| Summary: | 碩士 === 國立雲林科技大學 === 電子與資訊工程技術研究所 === 86 === 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.
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