低溫複晶矽薄膜電晶體元件均勻性及類比緩衝電路設計之研究

Abstract

在本篇論文中，我們以低溫複晶矽薄膜電晶體為基礎，從元件結構以及類比緩衝電路設計兩方面來探討元件均勻性之問題。在此研究中，我們將藉由簡單地調整元件結構來提升低溫複晶矽薄膜電晶體之均勻性，並且提出一個對元件變異性具有補償能力之新式源極隨耦器形式的類比緩衝電路。 首先，我們針對元件結構之觀點來探討低溫複晶矽薄膜電晶體之元件均勻性。希望在不需要額外之光罩也無須改變傳統製程之前提下，藉由元件結構上之簡單調整來提升元件均勻性。在此研究之中，我們採取了多通道之元件架構。根據實驗之結果，我們發現多通道結構可以提升低溫複晶矽薄膜電晶體之截止電壓(threshold voltage)及次臨界擺幅(subthreshold swing)之均勻性，然而對於載子移動率(mobility)及漏電流(leakage)之變異性並無改善。針對此結果，我們也進一步進行機制之探討，認為機率是造成多通道結構均勻性提升最主要之原因。此外，我們將多通道結構引入傳統源極隨耦器形式的類比緩衝電路之驅動電晶體中，藉由觀察類比緩衝電路之輸出特性來驗證多通道結構對薄膜電晶體均勻性提升之能力。由量測結果可以看到具有多通道結構之源極隨耦器形式的類比緩衝電路，其輸出電壓之變異性大約只有傳統源極隨耦器形式之類比緩衝電路之ㄧ半，證實多通道結構的確對元件之非均勻性具有改善作用。 在類比緩衝電路設計之研究中，我們將設計一個新式源極隨耦器形式之類比緩衝電路以消除因元件變異性造成之電路輸出電壓變異的問題。我們先從不同形式之類比緩衝進行比較、評估。雖然運算放大器形式之緩衝電路被廣泛應用於傳統單晶矽之類比積體電路，但礙於複雜之電路架構，較嚴重之輸出電壓之變異性以及較高的功率消耗，以低溫複晶矽薄膜電晶體為基礎之運算放大器形式的緩衝電路並不適合於系統面板(system-on-panel)之應用。相較之下，源極隨耦器形式之類比緩衝電路具有電路架構簡單，對元件變異性有較高之免疫力，以及功率消耗較低等優點，對未來系統面板之應用提供了較佳的選擇。 在設計源極隨耦器形式之類比緩衝電路方面，首先我們針對傳統架構之源極隨耦器進行電路模擬之探討。根據模擬的結果，我們發現傳統的源極隨耦器具有輸出不飽和以及嚴重的輸出變異之問題。因此我們提出了一個新式的源極隨耦器形式之類比緩衝電路來解決這些問題。我們所提出的補償電路由兩個N型薄膜電晶體，一個儲存電容及四各開關所組成。在此類比緩衝電路之中，我們藉由加入一個主動負載來抑制傳統源極隨耦器中，因為低溫複晶矽薄膜電晶體較大的次臨界電流所引起輸出不飽和的現象。並且藉由儲存電容以及適當的驅動過程來進行元件變異性之補償。經由模擬以及實際量測結果證實，我們所提出的電路架構確實可以有效地補償元件變異性之問題。 此外，在設計類比緩衝電路的過程之中，我們也發現主動負載之閘極偏壓對電路輸出偏差具有顯著的影響。因此在此部份之研究中，我們也對主動負載之偏壓效應做了詳盡的探討。從偏壓效應的探討之中可以得知，我們所提出來的源極隨耦器形式之類比緩衝電路，可藉由適當的偏壓設計達到具良好輸出特性以及低功率消耗之類比緩衝電路。

In this thesis, based on the low-temperature polycrystalline silicon thin film transistors (LTPS TFTs), the device uniformity issues of LTPS TFTs were investigated from the aspects of device structure and buffer circuits design. Simply modified device architecture is used to improve the uniformity of LTPS TFTs and a new source-follower type analog buffer with the capability of device variations compensating is proposed in this work. First, the device uniformity of LTPS TFTs is studied from the view point of device architecture. We purpose to improve the device non-uniformity of LTPS-TFTs by means of simply modifying the device architecture but without the use of additional masks and have no need to modify the standard process of TFT fabrication. The multi-channel structure with slicing layout method is used in this work. According to the experimental results, it is observed that the multi-channel structure can improve the non-uniformity of threshold voltage and subthreshold swing of LTPS TFTs, while has no effect on the transconductance and leakage current. The mechanism of the improving uniformity of multi-channel structure is discussed and it is considered that probability effect is the most possible cause. In this part of research, in order to verity the capability of improving uniformity of multi-channel structure, the output variations of the conventional source follower with multi-channel structure are also studied. It can be seen that the output variations are reduced to about half of that of conventional source follower. It is evident that the device non-uniformity indeed can be improved by multi-channel structure. In the study of analog buffer circuits design, we intend to develop a new analog buffer with compensating configuration to eliminate the output variations resulting from device non-uniformity. First, differential types of analog buffers were compared and evaluated. Although op-amp type is most commonly used as the output buffer in single crystal silicon integrated circuits, the complicated circuit configuration, huge output voltage variation, and high power consumption of op-amp-type analog buffer using LTPS TFT make it not suitable for system-on-panel application. Source-follower-type analog buffer is considered a best candidate because of its simplicity, higher immunity to the device variations of LTPS TFTs and low power consumption. For designing the analog buffer with source-follower configuration, the output characteristics of conventional source follower is first studied by HSPICE circuit simulator. It is observed that output unsaturated phenomenon and severe output variation exist in the conventional source follower. In order to enhance the output performance of analog buffers, a new source-follower type analog buffer for is proposed to solve the problems of output unsaturated and variations. The proposed analog buffer is composed of two n-type thin film transistors, one storage capacitor and four switches. The active load is employed to suppress the unsaturated phenomenon of output voltage arisen from the significant subthreshold current of driving TFTs. The device variations compensating is performed by the storage capacitor and compensated operation of the proposed buffer. According to the simulated and measured results, it confirms that the output variation resulting from the variation of poly-Si TFT characteristics is successfully compensated in the proposed analog buffer. Furthermore, it is observed the bias voltage of the active load has significant influence on the output deviation of the proposed analog buffer. The bias effect is also discussed in this study. It is concluded that an optimum value of the bias voltage can be designed to achieve high output performance and keep low power dissipation of the proposed analog buffer.