Satoshi Takenaka

Conduction Mechanism of Leakage Current Observed in Metal-Oxide-Semiconductor Transistors and Poly-Si Thin-Film Transistors

It has been confirmed that the leakage current observed in Metal-Oxide-Semiconductor (MOS) transistors and polycrystalline thin-film transistors (poly-Si TFTs) corresponds to activation energies of the current. Two new models of the conduction of leakage current have been proposed on the basis of this correspondence. These models consist of two types of steps: the first step is the thermal emission of an electron occurring in the neighborhood of the drain electrode, and the second step is the tunneling of an electron through a trap state in the band gap. These models have enabled us to explain the conduction mechanism of the leakage current in various devices.

High mobility poly-Si thin film transistors using solid phase crystallized a-Si films deposited by plasma-enhanced chemical vapor deposition

Thin film transistors (TFTs) have been developed on quartz substrates by using large-grain-size polycrystalline silicon (poly-Si) films. The poly-Si films were fabricated by solid phase crystallization (SPC) of amorphous silicon (a-Si) deposited by plasma-enhanced chemical vapor deposition (PECVD). We have found that the dehydrogenation process is strongly correlated with the SPC of the a-Si, especially in nucleus generation time. The n-channel TFT mobility of 158 cm2/(V s) is obtained by using the SPC of the PECVD a-Si.

Study of Degradation Phenomenon Due to a Combination of Contamination and Self-Heating in Poly-Si Thin Film Transistors Fabricated by a Low-Temperature Process

We have studied the reliability of polycrystalline-silicon thin film transistors (poly-Si TFTs) fabricated by a low-temperature process below 425°C (low-temperature-processed poly-Si TFTs). When TFTs are contaminated with mobile ions, such as sodium (Na) or potassium (K), the transfer characteristics of TFTs show interesting behaviors due to a combination of contamination and self-heating. Since the temperature of TFTs during operation becomes high because of the high supply voltage and low heat conductivity of the glass substrate, the ions can easily drift corresponding to the polarity of the gate voltage. As a result, the transfer characteristics of n-channel TFTs are significantly shifted in the negative direction. Depending on the circumstances, degradation due solely to self-heating is induced simultaneously. In this case, the transfer characteristics are shifted in the negative direction at the first stage, and then the direction of shift is changed to the positive direction. The temperature of TFTs during operation becomes high compared with that of metal-oxide-semiconductor field effect transistors (MOSFETs), and TFTs are fabricated on glass substrates consisting of many component parts. Therefore, there is a possibility that this degradation is an essential issue in low-temperature-processed poly-Si TFTs.

Initial Growth of Polycrystalline Silicon Films on Substrates Subjected to Different Plasma Treatments

Undoped 0.2-µm-thick polycrystalline Si (poly-Si) films were deposited on fused quartz substrates by plasma-enhanced chemical vapor deposition from a SiH4–H2 mixture. All films were prepared under the same deposition conditions, just after the substrates were exposed for 1 min to CF4–He plasma excited with various rf powers. Poly-Si films with improved crystallinity and large grains were obtained when the films were deposited on substrates with the proper degree of surface roughness of uniform size and shape. These films were also found to have lower values of stress and higher values of g, as compared with those of poly-Si films on substrates with a flat surface or an extremely rough surface. The X-ray diffraction (XRD) spectra exhibited only the texture, and the intensity was proportional to the third power of the average grain size estimated from the width values of the XRD spectra. These results suggested that the growth of grains is three-dimensionally controlled and depends on the surface morphology of the substrates, while the concentration of grains per unit area is roughly independent of the morphology of the substrates.