低溫多晶矽薄膜電晶體元件特性應用於光感測器之研究

Abstract

本論文探討低溫多晶矽薄膜電晶體元件特性應用於光感測器之研究。由於在薄膜電晶體的SPICE電路模型中，並沒有對應的光漏電流模型可供模擬；同時，傳統定義暗態電流與照光電流比例的光敏感性參數並不適合用作分析光漏電機制。因此，本論文首先提出以Unit-Lux-Current (ULC) 參數來分析元件的光效應行為。此參數反應的是單位光通量致光電流的能力並且與元件的暗態電流無關。光漏電流的成因是由於空間電荷區產生額外的載子所致。根據加諸於半導體的電場導致庫倫位障降低的Poole-Frenkel效應與聲子共振穿隧效應；在橫向空乏區與閘極-汲極重疊空乏區產生的光漏電流行為以及與活化能相關的溫度等效應，都有詳細於文中所提出的物理模型與能帶圖解釋探討。而後也提出此光漏電參數的相關場效應與溫度效應的方程式去進一步研究其光漏電機制。 接著本論文也探討光效應與直流偏壓所額外產生不均勻缺陷的關係。由於矽材料能帶結構是屬於非直接能隙，根據動量守恆原理，矽材料吸收光所產生過多的電子-電洞對無法在價電帶與傳導帶直接複合，而是傾向透過能隙間的缺陷態作複合的動作。我們研究直流偏壓狀態下多晶矽薄膜電晶體主要的兩大劣化機制，熱載子效應與自發熱效應，這樣的劣化所導致的光漏電流亦會嚴重影響我們在設計感測電路時的困難度。當光照在經過熱載子劣化後的元件，在空乏區域裡的多數電子-電洞對從這些因熱載子劣化而額外存在的淺態能階(Tail state)中產生。而當元件經過自發熱劣化後，矽薄膜層因高溫釋放氫造成斷鍵而形成了深態能階(Deep state)，而又因為此能階分佈在能隙的中間處，對矽材料來說，它同時也提供了照光產生電子-電洞對最有效率的複合中心。在本論文中我們利用這兩種劣化情況去探討因缺陷所導致的相關光漏電行為，同時也修改缺陷非勻狀態的薄膜電晶體所對應的光漏電方程式。這些經驗方程式的建立將有助於模型化與模擬低溫多晶係薄膜電晶體電路操作在直流偏壓劣化後的光效應行為分析。 接下來由於低溫多晶矽薄膜電晶體是上閘極結構，操作時它同樣會受到來自高強度環境背光源的影響而導致光漏電流的發生。基本上能夠採用正面照光的方式分析背面照光的光漏電流。然而不同的是，當光的來源為背光時，會有大量的電子-電洞對產生在元件多晶矽薄膜層的底部。因此將會有殘留過量的電洞累積因而形成正電位通道。在本文中我們同樣也探討背光致光漏電流的各式偏壓效應、溫度以及缺陷密度等影響因子。同時，我們利用缺陷在能階的分佈與背光導致電流關係的新觀點進一步去確認先前研究所未見之自發熱效應淺態能階的存在與位置，進而建立元件完整且正確的劣化模型。 本論文最後也提出了低溫多晶矽薄膜電晶體自我閘極遮蔽的三維嵌入光偵測的概念。這個系統利用了光源相對閘極汲極側與閘極源極側的正反向量測來建立感測方向，因而提供元件在照光下感測的不匹配特性。由於此概念並未改變元件製作流程，是可以期待整合至面板的偵測系統的。接者提出利用低溫多晶矽薄膜電晶體設計的一個源極隨耦態的電路去做環境光的感測。同時各種可能產生的元件變動性被考慮並利用統計性與補償電路的方式校正，此系統同樣也有著能整合至面板偵測系統的潛力。

This work focuses on the characterization of low-temperature poly-silicon thin film transistor for sensor application. Due to in the SPICE models of TFT, there is no photo leakage current model for simulation. Meanwhile, photosensitivity (RL/D) defined as the ratio of the current under illumination to the current in the dark is not proper to be used to analyze photo leakage mechanism. Thus, a new parameter, Unit-Lux-Current (ULC) is firstly used to analyze the effects of illumination on LTPS TFTs. It reflects the ability of photo leakage current induced per unit-photo flux and independent of the dark current. The amount of the photo current should be associated with the carrier generation in the space charge region. Based on the Poole-Frenkel effect lowering of a Coulombic barrier and phonon-assisted tunneling due to the electric field applied to a semiconductor, the ULC can be taken into account both the leakage current induced in the lateral depletion and in the gate-drain overlap depletion regions. We further take into account the temperature effect of ULC. The temperature effect can be identified by activation energy. An equation is provided to properly describe model for mechanism ULC behavior under various bias and temperature conditions for further exploration of photo leakage mechanism. Then we focus on the relation between photosensitivity and additional non-uniform defects. The energy-band structure of Si material is indirect band-gap. The excess electron-hole pairs induced by the absorption of light would not be recombined from band to band directly due to the momentum conservation principle. Thus silicon is more likely to recombine through localized traps. We investigate the two main degradation mechanisms for the poly-Si TFTs under DC operation, namely the hot carrier effect and the self heating effect. Due to such degradation, the photo-induced leakage current is strongly influenced which is difficultly designed for sensing circuits. When the LTPS TFT devices after hot carrier stress are under optical illumination, the numerous electron-hole pairs from additionally created shallow tail states are generated in the lateral depletion region. Then devices after self heating stress, the high temperature in the poly-Si film can release hydrogen and cause plenty of dangling bonds as deep states. These deep states near mid level, in the lateral depletion region, can recombine the electron-hole pairs generated by irradiation. In this work, we apply both stress conditions deliberately to manipulate the defect-related photo behaviors and modify ULC equations in TFTs. The empirical equation of ULC provides a potential modeling for simulation of LTPS TFT circuitry considering the photo effect after DC stress. Next, because LTPS TFTs are top gate structure, it is suffer from undesirable photo leakage current under a high back illumination environment. We can analyze such photo-induced current like as front light case. However, when the light is emitted from back-light, plenty of electron-hole pairs are generated in the bottom of poly-Si film. Therefore, the residual excess holes are accumulated in the poly-Si film to form the floating body with a positive channel potential. We also present detail studies on the factors that affect the photo leakage current like bias condition, temperature, and defect states of the LTPS TFTs. Meanwhile, we provide new insight which use energy level of trap defect behaviors connected with photo induced current to further make sure the existence of tail state after self heating degradation. Furthermore, a more accurate model after self-heating degradation is proposed. A three dimensional embedded optical sensor employs low temperature poly-silicon thin film transistor which used gate metal shielding by itself characteristics was proposed. The system connect with forward and reverse measurements can be used to set up sensing direction. It provides sensing disparity characteristics of adopted devices under illumination. It’s expected the integration of sensing system onto the panel without extra components sensors and extra change in the fabrication process. Then a circuit of source follower type based on the LTPS TFTs which can sense the illumination condition is proposed to be used as an ambient light sensor. Some kinds of variation effect can be calibrated by statistical and compensation circuit methods. This approach would provide the possibility for the light sensor array integrated in the pixels with the same device of LTPS TFTs.