臺灣博碩士論文加值系統

最近幾年砷化鋁鎵/砷化鎵異質接面雙極性電晶體(HBT*s)，在微波通信功率放大器的應用上，扮演一個很重要的角色。HBT*s具有高電流處理能力的優點，然而砷化鎵這種材料的熱導性卻相當差(砷化鎵的熱導係數只有矽的三分之一)，因此時常導致HBT晶格溫度的嚴重上升。這種造成電晶體溫度上升的機制稱為自發熱效應(self-heating effect)，這種熱效應降低了元件在高功率操作下的特性。本論文的主題之一，就是要探討熱效應對砷化鋁鎵/砷化鎵異質接面雙極性電晶體的影響。對於現代微波應用上所使用的HBTs，都是採用多射極結構。這種多射極結構可使每根射極所承載的電流減小，因此所產生的熱功率也較小，所以多射極結構自發熱效應(self-heating effect)的影響比單一射極結構來得小。然而，當多射極HBTs操作在高功率的工作環境下，由於射極本身產生的溫度上升外，再加上射極間的熱耦合，將導致中間的射極溫度較高。因為基-射極接面電流具有正溫度相關性，所以中間的射極將傳導較多的電流，因而造成中間的射極溫度明顯上升，這就是所謂的熱跑脫(thermal runaway)。由於電熱間的正回饋，最後將導致溫度無法達到穩態，而燒壞電晶體。對於多射極HBT*s，只要任何一根射極損壞，整個元件就無法正常工作。解決溫度不穩定的其中一個方法就是在射極上加ballasting電阻。雖然可藉由加ballasting電阻使元件溫度下降，但這種方法卻要付出使電晶體輸出功率及速度變差的代價，且這個ballasting電阻值的大小並不是很容易設計找到。在本論文我們提出另一種元件結構的設計方法，藉由調整射極間的間距以使所有射極溫度分佈真正達到均等，且不會有ballasting電阻的缺點。

In recent years, the AlGaAs/GaAs Heterojunction Bipolar Transistors (HBT*s) play an important role in microwave and millimeter-wave power applications. The HBT*s very high current-handling capability and very poor thermal conductivity of GaAs (the thermal conductivity of GaAs is only 1/3 of that of silicon), however, often lead to a significant increase in the lattice temperature of the HBT. This mechanism is called self-heating effect. The effect limits the device performance under high power operation. One objective of the thesis is to investigate the thermal effect problem in the AlGaAs/GaAs HBTs. For modern microwave HBTs, a multiple emitter finger structure has frequently been used (multifinger HBT). Such a structure allows less current to be carried and thus less heat power to be generated in each HBT unit cell, thus making the self-heating effect less prominent compared to its single-emitter finger counterpart. However, when the multifinger HBTs are operated under high power conditions, the heat generated in each finger and the thermal coupling among fingers result in a higher temperature at the center fingers. Because the B-E junction current has a positive temperature coefficient, the center fingers will conduct more current and further increase its temperature, which gives rise to a "hot spot." This is known as thermal runaway. The resulting electrical and thermal positive feedback can finally cause thermal instability. For multifinger HBT*s, if any single emitter fails, the entire device fails. One of the solutions to improve the stability is the use of emitter ballasting resistance. Though device temperature can be reduce by the ballasting resistance, the thermal stability is improved at the expense of power and speed. Thus the optimal value of this resistance is very critical and should be carefully designed. In this thesis, we propose a novel device layout of non-uniform finger spacing to achieve a true uniform temperature distribution with minimum loss in power and speed performances.