發光二極體晶片之熱電耦合分析; The Analysis on LED chips by the thermal-electrical coupling method

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題名:	發光二極體晶片之熱電耦合分析;The Analysis on LED chips by the thermal-electrical coupling method
作者:	胡凡勳;Farn-Shiun Hwu
貢獻者:	機械工程研究所
關鍵詞:	接面溫度;數值模擬;交流電發光二極體;發光二極體;AC LED;LED;numerical Simulation;junction temperature
日期:	2009-07-06
上傳時間:	2009-09-21 11:56:58 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	發光二極體(LED)之用途日漸廣泛，交流電發光二極體(AC LED)技術也於近年日漸成熟並商品化。AC LED之發熱情形與傳統LED在直流電操作下之穩定熱源有所差異，而現階段對於AC LED之相關研究甚少提及其發熱情形與接面溫度量測之方式。為了量測AC LED之接面溫度，本文提出一種搭配實驗與數値修正的方法來模擬AC LED在AC和DC操作條件下的溫度分布。並找出AC LED在DC操作下所量測到的接面溫度和封裝基板底部溫度之關係，這一關係亦可由數値模擬確認。且數値計算所得的基板溫度與實驗的觀察具有一致性，其溫度變化並不像交流電流的作用那麼敏感，此或許為基板之質量相對晶片質量而言大了許多。在數値模擬中可以明顯看到AC LED接面溫度的振盪；然而，AC LED接面溫度的振盪情形卻很難被量測到。因此，本研究提出一公式來計算AC LED的接面溫度振盪範圍。而隨著發光二極體的操作功率日漸升高，其晶片之發熱越來越大，而溫度對於晶片的各項電光特性皆有所影響，故爲深入了解LED晶片中的電熱特性，本文建立三維數值模擬的熱電耦合模型來進行高功率垂直電流注入(vertical)發光二極體晶片之電場與溫場模擬及分析。另外對於垂直電流注入LED晶片中，n 電極與電流阻擋層的大小對於熱與電特性的影響進行探討。藉由數値模擬所計算出的LED順向偏壓與相關文獻的實驗數據相當吻合，也觀察出熱與電之對於LED晶片的效能具有耦合的影響。在沒有電流阻擋層的情形時，活化層的電流密度分布與溫度分布、LED晶片的順向偏壓和焦耳熱所佔的百分比，隨著n電極面積的減小而增加。在加入電流阻擋層後，在晶片大小為600×600 μm2的情形下，有效出光區域的電流密度在L = 500 μm時變得比較均勻。較均勻的溫度分布是出現在L = 200 μm的條件下，而L =300 μm時則可得到最大的功率轉換效率(Wall-plug Efficiency)。而針對LED晶片n-GaN層的導電率與厚度等參數對其電及熱特性的影響加以分析後發現導電率對於電流密度的影響相當敏感，在小的導電率條件下電流壅塞在n電極下方區域的情形非常嚴重。而當導電率增大至σ=5×104 S/m時，電流密度的分布則相當均勻。而n-GaN層厚度的增加對於活化層電流密度的影響並不顯著，且需考慮n-GaN層厚度增加後對光學的影響，以及製程的時間成本因素，使得一般較少藉由增加n-GaN層厚度來進行電流均勻性的改善。另外，將n-GaN磊晶層的導電率與厚度相乘後發現當其積為固定値時，電流密度的分布情形亦相同。可以推論得n-GaN的導電率與厚度的乘積若大於一特定値，晶片活化層之電流密度分布將會相當均勻。 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.
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