應用於奈米線場效電晶體感測器之感測電路設計

Abstract

矽奈米線場效電路電晶體(SNWFET)為一種生物感測器，針對待測物濃度的不同產生不同的電流，由於其電流變化通常在奈米等級左右，用來測量的儀器具有昂貴不易攜帶的缺點，因此本篇論文提出了兩種放大器介面電路設計，將奈米線場效電晶體感測器在不同濃度或酸鹼值的環境下，所改變的電流變化轉換成適當的電壓值，藉此回推此時環境濃度或酸鹼值的變化，並且結合人機介面，替代昂貴的電流量測儀器。

本論文根據楊裕雄教授設計的SNWFET感測器的特性，設計放大電路。第一種設計為單端(Single End)放大器，是利用轉阻放大器(transimpedance amplifier)直接將輸入電流放大，依照所獲得的絕對電壓值反推待測物濃度。並使用Auto-zeroing放大器、迴授電容、以及二階濾波器來降低元件訊號飄移(offset)及雜訊影響。

由於製作過程、環境因素很大的影響SNWFET感測器的性能，因此量測反應前後的相對電壓/電流變化值會比量測絕對電壓/電流更為重要，為此本論文提出第二種迴授補償(Differential mode)的感測電路設計，利用增加一個參考的SNWFET感測器來降低環境因素的影響，進而獲得待測物所造成的電壓差值。在此電路設計中，我們同時藉由迴授控制設計、切換電容設計，來降低兩個SNWFET感測器彼此間的不對稱、放大器雜訊飄移等所造成的誤差，並藉由迴授電路的穩定性分析，獲得電路參數設計的最佳值。

本研究最後整合感測電路、Arduino(單晶片微電腦)與LabVIEW(程式語言)來完成此介面電路設計。使用Arduino來控制電路內部元件包括：數位電阻、數位類比轉換器、多工器等晶片，使感測電路可連結數個SNWFET、並可根據測量的濃度來調整放大器增益值，最後使用LabVIEW進行人機介面設計，透過電腦端輸入控制參數並顯示量測之電壓/電流/濃度值。所完成的單端感測電路可測量到的電流變化為2.5pA~2.5mA，頻寬10Hz，對應楊教授設計的SNWFET感測器性能，則此感測電路可偵測的濃度範圍約是0.1fM~10pM，如果以最小可量測的電流2.5pA計算，則理想的單端放大器感測電路解析度為0.108fM，但如果考慮誤差電流44.8pA的情形下，則感測電路的解析度為1.9fM；而在第二個迴授補償放大感測電路中，頻寬24Hz，對應楊教授設計的SNWFET感測器性能，則此感測電路可偵測的濃度範圍約是0.1fM~10pM，如果以最小可量測的電壓4.8mV計算，則理想的迴授補償放大感測電路解析度為0.229fM，但如果考慮誤差電壓0.04V的情形下，則感測電路的解析度為1.9fM。

Silicon Nanowire field-effect-transistors (SNWFET) are biosensors which produce different currents when immersed to different concentration of solution samples. Since the current output of SNWFETs normally varies from femto amperes to micro amperes, they are connected to high-end apparatus, which can detect tiny current variation in a large range, in practical sensor applications. Unfortunately, these high-end apparatus are often expensive and difficult to carry, which may limit the promotion of SNWFET sensors. The aim of this research is to design suitable interface circuits to replace the high-end apparatus in SNWFET sensor applications.

In this research, we proposed two circuit designs based on the characteristics of SNWFETs designed/ fabricated by professor Yang’s research group [1]. The first design is referred to as “single-end amplifier” in this report, which consists of two-stage transimpedance amplifier, auto-zeroing amplifier, feedback capacitor, low pass filters, multiplexer, digital potentiometer, and etc. It can measure 16 SNWFETs simultaneously and convert pico ampere currents into voltage for subsequent signal processing.

Because the performance of SNWFETs can be greatly affected by the fabrication process and testing environment, the relative value of the sensor measurement taken before and after the chemical reactions is more important than the absolute value of the sensor measurement taken after the chemical reactions. Thus, this research proposed the second design which is referred to as the "differential-mode” interface circuits in this report. The differential-mode sensing circuit employs an additional reference SNWFET device to form a differential mode measurement. Besides, both the feedback control and switching capacitor techniques are employed to compensate the mismatch between two SNWFET and the noise/drift of the circuit components.