In this thesis, novel technology for manufacturing high-performance hydrogenated amorphous silicon (a-Si:H) TFT is developed . In the bottom gate light-shied a-Si:H TFT structure, the side edge of a-Si:H island is capped with extra deposition of heavily phosphorous-doped a-Si layer. The new structure a-Si:H TFT process steps is almost unchanged. The masksteops of fabrication new structure TFT are the same as the inverter-staggered TFT. Such an ingenuity can effectively eliminate the leakage path between the parasitic contacts between source/drain metal and a-Si:H at the edge of a-Si:H island. a-Si:H is a well-known photosensitivity material. For driving LCD the TFT must be operated with illuminated environment. It will cause the leakage current. The new TFT structure is similar to the light-shield TFT proposed by Akiyama in 1989. So the new structure TFT can not only reduce the schoktty emission leakage current but also the photo-leakage current. In addition, electrical performance of the novel a-Si:H TFT device exhibits superior effective carrier mobility, as high as 1.05 cm2/Vsec due to the enormous improvement in parasitic resistance. The impressively high performance provides the potential of our proposed a-Si:H TFT to apply for AMLCD and AMOLED technology.

In recently yesrs,Thin-film transistors (TFTs) including an active layer of amorphous silicon or polycrystalline silicon have been widely employed as the pixel-driving elements of a liquid crystal display (LCD). Particularly, a-Si:H TFT is advantageous to the production of large screen displays and facilitates mass-production. a-Si:H has high photoconductivity which results in high off-state leakage currents of a-Si:H TFT under light illumination . Particularly, the off-state leakage current under light illumination is a serious problem in the projection and/or video displays which require high intensity backlight illumination.As the resolutions is higher , the TFT¡¦s performance must be higher to achieve the short charge time each line can charge. The performance includes mobility ,on current, off current, photo leakage current, threshold voltage ,and subthrehold swing. Furthermore, the to improve the mobility of thin-film transistors (TFT) to enable total integration of peripheral electronics in flat panel displays and imagers has led to recrystallized polycrystalline silicon (poly-Si) as the material of choice. However, laser recrystallized polycrystalline silicon suffers from high cost , complex processing, and significant nonuniformity over a large area. Indeed, the direct deposition of good-quality low-temperature poly films is highly desirable and constitutes a promising alternative. In this thesis, we use HDPCVD to fabricate direct deposition poly-TFT successfully.Through plasma passivation, we improve the characteristic of device. The photo-Leakage current have been reduced obviously to our device under light illumination, and is benefit to higher intensity light of large screen display. And our TFT device exhibits stable characteristics with voltage and current stress , and it¡¦s also confirmed that the device is reliable. On the characteristic of device, the direct-deposited poly TFT device exhibits higher effective carrier mobility than that of conventional one. For that reason, the high performance provides the potential of the direct-deposited poly TFT to apply for AMLCD and AMOLED technology.

The hydrogenated amorphous silicon thin-film transistors (a-Si:H TFTs) have been widely used as switching device for large-area electronics such as active matrix liquid crystal displays (AM-LCDs). a-Si TFT is particularly advantageous to the production of large screen displays and facilitates mass production. When employing an a-Si:H layer, the main objectives are to enhance the field effect mobility and to reduce the off-state current under light illumination. The increase of field effect mobility results in wide application of a-Si:H TFTs in high resolution LCDs. On the other hand, a-Si:H has high photoconductivity which results in high off-state current of a-Si:H TFT under light illumination. The off-state leakage current under light illumination is, in particular, a serious problem in the projection and/or multimedia displays that require high intensity backlight illumination. Minimizing the off-current increase by a-Si photosensitivity is an important design consideration for achieving highimage-quality LCDs. TFT off-current increase by photoillumination of a-Si decreases the charge stored on the pixel during the TFT off-time, and results in gray-scale shading, flicker, crosstalk and other display nonuniformity in the LCD. The fluorine incorporated amorphous silicon [a-Si:H(:F)] and amorphous silicon (a-Si:H) were illuminated with backlight to investigate electrical characteristics. The effect of different [SiF4] / [ SiH4] ratio on the performance of a-Si:H(:F) TFTs was also studied. We found the density of states in the gap of a-Si:H(:F) will be modified by the introduction of F into a-Si:H and resulting the shift of the Fermi level toward the valence band edge. The density-of-states increasing cause more recombination centers for electrons and holes to increase the carrier recombination rate. The shift in the Fermi level leads to a reduction of the photoconductivity of a-Si:H(:F). Due to these two important factor, the photo leakage current decreases.

