當p電極的側向長度越長時，越容易將電流聚集至n電極周圍，造成熱點現象進而使得晶片的接面溫度升高;當p電極的縱向長度上下兩端各增加40 μm時，能降低0.9 K的接面溫度。又當p電極上下端各縮短40 μm時，晶片的平均接面溫度則大約升高2 K。將原本的n電極形狀修改成與p電極相同的長條狀時，可增加每顆微晶粒的電流均勻性，且經修改後的n電極與p電極的上下兩端同時往縱向延伸40 μm時， 達準穩態後之接面溫度比原設計降了4.6 K。推測電極的縱向長度越長，能達到較佳的效益。

關鍵字

接面溫度 ； 全波電壓 ； 有限元素法 ； 高電壓發光二極體

並列摘要

High voltage light-emitting diode (HV LED) is a novel chip array formed as a series connection structure by joining multi-microchips. HV LED can be operated not only by using the DC voltage directly but also by the full-wave voltage converted from the AC one via the full-wave rectification. HV LED technology can be used to drive the LED at high operated voltage so that the current in each micro-chip can be decreased. Since the HV LED is based on designs of multi-microchips and small working current, the current spreading of LED chip will be more uniform.
In this study, we propose and establish a numerical model using finite element method. This numerical model can be used to calculate and analyze the distributions of current density and temperature in HV LED under two operating situations, DC or full-wave. Moreover, the simulation results of our proposed model were verified by two measuring experiments, temperature and optical measurements. The simulation results of the current density distribution in the active layer are agreed with the measured ones obtained by the optical experiment. We found that the phenomenon of current crowding will appear more obviously around the electrode edge when the input power is increased.This phenomenon also causes that the temperature near the crowded area is raised. When the input power is at 280mW, the light output powers of the HV LED by operating at DC and full-wave voltages are 45.63mW and 54.48mW, respectively. Because the HV LED driven by full wave voltage can decrease the heat accumulation, they will have lower junction temperature. Thus, the full-wave driving condition is better than the DC one under the same input power.
Finally, our proposed model can be further used to analyze the design of the electrode pattern. For the p electrode design, the current crowding effect will appear at the n electrode edge when the lateral length of p electrode is increased. Then the hot spot phenomenon at the n electrode edge is generated, the junction temperature in LED chip will be also increased due to the more serious current crowding at the n electrode edge. When the p-electrode is enlarged to the length of 40μm, the junction temperature can be reduced about 0.9K. When the length of p-electrode is reduced as 40μm, the junction temperature will be arisen about 2K. When we change the n-electrode pattern like the long narrow strip one, the current density distribution is more uniform on each micro-chip. Moreover, the p-electrode and modified n-electrode are both extended as 80μm; the junction temperature in LED chip at quasi-steady state is 340.5K (4.6K lower than the conventional one). The more extended length, the better thermal and electrical behaviors.

並列關鍵字

high voltage LED ； finite-element method ； junction temperature ； full-wave voltage

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