The current interest in transparent amorphous oxide semiconductor (AOS) thin film transistors was performed in operation of amorphous indium gallium-zinc oxide, a-IGZO, TFTs on flexible, room temperature and polymer substrate. AOS area has rapidly devel-oped, with a-IGZO TFT addressed active matrix displays. AOS are a new area of materials with electrical and optical characteristics uniquely suited to transparent, flexible and large area electronics. The a-IGZO TFTs have considerable attracted interest due to a room tem-perature processing, a visible transparency and a large area uniformity. These are ideal prop-erties for using them as active layers in TFTs which form the backbone of active matrix dis-play and large area electronic applications. Nevertheless, for these devices to be commercial-ly viable for flexible electronics, new device design concepts with an electrically analysis of device operation and reliability should be considered. In this dissertation, a-IGZO TFTs on various substrates were studied in the respect of material characteristics, the design of de-vice and its electrical performances. One of the key features of a-IGZO TFTs have a much higher field-effect mobility (μFE = >10 cm2/V·s) compared to a-Si:H TFTs, a low threshold voltage exhibiting in en-hancement mode operation, excellent switching properties, and a small parasitic series re-sistance employing IZO as source/drain electrode without the additional contact doping process. Process optimization of a-IGZO TFT was performed in scaling channel length, temperature-dependence and oxygen dispersion on active surface. A bendable a-IGZO TFTs and inverter circuits was demonstrated on a thin glass sub-strate. From the bending tests on the TFTs, VTH was negatively shifted as an increase of the bending strain for the symmetric gate overlap sample, while the TFTs showed relatively sta-ble operation against mechanical strain for the asymmetric gate overlap sample. Owing to the high temperature thermal annealing process, the a-IGZO TFTs showed very good bias stress stability under prolonged positive and negative stress test. Therefore, transparent, flexible, and stable TFTs can be realized using the a-IGZO TFTs on the thin glass substrate which can open a new topic for flexible display applications. Finally, we demonstrated high performance and flexible a-IGZO TFTs with hole-array on polyimide substrate and investigated the variation in the electrical characteristics as a function of hole area and radius. The a-IGZO TFTs with hole-array device performance shows good electrical characteristics and the mechanical strain reduced remarkably in hole-array structure as compared with the TFT without hole-array. Electrical stability measure-ments of the flexible devices with hole-array structure under tensile and compressive me-chanical strain showed no appreciable change in the I-V characteristics during bending. The electrical characteristics under mechanical bending suggest that carrier transport was unaf-fected during mechanical strain. Testing under dc gate bias conditions, the electrical stabil-ity of the TFTs showed a positive VTH shift of 3.8 V after 3600 s without any change in subthreshold-swing (S.S.). The a-IGZO TFTs with hole array structure exhibits high on/off ratio of >106 and field effect mobility of >6 cm2/V·s even after high bending radius. The bending radius was set to 100 mm by considering minimum bending radius (tensile strain of 0.22 % perpendicular to the channel current flow). The a-IGZO TFTs with hole-array re-markably reduced more electrical failure than TFT without hole-array samples since discon-nected micro-cracks induced the release of mechanical strain. Thus, proposed hole-array structure of a-IGZO TFTs can give an important merit to use flexible devices. ⓒ 2017 DGIST
Table Of Contents
I. INTRODUCTION 1--
1.1 Background and Motivation 1--
1.2 Flexible Electronics 6--
1.3 Organization of Dissertation 9--
II. PROCESS OPTIMIZATION AND ELECTRICAL CHARACTERIZATION OF a-InGaZnO THIN FILM TRANSISTOR 11--
2.1 Introduction 11--
2.2 Scaling channel length effect of a-IGZO TFTs with a-IZO electrodes 12--