新穎浮動副閘極結構之複晶矽薄膜電晶體

Abstract

複晶矽薄膜電晶體已經廣泛的使用在液晶顯示器中的畫素控制元件，而將驅動電路整合在同一玻璃基板上是未來的趨勢。對於驅動電路工作來說，不同於畫素控制元件，它必須有高的驅動電流和低的關閉態漏電流。已經有很多種提升驅動電流技術提出，但如此一來關閉態漏電流也相對的大增。所以本篇論文提出了一種具有浮動副閘極新穎的薄膜電晶體結構。當元件工作在關閉態時，就會像偏移結構的元件一樣的關閉態特性，而當元件工作在開啟態時，會和傳統元件一樣的開啟態特性。新結構的開啟態漏電流會比傳統元件的漏電流低上兩個數量級以上，而其開啟態電流會比偏移結構元件高一個數量級以上。因為此結構可以降低在關閉態時汲極附近的電場，而在開啟態時可經由浮動閘極的耦合不使電場降低太多。總和來說，在元件的開關電流比可以增加兩個數量級以上，且在製作此新結構元件不需要增加多餘的光罩，和傳統結構的四道光罩相同。

Polysilicon thin film transistor (poly-Si TFT) has been used widely as the pixel switching devices in Active-Matrix Liquid Crystal Displays (AMLCD). It’s said that system on panel (SOP) that integrates all driving circuits on the glass is the tendency in the future. For circuit operation, distinct switching device, it must have low leakage current and high on state current. Various techniques have been reported to raise the driving current but the leakage current also increases at the same time. So we propose a novel poly-Si TFT with a floating subgate structure in this thesis. The device operates as an offset gated structure in the OFF state, while acting as a conventional non-offset structure in the ON state. The OFF state leakage current of the new TFT is two orders of magnitude lower than that of the conventional non-offset TFT, and the ON current of the new TFT is one order of magnitude higher than that of the offset TFT. With these novel TFTs structures, we can effectively reduce off-state leakage current resulting mainly from a higher electric field near the drain and the turn-on current will not be degraded. The ON / OFF current ratio of the new TFT is greatly improved by two orders of magnitude. Beside, no additional photo-masking steps are required to fabricate the subgate of the new TFT and its fabrication process is fully the same as the conventional non-offset TFTs.