具輕摻雜汲極結構之複晶矽薄膜電晶體之通道延伸效應研究

Abstract

多晶矽薄膜電晶體在面板技術的應用上，由於具有高遷移率，有機會整合面板周邊電路，實現系統面板（System on Panel）的目標。在實際的應用上，多晶矽薄膜電晶體通常會使用輕摻雜汲極結構抑制漏電流。然而該結構在元件導通時，會有通道延伸的效應發生而影響元件特性。在本論文中，我們將分別針對多種影響通道延伸效應的因素進行相關的分析與探討。最後，提出一個方向用以設計能同時兼顧漏電與通道延伸效應的輕摻雜汲極結構。 首先，我們利用 Silvaco 元件模擬軟體，針對元件結構的影響進行模擬與比較，觀察到介於閘極及汲極的垂直電場大小決定了通道延伸效應的程度。其次，比較環境溫度所帶來的影響，得到溫度的提升有助於減少通道的延伸。接著，我們改變薄膜缺陷的態密度，針對輕摻雜汲極結構在不同摻雜濃度下的元件進行模擬與分析，發現到摻雜濃度與缺陷態密度對於通道延伸效應的影響存在相關性。整合上述的結果，我們對照前人提出的薄膜電阻模型與 Silvaco 的模擬結果，進而推論輕摻雜汲極的電阻值為造成通道延伸效應的另一主要原因。 最後，我們由上述的研究中瞭解，若要縮短通道延伸的長度，最簡單的方法即是增加輕摻雜汲極結構的摻雜濃度，但也因此會降低輕摻雜汲極結構抑制漏電的能力。所以藉由同時考量元件導通時的通道延伸長度與關閉時汲極端的橫向電場，進而得到能兼顧抑制漏電及通道延伸的最佳摻雜濃度。

Polycrystalline silicon thin film transistors (poly-Si TFTs) have been studied extensively for their application on system-on-panel (SOP) technology due to the high mobility. For actual applications, lightly-doped drain (LDD) structure is usually applied to poly-Si TFTs. When the TFTs turn on, the devices have channel extension effect to affect the electric characteristics. In this thesis, we will study on the influence factors on the LDD channel extension. And purpose a notion to design the optimal LDD which can get a balance between leakage current and LDD channel extension. In the beginning, we use Silvaco, the device simulation software, to simulate the influence of device structure. The result reveals that the vertical electric field between gate and drain determines the degree of channel extension. Second, to simulate the effect of temperature and get the relation which is that adding the temperature is helpful to reduce the channel extension effect. Then, change the density of states (DOS) of the thin film. The simulation focuses on the relation between DOS and different LDD doping concentrations. We find that the DOS and LDD doping concentration have specific effect on the channel extension. Base on previous results, we compare the thin film resistivity model with Silvaco simulation results. And get the conclusion which is the resistivity is other main reason causes the LDD channel extension. Finally, through above studies, we know the simplest method to reduce the extension length is to increase the doping concentration in LDD. However, this manner causes the ability of decreasing the leakage current at the same time. Hence, by the way of considering the extension length as the device is turning on and the parallel electric field as the device is turning off. We can get the optimal LDD doping concentration to get a balance between these two problems.