大面積發光二極體之研究

Abstract

本文中，大面積發光二極體之研究分為兩個步驟。由於所用的LED磊晶片是以藍寶石為基板，因此在LED的結構設計上必須使n型與p型兩電極在晶片的同一邊。而在這樣的結構設計中，側向電流是否能均勻分散對LED的效能就會造成極大的影響，特別是在大面積LED上。 首先，可以根據LED的結構將其簡化為一等效電路模型。藉由這樣的模型，我們發現在LED結構中 rn、rp、tp、 tn 是影響電流能否均勻分散會的重要參數。若能使rn減小，rp、tp、 tn增加，則可提高transfer length Lt 使加大LED面積後電流仍能均勻分散。 接著，我們將大面積LED以高電流密度操作。雖然大面積LED的series resistance 已經明顯的較conventional LED小，但是當大面積LED的操作溫度高於500℃後specific contact resistance 會明顯的增加造成joule heating 的影響加劇而使得LED的contact metal受到破壞。因此未來的發展方向將朝向更換contact metal為高溫熱穩定金屬，如W、Ti/W、WSix，改善腳架（lead frame）的散熱效果，甚至以flip chip的方式來解決大量散熱的問題。

Most commercial LED suppliers use sapphire substrates, which are significantly less expensive than SiC substrates. However, using sapphire as a substrate poses devices design challenges, especially for large area LEDs. In this work, we study the pattern design of a large area LED first. According to the lateral current path, the LED structure could be simplified to get an equivalent circuit model. The model is made of the contributions from different parts of the structure. By virtue of the model, it is obvious the thickness and resistivities of the TCL, p-type GaN, and n-type GaN must be considered as well as the contact pattern design. As a result, if the rn, rp, tp, and tn could be adjusted suitably, the transfer length Lt will be raised and the current spreading will be much uniform. Moreover, we make large area LEDs operated at high current density. Although the series resistance of a large area LED has been lowered, yet the significantly raised specific contact resistance make a vicious circle and then, cause damage mainly to contact metal of LEDs when the operating temperature is over 500℃. Therefore, we propose some suggestions and future works to solve the problem about heat-sinking in chapter 3. The contact metals (Ti/Al) can be replaced by W, Ti/W or WSix. The heat-sinking of lead frames must be powerful to reduce the increment of temperature at high injection current operation. Moreover we could reverse LED chips to form the flip chip LEDs, which could transfer a great deal of heat generated in LEDs and enhance the luminous intensity obviously.