Hung-Wei Li

Bias-induced oxygen adsorption in zinc tin oxide thin film transistors under dynamic stress

This study investigates the effects of bias-induced oxygen adsorption on the electrical characteristic instability of zinc tin oxide thin film transistors in different ambient oxygen partial pressures. When oxygen pressure is largest, the threshold voltages showed the quickest increase but the slowest recovery during the stress phase and recovery phase, respectively. This finding corresponds to the charge trapping time constant and recovery time constant, which are extracted by fitting the stretched-exponential equation and which exhibit a relationship with oxygen pressure. We suggest that the gate bias reduces the activation energy of oxygen adsorption during gate bias stress.

Environment-dependent thermal instability of sol-gel derived amorphous indium- gallium-zinc-oxide thin film transistors

The environment-dependent electrical performances as a function of temperature for sol-gel derived amorphous indium-gallium-zinc-oxide (a-IGZO) thin film transistors are investigated in this letter. In the ambients without oxygen, thermal activation dominates and enhances device performance. In oxygen-containing environments, mobility and drain current degrades and the threshold slightly increase as temperature increases. We develop a porous model for a-IGZO film relating to the drain current and mobility lowering due to film porosity and oxygen adsorption/penetration. It also relates to the threshold voltage recovery at high temperature owing to the varying form of adsorbed oxygen and the combination of oxygen and vacancies.

A low-temperature method for improving the performance of sputter-deposited ZnO thin-film transistors with supercritical fluid

A low-temperature method, supercritical CO2 (SCCO2) fluid technology, is employed to improve the device properties of ZnO TFT at 150 °C. In this work, the undoped ZnO films were deposited by sputter at room temperature and treated by SCCO2 fluid which is mixed with 5 ml pure H2O. After SCCO2 treatment, the on/off current ratios and threshold voltage of the device were improved significantly. From x-ray photoelectron spectroscopy analyses, the enhancements were attributed to the stronger Zn–O bonds, the hydrogen-related donors, and the reduction in dangling bonds at the grain boundary by OH passivation.

High-stability oxygen sensor based on amorphous zinc tin oxide thin film transistor

This research presents a sol-gel derived zinc tin oxide thin film transistor (TFT) as a high-stability oxygen sensor. Due to its high sensitivity, oxygen has been traditionally regarded as having a negative influence on the electrical characteristics of zinc-based TFTs; however, TFTs can also act as an oxygen sensor. After illumination with visible light in oxygen-rich ambient, a significant increase in drain current of nearly 104 times occurs with fixed gate and drain voltages. It is expected that an optimized method of illumination can help to reset the electrical characteristics or distinguish the on/off state of this reliable oxygen sensor.

Anomalous Capacitance Induced by GIDL in P-Channel LTPS TFTs

In this letter, a mechanism of anomalous capacitance in p-channel low-temperature polycrystalline silicon thin-film transistors (LTPS TFTs) was investigated. In general, the effective capacitance was only the overlap region and independent with the frequency in LTPS TFTs under the off state. However, our experimental results reveal that the capacitance was related with the leakage current and that it was dependent with the measurement frequencies when operated at the off-state region. The increase of the capacitance value is verified to be due to the increase of the electron capacitance originating from a gate-induced drain-leakage (GIDL) one. Nevertheless, the GIDL-induced electron capacitance can be suppressed by employing band-to-band hot electron stress.

Analysis of Degradation Mechanism in SONOS-TFT Under Hot-Carrier Operation

This letter investigates the degradation mechanism of polycrystalline silicon thin-film transistors with a silicon-oxide-nitride-oxide-silicon structure under off -state stress. During the electrical stress, the hot hole generated from band-to-band tunneling process will inject into gate dielectric, and the significant on-state degradation (more than 1 order) indicates that the interface states are accompanied with hot-hole injection. In addition, the asymmetric I- V characteristics indicate that the interface states are located near the drain side. Moreover, the ISE-TCAD simulation tool was utilized to model the degradation mechanism and analyze trap states distribution. Although both the vertical and lateral electrical fields are factors for degradation and hot-hole injection, the degradation is mainly affected by the lateral electrical field over a critical point.

The suppressed negative bias illumination-induced instability in In-Ga-Zn-O thin film transistors with fringe field structure

This study investigates the suppressed negative gate bias illumination stress (NBIS) -induced instability of via-type amorphous indium-gallium-zinc-oxide (a-IGZO) thin film transistors (TFTs) with fringe field (FF) structures. The less negative threshold voltage shifts of devices after NBIS are showed when device has larger FF structures. This finding is attributed to more dispersive distribution of photo-generated holes in the width direction of a-IGZO during NBIS, which reduce the hole trapping phenomenon in the front channel interface. The a-IGZO TFT with FF structure is expected to be an effective method to increase the electrical reliability of devices after NBIS.

Charge-Trapping-Induced Parasitic Capacitance and Resistance in SONOS TFTs Under Gate Bias Stress

This letter investigates the charge-trapping-induced parasitic resistance and capacitance in silicon-oxide nitride-oxide-silicon thin-film transistors under positive and negative dc bias stresses. The results identify a parasitic capacitance in OFF-state C-V curve caused by electrons trapped in the gate insulator near the defined gate region during the positive stress, as well as the depletion induced by those trapped electrons. Meanwhile, the induced depletions in source/drain also degraded the I-V characteristic when the gate bias is larger than the threshold voltage. However, these degradations slightly recover when the trapped electrons are removed after negative bias stress. The electric field in the undefined gate region is also verified by TCAD simulation software.

Elimination of Photoleakage Current in Poly-Si TFTs Using a Metal-Shielding Structure

A technology to eliminate the photoleakage current of poly-Si thin-film transistors (TFTs ) with top gate structure has been developed. A thin metal film is formed on the glass substrate to be used as a light-shielding layer. The light-shielding layer, buffer layer, and active island are patterned employing the same mask. The leakage current and the variation of subthreshold swing in the proposed devices are suppressed completely under illumination. Due to the parasitic capacitance in the overlap region between the drain side and the metal-shielding layer, a floating voltage coupled from drain bias influences the threshold voltage of the proposed poly-Si TFTs.

Reduction of photoleakage current in polycrystalline silicon thin-film transistor using NH3 plasma treatment on buffer layer

The technology of polycrystalline silicon thin-film transistors (poly-Si TFTs) with low photoleakage current is developed in this work. The electrical characteristics of poly-Si TFTs under illumination were significantly improved employing the NH3 plasma treatment on the buffer layer, with no need for complicate device structure and additional masks. The trap states that originated from the plasma bombardment on the interface between the poly-Si layer and buffer oxide can effectively recombine the light-induced electron-hole pairs. The fewer residual electron-hole pairs lead to the lower photoleakage current and improved subthreshold swing, as well as maintaining good electrical characteristics in the dark sate.