先進金氧半場效電晶體考慮溫度相依之高頻小訊號及雜訊特性分析

Abstract

本篇論文對於現今各種的平面金氧半場效電晶體(MOSFET)作了完整的高頻小訊號及雜訊特性分析和模型化工作，這些元件包括傳統MOSFET(bulk MOSFET)、絕緣層上矽MOSFET(SOI MOSFET)、絕緣層上矽動態起始電壓MOSFET(SOI DT MOSFET)和應變MOSFET(strained MOSFET)等。建立於傳統MOSFET架構下的等效射頻小訊號電路模型將被適當地修改以考量存在於各別元件的寄生效應，並藉由在適當的位置擺入各雜訊源，我們便可以建立各元件對應的高頻雜訊模型。此外，本篇論文也首次探討到各元件在溫度變化下的高頻行為表現。 部分空乏SOI MOSFET中性體區(neutral-body)的寄生效應，會影響到元件的輸出特性，其影響甚至可到數個GHz的操作頻率。由於通道電導高溫時減小，造成了bulk MOSFET和SOI MOSFET的通道雜訊都呈現具負溫度係數的特性。此外，SOI元件中的自發熱效性(self-heating effect)和浮體效應(floating-body effect)會使得其雜訊因子(noise factor)高於傳統MOSFET。不利於高頻雜訊的浮體效應(在低閘電壓下較明顯)可藉由升溫加以抑制，而自發熱效應則因為高閘極電壓伴隨的低電導而可扺掉一部分效應。 SOI DT MOSFET的基底寄生元件和串聯電阻對最大震盪頻率(fmax)的影響比對截止頻率(ft)來的大。此外，在一般操作所使用的低閘極和低汲極偏壓(VDD)下，由於轉導(gm)會隨溫度上升而上升，造成ft和fmax都和溫度成正相依關係。實驗結果顯示當DT MOSFET的偏壓朝弱反轉方向減小，由於轉導平方項的正溫度相依情形相對於通道雜訊來得大，我們發現其等效熱雜訊電阻Rn將具有負的溫度係數。進一步的研究發現，基底的連接電阻Rb所產成的雜訊會不利於最低雜訊指數NFmin，且低VDD偏壓下的較大Rb對於NFmin的溫度相依情形反而影響不大。 舒張形變n型MOSFET(tensile-strained nMOSFET)因為擁有較大載子移動率及和傳統MOSFET相近的飽和電壓，所以以相同的偏壓條件來說，會有較大的通道雜訊。然而，實驗結果顯示對於相同的功率消耗條件來說，其較大的轉導會使得tensile-strained nMOSFET 在ft、fmax、NFmin和Rn的表現上都會比傳統MOSFET來的優異。 最後，本論文探討65奈米MOSFET應用於毫米波時的雜訊表現。實驗結果顯示，持續上升的通道雜訊對整體毫米波雜訊模型化工作和特性分析的影響愈來愈重要。除此之外，以寄生效應來看，閘極電阻對毫米波的雜訊影響程度比基板電阻來的大。

This dissertation provides a comprehensive high-frequency small-signal and noise characterization and modeling for various kinds of modern planar MOSFET devices, including the bulk MOSFET, silicon-on-insulator (SOI) MOSFET, partially-depleted SOI dynamic threshold voltage (DT) MOSFET, and strained MOSFET. The traditional RF small-signal equivalent circuit for the bulk MOFET will be modified to include existing parasitic components present in each kind of MOSFETs. Based on each tailored small-signal model, the corresponding high-frequency noise model can be built by adding the noise sources in place. For the first time, the temperature dependence of the high-frequency performance will also be discussed. The SOI MOSFET has the inherent neutral-body effect, which will be found to influence the output characteristic even in GHz applications. The channel noise Sid has been shown to have a negative temperature coefficient for both the bulk and SOI MOSFETs due to the lowered channel conductance at high temperature. Besides, the self-heating effect (SHE) and the floating-body effect (FBE) of the SOI MOSFET would make its noise factor higher than the bulk MOSFET. It shows that the FBE, which dominates at low VGS regime, can be suppressed by elevating the ambient temperature, while the SHE, obvious at high VGS, would be partly counterbalanced by the lowered channel conductance at high temperature. The body-related parasitics and the series resistance of the SOI DT MOSFET are found to have more impact on fmax (maximum oscillation frequency) than ft (cut-off frequency). Besides, in the normal bias condition - low gate and drain voltage (low VDD) regime, both ft and fmax have positive temperature coefficients due to the increased gm (trans-conductance) at high temperature. We also show that the DT MOSFET would get a negative temperature coefficient for equivalent noise resistance Rn towards the weaker inversion region due to the much higher gm2 than Sid with increasing temperature. Furthermore, our research results show the noise arising from the body resistance Rb can degrade the minimum noise figure NFmin, and the larger Rb encountered in the low VDD regime would have less impact on the temperature dependence of NFmin. The tensile-strained nMOSFET presents larger Sid than the control device due to its enhanced mobility and nearly the same saturation voltage for a given bias point, and has the same temperature dependence of Sid as the control device. However, our measured data indicates that the enhanced carrier trans-conductance in the tensile nMOSFET would contribute to better ft, fmax, NFmin and Rn than the control device for a given DC power consumption. Finally, for the emerging millimeter-wave applications, we examine the millimeter wave noise behavior of 65nm MOSFETs. The experimental results show that the continually increasing Sid makes it play a more and more important role in the millimeter-wave noise modeling and characterization. Besides, compared to the substrate resistance, the gate resistance has more impact on the noise parameters in the millimeter-wave frequency.