五環素有機薄膜電晶體之可靠度分析

Abstract

有機薄膜電晶體，因其具有低溫製程、低成本和製程簡單的優勢，所以有很好的潛力應用在如可撓曲式面板、感測器和RFID及其他的電子元件。近來的文獻及研究著重在DC gate bias stress without drain bias，而對於電子元件實際操作上， drain bias 及操作AC signal的研究比較缺乏。因此此論文將對於DC gate bias with drain bias stress 及AC bias stress做深入的研究。 □□漸□A在DC gate bias stress without drain bias部分，□Vth 隨著時間的關係，符合Stretched-exponential equation，與現有的文獻一樣的結果。而當Drain bias施加後，□Vth 會隨VD增加而減小，我們分析得到當VD增加時，通道內載子濃度的減少，由於載子濃度與□Vth 成正比的關係，造成□Vth 的變小。因此我們對載子濃度歸ㄧ化後可得DC gate bias stress with drain bias中□Vth 隨時間的關係亦符合Stretched-exponential equation。 在AC stress部分，在同樣的duty ratio下□Vth 會隨著頻率越高□Vth 越小，我們分析發現pentacene本身的電容與電阻以及與金屬間的接觸電阻所等效的RC電路，造成通道內載子濃度因RC delay time隨著pulse width的變小而減少，使得□Vth 變小。同DC bias stress的部份，亦可對載子濃度歸ㄧ化後可得在AC bias stress中□Vth 隨時間的關係亦符合Stretched-exponential equation。

As organic thin film transistor has lots of advantage of low temperature fabrication、low cost and easy fabrication, it has good potential for application on flexible display、sensor、radio-frequency identification tags and other electronics. Although studies have been made on DC gate bias stress without drain bias, the drain bias and AC bias stress in actual operation of organic electronic device is unknown. So we focus on the study of DC gate bias with drain bias stress and AC bias stress in this thesis. First, in the DC gate bias stress without drain bias, the dependence of the threshold voltage shift on the stressing time is found to follow a stretched-exponential function. It is the same as the many recent researchs. When the drain bias is applied, the threshold voltage shift will decrease and be suppressed as the drain bias becomes more negative. Since the threshold voltage shift is proportional to the carrier density, as the drain bias becomes more negative, the carrier concentration in channel is decrease such that the threshold voltage shift is reduced. Therefore, a channel charge normalization factor was used to describe the influence of drain bias and to modify the conventional stretched-exponential function. The threshold voltage shift has strong frequency dependence-the higher frequency same duty ratio, the smaller the magnitude of threshold voltage shift. We used an simple RC equivalent circuit to simulate the device under negative pulsed bias stress. The RC circuit consists of the insulator capacitance Ci, pentacene capacitance Cs, and a effective pentacene resistance Rs for hole conduction and injection. Th effective channel carrier concentration considering the RC delay can successfully explain the dependence of threshold voltage shift on the pulse width of the stressing signal. The channel charge normalization also can be used as the DC bias effect modeto describe the influence of AC bias stress and to modify the conventional stretched-exponential function.