外加閘極電容對金氧半場效應電晶體次臨界特性的影響

The Effect of External Gate Capacitance on the Subthreshold Characteristics of MOS Field Effect Transistor

陶吉華(Ji-Hua Tao)

指導教授 ： 吳幼麟

國立暨南國際大學/科技學院/電機工程學系/碩士(2019年)

https://doi.org/10.6837/ncnu201900112

摘要

傳統的金屬-氧化物-半導體場效應電晶體(MOSFET)一直存在著所謂的波茲曼統治(Boltzmann tyranny)的限制，使得電晶體在室溫下至少需要60mV/decade的閘極電壓變化才能從關閉狀態切換至導通狀態。這導致了金氧半場效應電晶體的工作電壓無法進一步的縮小，進而限制了MOSFET功率消耗也無法加以縮小。為了減少或消除所謂的Boltzmann tyranny的限制，在電晶體的閘極串聯一個負電容是一個可行的方法。在理論及實驗上已經證明這個方法可以有效地將次臨界擺幅(Subthreshold Swing，或稱為次臨界斜率(subthreshold slope).)降低至60mV/decade以下。
通常負電容是利用鐵電材料來製作，這是因為鐵電材料具有極化電荷所造成的。部分的有機材料也具有鐵電特性，在本研究中我們採用聚偏二氟乙烯(polyvinylidene difluoride，簡稱為PVDF)為絕緣層之金屬-絕緣層-金屬(metal-insulator-metal, MIM)結構的負電容。
由於負電容並不容易以一般的量測儀器量得，故在本篇論文中，我們嘗試研究將正電容與負電容分別串聯在金氧半場效應電晶體的閘極來量測其次臨界斜率並互相比較，透過次臨界斜率的改善來確認負電容的存在。而本篇論文所使用的電晶體為市售的金氧半場效應電晶體，其編號為HCF4007UBE。依據我們實驗所得的結果，使用PVDF為絕緣層的負電容在串聯於金氧半場效應電晶體閘極後，確實可降低金氧半場效應電晶體的次臨界斜率。

關鍵字

有機鐵電材料 ； 波茲曼統治 ； 負電容 ； 次臨界斜率 ； 金氧半場效應電晶體

並列摘要

Conventional metal-oxide-semiconductor field-effect transistor (MOSFET) has been facing a fundamental limit called Boltzmann tyranny, which is a result of Boltzmann distribution of electrons at the source/channel interface, making at least a gate voltage change of 60 mV/decade at room temperature is required to switch the transistor from off state to on state. This limits the operating voltage of MOS FETs being scaled down further, and restraining the power dissipation in MOS FETs from becoming less. One possible way of eliminating the Boltzmann tyranny is to use a negative capacitance capacitor connected to the transistor gate in series. It has been proven both theoretically and experimentally that a serially connected negative capacitance capacitor to MOSFET transistor gate can effectively reduce the subthreshold swing to below 60 mV/decade.
Usually, negative capacitance capacitor can be fabricated by using a ferroelectric dielectric layer because of the existence of polarization charge inside the ferroelectric dielectric. It is noted that some of the organic materials are also ferroelectric in nature. In this wrok, we adopted an organic material polyvinylidene difluoride (PVDF) as the insulator layer of the negative capacitance capacitor with metal-insulator-metal (MIM) structure
Since negative capacitance is not easy to measure by using common measuring equipment, we try to investigate the negative capacitance effect by comparing the effect of connecting a capacitor with either normal capacitance or negative capacitance in series with the gate of a MOSFET and checking the subthreshold slope changes in this work.
Commercially available MOSFETs HCF4007UBE are purchased directly from vendors and are used in this work. From our experimental results, it is found that a negative capacitance with PVDF dielectric can truly reduce the subthreshold slope of the MOSFETs.

並列關鍵字

organic ferroelectric materials ； Boltzmann tyranny ； negative capacitance ； subthreshold swing ； metal-oxide-semiconductor field-effect transistor

參考文獻

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