Kaichi Fukuda

Threshold voltage instability of a-Si:H TFTs in liquid crystal displays

Abstract A method is presented for simulating the threshold voltage (Vth) shift of a-Si:H TFTs in liquid crystal displays. The Vth shift after a certain operation period can be obtained by summing up each Vth shift, which is caused by pulsed voltage stress between the gate and source, step by step along with the pulse sequence. The estimated lifetime of 10 years at 50°C for one of the prepared samples, assuming a life end of ΔVth≦2V, is quite satisfactory for practical application in LCDs.

Properties of a-SiN0.7:H Films in High Electric Field

To understand the mechanism of transport in a high electric field, various properties of a-SiN0.7:H films deposited by rf glow discharge were investigated. The results are explained by a model in which two kinds of centers with different barrier heights are subject to Poole-Frenkel ionization. Either one of these centers contributes to the current, depending on the temperature. Above room temperature, the center contributing to Poole-Frenkel current is the neutral Si dangling bond, where the Fermi level is located. Below room temperature, the center can be associated with the negatively charged Si dangling bond, which behaves as an electron trap for photoconductivity. Based on this understanding of energy levels subject to Poole-Frenkel ionization, field enhancement of the photocurrent was expected and actually observed for the first time.

Importance of first layer thickness on TFT characteristics using a-Si:H deposited by 2-step process

In order to increase the throughput during thin film transistor-liquid crystal display manufacture, we investigated a 2-step deposition process for hydrogenated amorphous silicon. The first hydrogenated amorphous silicon layer of an inverted staggered type thin film transistor was deposited at a lower deposition rate to improve the interface with the gate insulator layer and the second hydrogenated amorphous silicon layer was deposited at a higher rate to improve the throughput. It was found that the mobility values of thin film transistors increased with increasing first layer thickness and reached a saturation value after a certain first layer thickness. When the first hydrogenated amorphous silicon layer quality was improved by decreasing its deposition rate, a thicker first layer hydrogenated amorphous silicon was needed to reach a mobility saturation.

Switching performance of high rate deposition processing a-Si:H TFTs

Abstract We studied the switching performance of thin film transistors fabricated using high deposition rate hydrogenated amorphous silicon deposited at high temperature. High field effect mobility was obtained with material deposited at rates close to 100 nm/min. The mobility decreased, the output characteristics showed current crowding, and capacitance-voltage ( C - V ) characteristics showed frequency dispersion and did not respond to high frequency when the hydrogenated amorphous silicon deposition rates were further increased. We also studied the performance of thin film transistors made using a 2-step deposition process for the hydrogenated amorphous silicon in an attempt to further increase the effective deposition rate. By using the 2-step process for hydrogenated amorphous silicon deposition, both static and dynamic mobility were improved and the charging of the pixel was not seriously affected for material deposited at close to ∼ 200 nm/min.

A new a-Si TFT with SiO 2 /SiN/sub x/ gate insulator for 10.4-inch LCDs

A novel a-Si TFT (thin-film transistor) with a composite gate insulator of CVD (chemical-vapor-deposited) SiO/sub 2//PE-CVD (plasma-enhanced CVD) SiN/sub x/ has been developed. This TFT has been applied to 10.4-in-diagonal LCDs (liquid crystal displays). Because of the high quality of CVD SiO/sub 2/, Vth drift, which was often observed after prolonged application of gate bias, was remarkably reduced compared to PE-CVD SiO/sub x/N/sub y/. Also, the degradation of subthreshold characteristics on a-Si TFT (often observed after long-term operation of LCDs at high temperature) was improved. Selective etching technologies of SiN/sub x/ against SiO/sub 2/, which is one of key issues in connection with obtaining high production yield, have been developed.<<ETX>>