五環素有機薄膜電晶體可靠度之研究

Abstract

摘要 在有機薄膜電晶體的技術演進中，其載子遷移率表現出來的電性已經迎頭趕上複晶矽薄膜電晶體的電性了。而其中的五環素為電洞導通的有機半導體，載子遷移率最高已高過 1cm2/V s，可說已到了單晶有機的傳輸極限了。不過儘管電性已有大幅改善，但是在接面電阻，導通機制和電性上的不穩定這三個方面還未作有系統的探討。 因而在本論文裡，首先我們會利用四點探測法來了解接面電阻。而四點探測是在上源極和汲極結構時，同時製作上去的， 而非額外多了兩根測電位的探針製作步驟。因此我們用原本兩個電極(源極和汲極)注入並接收電流，另外通道中的兩個電極來感測電壓。這手法跟最近報導的四點探針電晶體很像。而相較於傳統的R對L作圖法求接觸電阻，四點探測主要優點是可在單一顆元件上單獨量到薄膜電阻和各接觸電阻，並使得元件元件間在既存在的電阻差異下更容易對此元件作評估。因此我們可利用它來作進一步作現象分析。 接著我們作變溫量測，此可在遷移率與溫度有關的半導薄膜上作量測。在與溫度相關的遷移率中，蘊育著半導體和傳導機制有關或和缺限分布有關的資訊。有一些模型已提出來解釋這些半導晶體中熱活化的載子遷移率現象，然而“Multiple trapping and release model(MTR)”是最為被大家所廣泛接受的。因此我們再利用橫向電導，可同時來觀測Meyer-Neldel relationship的電性傳輸行為。而我們也適當的利用Multiple trapping and release model(MTR)來解釋變溫量測中過程的熱活化現象。 最後，我們在連續量測中了解到有機薄膜電晶體電性上的不穩定性，進而在定汲極電流的運作下，來探討這五環素有機薄膜電晶體的特性。在此我們也發現極化現象在定電流運作過程中扮演很重要的角色。因此提供簡單的能帶圖來陳述有機薄膜電晶體的特珠殊行為，而這些是載子受到束縳而在臨界電壓和遷移率上的不穩定現象也都一一會被探討。

Abstract

Recent progress in organic thin film transistor (OTFT) technology has led to charge carrier mobility comparable to amorphous silicon. Pentacene, a hole conductor, has shown the highest OTFT mobility(>1 cm2/V s) to date, and has nearly reached the intrinsic transport limit of organic single crystals. Despite the progress in realizing OTFTs with good properties, three important aspects of OTFTs have not been systematically studied: contact resistance, conduction mechanism, and electrical instability. In this thesis, at first, we use an improved four-probe method for ascertaining contact resistance. The four-probe method is based on the standard OTFT source and drain geometry, but with two additional mid-channel voltage sensing probes. Thus, we use two electrodes to inject and receive current, and two to sense voltage in the OTFT channel. The technique we employ is similar to other recently reported four-probe transistor devices. The major advantage of the four-probe method over conventional RvsL plots is that it allows the film and each contact resistance to be measured independently in a single device, which facilitates assessment of device-to-device variation in these resistances. Hence, we use it to further analyze the phenomenon. Variable temperature TFT measurements are adapted later. It can be used to determine the temperature dependence of the mobility in a semiconductor thin film. The temperature dependence of the mobility can yield information about the conduction mechanism and trap states in the semiconductor. Several models have been proposed to explain thermally activated mobility in crystalline organic semiconductor films, but the multiple trapping and release (MTR) model is the most widely accepted.. Thus, A Meyer–Neldel relationship was simultaneously observed for electrical transport ,using a transconductance. we properly use the multiple trapping and release model to explain the thermally activated phenomenon from the variable temperature measure.. Finally, we understand the instability of electrical characteristics in a continued series measurements and investigated the characteristics of bottom-contact pentacene-based OTFTs under drain current stress conditions. Here, we find that polarization phenomenon plays an important role during stress, and thus we provide the rough energy band figure to depict these special properties of OTFTs. The effect of these charge trapping instability on the measured threshold voltage and mobility of the transport studies would be discussed.