대면적, 고해상도 AMOLED 디스플레이를 위한 N-channel TFT를 이용한 화소회로 설계

Abstract

Active-matrix organic light-emitting diode (AMOLED) displays have been widely developed as ideal flat-panel displays due to their wide viewing angle, high brightness, fast response time, compactness, light weight, vivid color, and high contrast ratio. Despite these advantages, however, AMOLED displays still have problems with regard to large-display applications, such as in computer monitors and televisions. The main issue of large AMOLED displays is their backplane, for which hydrogenated amorphous silicon (a-Si:H) and low-temperature polycrystalline-silicon (LTPS) are currently being used. The a-Si:H TFT technology has the advantages of uniform electrical characteristics (e.g., mobility and threshold voltage), low-cost process, and large glass size because it does not require the ion-doping process and crystallization, but a-Si:H TFTs have critical drawbacks when used in the actual backplanes of AMOLED displays. As the field effect mobility of a-Si:H is very low, it is not suitable for driving large and high-resolution AMOLED displays. Moreover, a-Si:H TFTs have poor stability: their threshold voltages shift under constant current stress. On the other hand, LTPS TFTs have high field effect mobility and excellent stability, but crystallization is needed to convert a-Si:H to LTPS TFTs, and this process has a high cost. Moreover, as LTPS TFTs have grain boundaries, their threshold voltages are not uniform. This is overcome by using complex compensation pixels, which have a low yield and a high cost. Therefore, a-Si:H and LTPS backplanes are not suitable for large AMOLED displays. Recently, indium gallium zinc oxide (IGZO) TFTs were developed for use in the backplanes of AMOLED displays because they have various advantages compared to a-Si:H and LTPS TFTs. IGZO TFTs have higher field effect mobility than a-Si:H TFTs. As such, they have good driving capabilities, which can enable them to drive large displays. IGZO TFTs also have higher current on/off ratios as well as better stability under constant current stress than a-Si:H TFTs do. Moreover, they can be fabricated using a lower-temperature process and have less processing steps compared with LTPS TFTs. In this thesis, an AMOLED pixel circuit using IGZO TFTs is proposed to compensate for the threshold voltage variation of IGZO TFTs because such TFTs have threshold voltage nonuniformity with the measurement position in a glass substrate. Based on the simulation results, the maximum deviation of the emission current of the pixel circuit with a threshold voltage variation of ±0.5 V was determined to be less than 74.7 nA.