Amorphous In-Ga-Zn-O Thin Film Transistors for Active-Matrix Organic Light-Emitting Displays.

Abstract

Active-matrix organic light-emitting display (AMOLED) is now generally viewed as the next generation display because of its vivid color, high contrast ratio, thin/light module, and low energy consumption. So far, most reported pixel circuits are either based on low temperature polysilicon (LTPS) thin film transistors (TFTs) or hydrogenated amorphous silicon (a-Si:H) TFTs. Both backplane technologies have their own shortcomings, such as nonuniformity of LTPS TFTs, low field-effect mobility and threshold voltage instability of a-Si:H TFTs. As a result, TFTs based on other semiconductor materials have been explored as an alternative approach to realize reliable, high resolution AMOLEDs. Among all, amorphous In-Ga-Zn-O (a- IGZO) TFTs possess certain advantages including visible transparency, low processing temperature, uniformity over large area, and good electrical performance, which make them very attractive for AMOLEDs. The focus of this work has been to provide a more thorough understanding of the device performance of a-IGZO TFTs, along with the underlying semiconductor physics and their possible application to AMOLEDs. Firstly, the electronic structure of crystalline In-Ga-Zn-O was studied by ab initio quantum mechanics calculation. Then the electrical properties of a-IGZO TFTs were described, including the gate voltage dependent field-effect mobility and source/drain contact resistance. The operation principles of a-IGZO TFTs were further investigated by the channel region surface potential profile obtained by scanning Kelvin probe microscopy. The effect of temperature on the electrical properties of a-IGZO TFTs was investigated. The thermally activated drain current was explored, and the density of deep states profile was calculated from measured data. Current temperature stress measurements were performed on a-IGZO TFTs. Several factors were considered when investigating the electrically stability of the devices, including the stress time, stress temperature, stress current, and TFT biasing conditions. Finally, a-IGZO TFT SPICE model was developed based on the RPI a-Si:H TFT model. Several voltage- and current-programmed AMOLED pixel circuits were simulated. The effect of threshold voltage variation on the pixel circuit performance was investigated, and the potential advantages of using a-IGZO TFTs were discussed.