| Summary: | 碩士 === 國立交通大學 === 光電工程學系 === 98 === a-IGZO is the high-potential material for optoelectronic application, display specially. Therefore, the photo-response to visible light of a-IGZO transistor must be understood. In this study, we discuss the photo-response of a-IGZO TFT under illumination with various wavelengths and find out that a-IGZO TFT is strongly wavelength and operation mode dependent. This study could provide a useful direction for future system design. In order to reform the transparent a-IGZO thin film transistor to become a visible light photo-sensor with adequate sensitivity, we introduce a narrow bandgap polymer semiconductor, P3HT, capping onto the active layer of bottom-gate a-IGZO TFT to form a photo-transistor. The large photocurrent of P3HT-capped a-IGZO photo-transistor may be caused by the light-induced threshold voltage shift. By a series of experiments made in different operation modes under illumination, the reasonable mechanism of light-induced threshold voltage shift is proposed. The excitons are generated in P3HT by illumination and then are dissociated by the build-in
electric field at P3HT/IGZO junction. The electrons dissociated from excitons drift into IGZO and then be trapped or accumulate at the back channel in IGZO TFT during illumination.
Furthermore, in this study, it was found that the threshold voltage position is changed by introducing the capping layer. It is speculated that electric dipoles are formed during the process of fermi-level equilibration while the junction between capping layer and IGZO form. We demonstrate that the threshold voltage position of a-IGZO TFT could be effectively adjusted by capping layer with various fermi-levels. By the electric dipoles with various magnitudes and different polarities forming between IGZO back channel and capping layer with various fermi-levels, the body voltage could be adjusted (body effect) and then affect the device threshold voltage. In this study, we propose a novel structure with capping metal layer onto the
active layer of bottom-gate a-IGZO TFT to provide a powerful solution of enhancement of device performance and threshold voltage modulation that would not cause current leakage and performance degradation. In addition, the device mobility increases significantly after introducing the metal capping layer. In summary, capping metal layer seems a simple and effective approach to fabricate a feasible metal oxide transistor.
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