| Summary: | 碩士 === 國立中山大學 === 物理學系研究所 === 103 === With the arrival of wisdom of the ages, smart phones have been already essential necessary for everyone in the present day. Due to the requirement of high resolution for smart phones, Low-Temperature Polycrystalline-Silicon (LTPS) thin film transistors have been restudied again owing to its higher carrier mobility. Furthermore, the displays also turn towards large-scale development in the resent years. Since amorphous idium-gallium-zinc-oxide (InGaZnO) TFT possesses lots of advantages, such as better uniformity, higher carrier mobility and lower process temperature, it becomes to a popular research topic.
This study focuses on the hot-carrier and self-heating effects in LTPS-TFTs and a-IGZO TFTs. First, the comparison of normal type and GOLD (Gate overlapped lightly doped drain) structures are carried out. It is found that the GOLD structure has a better electrical characteristic than that of normal type due to higher coverage area of metal gate. In order to investigate the reliability of current operation for GOLD structure, the effects on hot-carrier and self-heating stress are discussed in the next section. Accoring to results, it is found that the degree of degradation becomes more severe as gate bias increases regardless of hot-carrier and self-heating stresses. However, the degradation behaviors after hot-carrier and self-heating stresses are consisten with each other. These degradations are further found to depend on device width, representing that they are belonged to a classical self-heating-induced degradation behaviors. Therefore, these instabilities after hot-carrier and self-heating stresses can be both attributed to the high channel current operation-induced self-heating effect. Furthermore, the degradation behaviors become to be dominated by another mechanism as the SiNx thickness of gate insulator increases. Except the electron-trapping into the SiOx layer through thermonionic-field emission, the phenomenon about hole-injection becomes to more serious in thicker SiNx thickness of gate insulator devices, resulting in the instability of threshold voltage becomes more severe.
In the final section, hot-carrier effects and self-heating effects in a-IGZO TFTs are also well discussed. Under hot-carrier stress, devices were electrically stressed by different drain voltages at fixed gate voltage and different gate voltages at fixed drain voltage. Both these two kinds of stresses show on-current degradation as gate and drain biases of stresses increases, hence, it can be considered that the degradation behaviors are related to both increase of channel electrons and lateral electric field in the channel.
On the other hand, we focus on degradation behaviors of self-heating effects in a-IGZO TFTs. Devices were stressed by different drain voltages at fixed gate voltage and all the drain voltage are less than gate voltage. After the series of stresses, both threshold voltage shift and on-current degradation can be observed under source and drain interchange mode. Accordingly, it is indicated that the degraded mechanisms of hot-carrier-induced trap state generation had been also occured in that of self-heating stress. However, the additional VT-shift might be attributed to higher channel current-induced Joule-heating generation near the drain side during self-heating stress. Because the charge-trapping effect is a thermionic-field emission process, the higher channel temperature enhanced more electrons trapped into the gate insulator and induced extra VT-shift under source/drain interchange mode.
Keyword: Thin Film Transistor, InGaZnO, Hot-Carrier Effect, Self-Heating Effect, Threshold voltage, Corner Effect, charge trapping.
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