The off-state leakage current under back light illumination is, in particular, a serious problem in the multimedia displays that require high intensity backlight illumination. The photo leakage current characteristic of flexible a-Si:H TFTs has been measured in this study . The device activation energy (Ea) of a-Si:H TFTs extracted from various temperature measurements are different from those of typical a-Si:H TFTs, because the Fermi level of a-Si:H TFTs are modulate by the density of states (DOS) in the a-Si:H band gap. The information on DOS is important for understanding the physical mechanisms responsible for the device behavior. It¡¦s related to the threshold voltage,iii subthreshold slope, field effect mobility and the stability of the TFTs. Experimental results show the photo leakage currents of a-Si:H TFTs under tensile stress are less than that of flattened a-Si:H TFTs stemmed the weak light intensity. In addition, the small shifts of threshold voltage and subthreshold swing are resulted from the smaller Ea in a-Si:H channel material.

Since the traditional CRT(Cathode Ray Tube) replaced by FPD(Flat Panel Display), e.g. LCD¡BOLED¡BPDP, FPD industry is regarded as the important one of global industry following Semi-conductor industry. The main stream of Large-Area Displays is TFT-LCD(Thin Film Transistor-Liquid Crystal Display) and it¡¦s applied a-Si:H TFT (the hydrogenated Amorphous Silicon Thin Film Transistor) as pixel-switch device on LCD. In a-Si:H TFT Cell process, the active region material(a-Si:H) with higher Photoconductivity results into higher off-state current under light illumination and that causes color performance discrepancy as incomplete On/Off operation of pixel-switch devices. As long as the introduction of F into a-Si:H modify the density of states in the gap of a-Si:H(:F), that may result the shift of the Fermi level toward the valence band edge and The density-of-states increasing. It¡¦s effective to decrease the photo leakage current. Due to electro-optical properties of liquid crystal(LC), to drive Pixel-switch device in TFT-LCD shall force On/Off voltage to change Twist Angle of LC is corresponding to have Stress on TFT device. According to DC Stress experiment results, it¡¦s found TFT device with SiF4 dopant can reach better reliability. This issue is aimed to research the photo leakage current variation of a-Si:H TFT at low temperature and ON/Off state effect by stress on TFT device.

In this dissertation, the electrical characteristics of generally used hydrogenated amorphous silicon (a-Si:H) TFTs in LCD and newly risen amorphous indium-gallium-zinc oxide (a-IGZO) TFTs were studied. For modern mobile display and large-size flat panel display application, the traditional thin-film transistor-liquid crystal display (TFT-LCD) technology confronts with a lot of challenges and problems. In general, flexible displays must exhibit some bending ability; however, bending applies mechanical strain to electronic circuits and affects device characteristics. Therefore, the electrical characteristics of a-Si:H TFTs fabricated on stainless steel foil substrates with uniaxial bending were investigated at different temperatures. Experimental results showed that the on-state current and threshold voltage degraded under outward bending. This is because outward bending will induce the increase of band tail states, affecting the transport mechanism at different temperatures. In addition, for practical operation, the electrical characteristics of a-Si:H TFTs under flat and bending situations after AC/DC stress at different temperatures were studied. It was found that high temperature and mechanical bending played important roles under AC stress. The dependence between the accumulated sum of bias rising and falling time and the threshold voltage shifts under AC stress was also observed. Because a-Si:H is a photosensitive material, the high intensity backlight illumination will degrade the performance of a-Si:H TFTs. Thus, the photo-leakage current of a-Si:H TFTs under illumination was investigated at different temperatures. Experimental results showed that a-Si:H TFTs exhibited a pool performance at lower temperatures. The indirect recombination rate and the parasitic resistance (Rp) are responsible for the different photo-leakage-current trends of a-Si:H TFTs under varied temperature operations. To investigate the photo-leakage current, the a-Si:H TFTs were exposed to ultraviolet (UV) light irradiation. It was found that the photo current of a-Si:H TFTs was reduced after UV light irradiation. The detail mechanisms on reducing/increasing photo-leakage current by UV light irradiation were discussed. Recently, the oxide-based semiconductor TFT, especially a-IGZO TFT, is considered as one of promising candidates for active matrix flat-panel display. However, the a-IGZO TFT exists significant electrical instability issue and manufacturing problems. As a consequence, we investigated the effect of hydrogen incorporation on a-IGZO TFTs to reduce interface states between active layer and insulator. Experimental results showed that the electrical characteristics of hydrogen-incorporated a-IGZO TFTs were improved. The threshold voltage shift (£GVth) in hysteresis loop is suppressed from 4 V to 2 V due to the hydrogen-induced passivation of the interface trap states. Finally, we reported the effect of ambient environment on a-IGZO TFT instability. As a-IGZO TFTs were stored in atmosphere environment for 40 days, the transfer characteristics accompanying strange hump were observed during bias-stress. The hump phenomenon is attributed to the absorption of H2O molecule. Additionally, the sufficient electric field is also necessary to cause this anomalous transfer characteristic.