矽鍺異質接面雙載子電晶體在射頻應用之雜訊模型化及特性化分析

Abstract

近年來隨著生活水準提升,無線通訊 ( wireless communication) 市場快速成長,無論是學術界或是工業界皆無不極力地發展無線通訊這高科技 o 而微波元件則是通訊系統中最重要的骨架 o 以矽為基底的矽鍺異質接面雙載子電晶體(SiGe HBT),除了比傳統的矽電晶體(Si BJT)有較高的效能外,也比三五族(III-V) 複合材料的微波元件有較佳的低成本效益 o 除此之外,無論是在高頻頻段抑或是低頻頻段的範圍操作下,矽鍺異質接面雙載子電晶體也被證實了有較佳的雜訊效能o 本篇論文之重點即是在研究以矽為基底的矽鍺異質接面雙載子電晶體的雜訊特性以及幾何結構效應的分析,進而提出在一個既定的製程裡,如何利用幾何結構的分析而能找到一個最佳化的雜訊效應同時滿足高頻頻段和低頻頻段的操做 o 此外也在高頻和低頻頻段裡個別提出了一個雜訊的小訊號模型來驗證雜訊的來源和合理性o 首先,在高頻雜訊分析中,幾何結構的效應證實了一個矽鍺異質接面雙載子電晶體若能同時擁有較短的射極寬度(Emitter Width)以及較長的射極長度(Emitter Length)即能在高頻頻段的操作下有較佳的雜訊特性o 而在低頻雜訊分析裡,我們發現低頻雜訊與射極的面積(Emitter Area)呈現一個反比例的關係o 隨著製程技術的進步,元件的尺寸越縮越小,若要達到一個較佳的雜訊特性同時在高頻和低頻段的操做,我們可以利用一個較小的射極寬度但是去調變射極長度來達到此目標 o 在另一方面,由於矽鍺異質接面雙載子電晶體擁有一個天生的雙異質接面,不僅是在射極-基極接面,並且在基極-集極接面 o 經過實驗的證實,低頻雜訊主要一部分源自於異質接面的結構 o 除了傳統的模型描述雜訊來源為射極-基極接面外,我們也加入了基極-集極的雜訊來源並且提出一個更完整的低頻雜訊小訊號模型來描述o 最後矽鍺異質接面雙載子電晶體(SiGe HBT)被證實了有較佳的雜訊性能,若能結合矽基底的金氧半場效電晶體(Si-MOSFET)而做成以矽為基底的BiCMOS製程,將提供給無線通訊市場一個更大的契機,並且可能是RF SOC (system on a chip) 的唯一途徑o

In recent years, with the improvement of living standard, the development of wireless communication has become the most important technology, not only in academic circles but also in the industries. Microwave transistors are the backbone of these modern wireless communication systems. The Si-based SiGe HBTs (hetero-junction bipolar transistors) have the better transistor performance than Si-BJT and have the lower cost beneficial results than III-V compound materials. In addition, for both high frequency and low frequency operation, SiGe HBTs were proved to have the better noise performance than other microwave transistors. The purpose of this thesis is to investigate the characteristic of noise in SiGe HBTs and the analysis of geometry effect on noise of SiGe HBTs. Then, we used the analysis of the geometrical scaling issues to find which device size optimizes noise performance simultaneously both in the high and the low frequencies. In addition, we proposed a small-signal noise model individually in high-frequency and low-frequency ranges and verified the rationality of the noise sources. First, in high-frequency operation, geometry effect shows that the shorter the emitter width and the longer the emitter length is the better for RF noise performance. On the other hand, in low-frequency range, the noise spectrum shows that it is inversely proportional to emitter area. As the emitter width reduced with the modern-scaling technique, we should increase the emitter length to maintain the low-frequency noise performance and reach the optimum high-frequency noise performance. Secondly, the inherent double hetero-structure of SiGe HBTs includes not only the emitter-base junction but also the base-collector junction. The sources of low-frequency noise are proved partially gotten from the hetero-structure through the experiments. Beside the traditional LF noise model which generally considered the noise source from the emitter-base junction, we added an extra electron-induced noise source from base-collector junction and described a more complete LF small-signal noise model. Finally, SiGe HBTs have been proved to have the better noise performance. If we combine the merits of SiGe HBTs and Si-MOSFET to form a SiGe-BiCMOS technology, it will provide a wider design window for the wireless communication system and it presents a unique opportunity for Si-based RF system-on-a-chip solutions.