偏壓對有機薄膜電晶體氣體感測器之影響以及相關機制

Abstract

近年來有機薄膜電晶體氣體感應器已經引起大量的研究，此成果之應用範圍包含食物品質檢測以及醫療用自我檢測儀器。為了達成能商業量產的價值，我們需要製造出成本低廉且高度靈敏的感測元件，為了達成這個目的，我們選用有機電子材料來製造氣體感測器。 現有的相關研究多著重在氣體感測器電流值的變化，對於其他參數如臨界電壓以及載子移動率卻缺乏討論。我們希望站在元件物理的基礎上更深入的探討氣體反應的相關機制。 在氣體感應的量測中我們觀察到造成電流值變化的主因是因為臨界電壓的漂移。隨著氨濃度的增加，臨界電壓值的移動也會更劇烈，但當氨氣移除之後臨界電壓有恢復到初始狀態的趨勢。故我們認為氨氣與五苯環分子的作用為一種吸附現象。在這實驗中我們發現一個很有趣的現象:當三顆特性相同的有機元件放在同一個有氨的環境中，不論放置在這環境中多長時間所量測到的初始臨界電壓都相近，這結果暗示我們氣體感測的行為與閘極偏壓有關。 在長時施加閘極偏壓的氣體感測中，固定施加的偏壓改變氣體濃度，當氨氣濃度越高臨界電壓變動也增加。若固定氣體濃度施加更高偏壓也會看到相同的情形。不論是在有無偏壓實驗條件下，氣體擴散模型可以成功解釋氨在五苯環薄膜中的擴散行為。

Electronic noses have attracted lots of interests in the recent years. It can be used from food-quality monitoring to medical self-diagnosis kits. For commercial purpose, we need to produce low-cost and high sensitivity electronic noses. To reach these targets, we used organic material to fabricate electronic nose. Most of the previous works focused on the electric current effect alone. In this work we also studied the effect on threshold voltage 、 mobility, from which we investigated the mechanism for gas sensing. In this experiment, we observed that the electric current variation was strongly influenced by the threshold voltage. When the ammonia concentration was increased, the threshold voltage became larger. But we noted that the threshold voltage tend to recover when the ammonia was removed. For this reason, we consider that the reaction between ammonia and pentacene is basically an absorption reaction. At the same time, we observed an interesting phenomenon: we measured three identical devices that were placed at the same distance from the gas source. When the measurement was applied on the three devices one after another along with the NH3 sensing time, we observed an interesting event- these three devices had similar initial threshold voltages no matter how long these devices had been exposed to the NH3 gas. This result implied that the sensing behavior may have a strong correlation with the gate bias. In the experiment of bias stress gas sensing, we applied a fixed gate bias and varied the ammonia concentration. The threshold voltage became larger with the ammonia concentration was increased. We observed the same result when we applied higher gate bias and added a fixed amount of ammonia. Our results showed that Fick’s second law can successfully explain the variations of threshold voltage.