本論文研究N型有機小分子材料(DFH-4T)成長在介電層(PMMA)的有機薄膜電晶體，並探討在不同薄膜厚度下的表面形貌及晶體結構變化對電晶體的電性表現之影響。由觀察電晶體電性表現結果，發現當DFH-4T膜厚由數奈米增加至數十奈米，載子遷移率會提升近兩個數量級，其中當薄膜成長至一特定厚度(約20 奈米)以上，電子遷移率便開始出現飽和的現象。進一步由AFM與XRD分析發現，DFH-4T薄膜厚度由數奈米至數十奈米會形成截然不同的表面形貌，但晶體結構並無明顯的差異，因此推論靠近介電層之表面形貌為主導電晶體電性之因素。為了進一步了解DFH-4T表面形貌對電晶體電性影響的微觀機制，本論文也進行低溫量測實驗，並利用mobility edge model理論分析薄膜的電子能態密度，建立不同表面形貌對電子缺陷態分佈的關聯性。本論文提供了完整的DFH-4T導電機制的研究，期望未來可利用此材料特性設計出更多元的有機元件。;In this thesis, the organic thin film transistors were fabricated with the N-type organic semiconductor (DFH-4T) deposited on the PMMA dielectric, and the effects of film morphology and microstructure of various thickness on the performance of transistors were investigated. From the results of transistor characteristics, we found that the electron mobility increases by two order of magnitude as the film thickness increases from few nanometers to few tenth of nanometers, but the mobility starts to saturate when the film is thicker than 20 nm. Further investigations from AFM and XRD analyses showed that as the thickness increases from few nanometers to few tenth of nanometers shows different surface morphology, but similar crystalline structure. Therefore, we speculate that the surface morphology close to the PMMA dielectric is the key factor for transistor performance. To further understand the microscopic mechanisms of DFH-4T surface morphology that affecting the transistor characteristics. We performed the low-temperature measurement and analyzed with mobility edge model, and we established the relationship between morphology and distribution of trap state. This thesis provides the comprehensive DFH-4T transport mechanism, which could applied to design other the organic device in the future.