| Summary: | 博士 === 國立清華大學 === 電子工程研究所 === 95 === A low-dielectric-constant (low-k) material, siloxane-based hydrogen silsesquioxane (HSQ), is investigated as a passivation layer in bottom-gate hydrogenated amorphous silicon thin film transistors (a-Si:H TFTs). The low-k HSQ film passivated on TFT promotes the brightness and aperture ratio of TFT-LCD due to its high light transmittance and good planarization. Also, the performance of a-Si:H TFT with HSQ passivation has been improved, compared to a conventional silicon nitride (SiNx) passivated TFT, due to that the hydrogen bonds of HSQ assist the hydrogen incorporation to eliminate the density of states between back channel and passivation layer. Experimental results exhibit an improved field-effect mobility of 0.57 cm2/Vs and subthreshold swing of 0.68 V.
Next, a novel spin-on low-k material, photosensitive Polymethylsilazane (PS-MSZ), directly patterned by i-line stepper has been investigated for BCE a-Si TFT passivation layer. The presence of PS-MSZ with good transmittance and planarization reduces the RC delay between gate and data-line, and promotes the aperture ratio of TFT-LCD panel. The direct patterning technique simplifies the process, decreasing cost without the vacuum system and etching steps. The TFT transfer characteristics are not significantly impacted by the PS-MSZ passivation layer. In addition, the PS-MSZ passivated TFT has low leakage current in reverse subthreshold region due to its spin-on deposition. On the other hand, another new direct pattern low-k material, polymer, will also be proposed to be a passivation layer and to compare with PS-MSZ film, which can also applied on TFT device.
On study the back channel effect, the mechanism of back channel leakage between a-Si:H film and passivation layer has been demonstrated. There are two factors affected back channel leakage very much. These factors are fixed charge and interface states. These factors will impact TFT reverse subthreshold characteristics (operation bias at small negative gate voltage) and lead to back channel effect. Fixed charges bend the band and accumulate electrons at back channel. We use forward and reverse sweep measurement at different temperature to confirm back channel leakage mechanism. When TFT device is operated at Vg= 20 ~ -20V, there is a different phenomenon which let threshold voltage increased and back channel leakage decreased. There are lots of electrons trapping in amorphous silicon film when the initial operating voltage is a large positive bias. These trapping electrons will block other electrons entering amorphous silicon film, including back channel region. These trapping electrons in amorphous silicon film made an additional barrier which blocked electron transition, especially at small gate bias operation. This kind of conduction mechanism is similar to space-charge-limited current conduction (SCLC). These trapping electrons in amorphous silicon film are temporary. Therefore, these trapping electrons will be excited at a high temperature environment. Once trapping electrons excited, the blocking barrier decreases and the device conduction mechanism is back to initial type. Therefore, for TFT transfer characteristics, fixed charges existing passivation layer and interface states is the main reason which result in back channel leakage.
The dual-gate a-Si:H TFT owns superior conducting ability than conventional TFT which contains an additional electron path at back channel. Positive back gate bias leads to an increasing in drain current. The dual-gate a-Si:H TFT also exhibits the better endurance against photo leakage current than conventional a-Si:H TFT. When dual gate driving bias becomes negative, conduction band and valence band are bending up by dual gate which lead the photo excited electron-hole pairs to be confined. The confined electron-hole pairs may be easily recombined by lots of DOS in a-Si:H film. Otherwise, under negative dual gate bias, Fermi level is near valence band. At this time, photo excited electron-hole pairs would probably be recombined immediately by those lots of traps when they generated.
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