次臨界互補式金氧半類比積體電路之匹配誤差分析

Abstract

操作於次臨界區的金氧半場效電晶體(MOSFET)近來發展出嶄新的應 用, 譬如在低電壓, 低功率類比計算電路與系統上. 我們已知運算互導放 大器(operational transconductance amplifier)是類比電路製作中使用 非常多的建構區塊, 譬如記憶體中的感測放大器, 類神經網路系統等; 在 大型類比電路中使用運算互導放大器的優點有:(i)電路架構簡單, 由電流 源, 輸入差動對及電流鏡等常用的小電路所組成; (ii)電路可用標準的互 補金氧半製程製作. 其中限制次臨界金氧半類比積體電路之準確度的基本 因子之一. 就是在相同之形狀和偏壓下的電晶體元件之間的渠極電流不匹 配. 因為與製程變化量有指數相依的關係, 電晶體元件在次臨界區通常存 在著比過臨界區有更大的電流不匹配. 而非零之偏移電壓是運算互導放大 器可被量測出的非匹配誤差. 在本篇論文中我們要探討的是量測與分析操作於次臨界區運算互導放大器 的偏移電壓非匹配效應; 在此我們用統計的觀點來分析次臨界偏移電壓非 匹配效應. 我們經由解析地推導出一個次臨界運算互導放大器偏移電壓的 非匹配誤差模型, 可由製程變異量, 元件面積, 與偏壓所組成的函數來表 達; 與實驗量測數據結果比較, 用元件非匹配誤差模型來粹取出的製程參 數變異量即可以成功地預測出次臨界運算互導放大器偏移電壓誤差的不匹 配效應. The advantages of using OTA's as building blocks for larger subthresholdanalog circuits are that (i) the circuit structures are simple ; and (ii) thecircuits can be implemented by a standard CMOS process. One of the fundamentalfactors that limit the accuracy of the subthreshold MOS analog integratedcircuits is the mismatch in the drain current between identically drawn andbiased devices . Owing to exponential dependencies on the process variations ,however , devices in weak inversion usually exhibit a dramatically large mis-match in current as compared with that in above-threshold. The non- zero offsetvoltage is measured of the mismatch in OTA's. In this paper we extensively measure and analyze the offset voltage mis-match of the operational transconductance amplifier( OTA) circuits operating inthe subthreshold region . We treat the subthreshold offset voltage mismatch ofOTA from a statistical point of view . A statistical subthreshold OTA offsetvoltage mis-match model is derived analytically as function of process para-meter variations , area , and bias . Compared to the measured data, the devicemismatch model can be used to extract the variations in the two procee para-meters such as flat band voltage and body effect coefficients.The subthresholdOTA offset voltage mis-match has further been successfully predicted by meansof those extracted process parameter variations.