Suhui Lee

Reduction of Negative Bias and Light Instability of a-IGZO TFTs by Dual-Gate Driving

Stability under negative-bias-illumination-stress (NBIS) of dual-gate (top- and bottom-gate) amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors is investigated. It is found that the negative threshold-voltage shift (ΔVTH) induced by NBIS is much smaller under dual-gate driving (when the two gates are electrically tied together) compared with single-gate driving. For a 20 nm-thick a-IGZO active layer, this is attributed to bulk accumulation, where electrons are accumulated across the entire depth of the active layer, which is responsible for the small negative ΔVTH after NBIS. Due to bulk accumulation, the Fermi level can be easily shifted by dual-gate driving as compared with the conventional single-gate driving, even after NBIS.

Methods and evaluations of MRI content-adaptive finite element mesh generation for bioelectromagnetic problems

In studying bioelectromagnetic problems, finite element analysis (FEA) offers several advantages over conventional methods such as the boundary element method. It allows truly volumetric analysis and incorporation of material properties such as anisotropic conductivity. For FEA, mesh generation is the first critical requirement and there exist many different approaches. However, conventional approaches offered by commercial packages and various algorithms do not generate content-adaptive meshes (cMeshes), resulting in numerous nodes and elements in modelling the conducting domain, and thereby increasing computational load and demand. In this work, we present efficient content-adaptive mesh generation schemes for complex biological volumes of MR images. The presented methodology is fully automatic and generates FE meshes that are adaptive to the geometrical contents of MR images, allowing optimal representation of conducting domain for FEA. We have also evaluated the effect of cMeshes on FEA in three dimensions by comparing the forward solutions from various cMesh head models to the solutions from the reference FE head model in which fine and equidistant FEs constitute the model. The results show that there is a significant gain in computation time with minor loss in numerical accuracy. We believe that cMeshes should be useful in the FEA of bioelectromagnetic problems.

Removal of Negative-Bias-Illumination-Stress Instability in Amorphous-InGaZnO Thin-Film Transistors by Top-Gate Offset Structure

A highly stable, dual-gate (DG) amorphous, indium-gallium-zinc oxide (a-IGZO) thin-film transistor (TFT) with an offset top-gate (TG) is reported. Given that both gates are opaque and electrically tied together, the TG functions as a lightshield and drain-current (IDS) enhancer as synchronized gate-voltage (VGS) sweep induces bulk-accumulation (BA) at positive voltages. It is demonstrated here that regardless of the offsets between the TG and source/drain electrode, this BA a-IGZO TFT is immune to negative bias and light-illumination stress (NBIS) when the TG covers at least 50% of the channel region. Therefore, high performance BA a-IGZO TFTs that are also immune to NBIS can be designed without introducing additional parasitic capacitance that occurs when the TG overlaps the source and/or drain electrode(s).

Conductivity images of biological tissue phantoms using a 3.0 tesla MREIT system

We present cross-sectional conductivity images of a biological tissue phantom obtained by using a 3.0 Tesla magnetic resonance electrical impedance tomography (MREIT) system. Inside the cylindrical phantom with 140 mm diameter and 140 mm height, biological tissues such as bovine tongue and liver, porcine muscle, and chicken breast were placed within an agar gelatin. Injecting current of 480 mA/spl middot/ms into the tissue phantom, we measured the z-component B/sub z/ of the induced magnetic flux density B=(B/sub x/, B/sub y/, B/sub z/). Using the harmonic B/sub z/ algorithm, we reconstructed cross-sectional conductivity images from the measured B/sub z/ data. Reconstructed images clearly distinguish different tissues in terms of both their shapes and conductivity values.

3-D diffusion tensor MRI anisotropy content-adaptive finite element head model generation for bioelectromagnetic imaging

Realistic finite element (FE) head models have been successfully applied to bioelectromagnetic problems due to a realistic representation of arbitrary head geometry with inclusion of anisotropic material properties. In this paper, we propose a new automatic FE mesh generation scheme to generate a diffusion tensor MRI (DT-MRI) white matter anisotropy content-adaptive FE head model. We term this kind of mesh as wMesh. With this meshing technique, the anisotropic electrical conductivities derived from DT-MRIs can be best incorporated into the model. The influence of the white matter anisotropy on the EEG forward solutions has been studied via our wMesh head models. The scalp potentials computed from the anisotropic wMesh models against those of the isotropic models have been compared. The results describe that there are substantial changes in the scalp electrical potentials between the isotropic and anisotropic models, indicating that the inclusion of the white matter anisotropy is critical for accurate computation of E/MEG forward and inverse solutions. This fully automatic anisotropy-adaptive wMesh meshing scheme could be useful for modeling of individual-specific FE head models with better incorporation of the white matter anisotropic property towards bioelectromagnetic imaging.

Some technical aspects of magnetic stimulation coil design with the ferromagnetic effect

The performance of the stimulation coil in a magnetic nerve stimulator can be improved by attaching a ferromagnetic structure to the coil. This reduces heat generation at the coil and increases magnetic field strength for a given unit of current. Some technical aspects of the design of a stimulation coil with a ferromagnetic structure have been studied. Finite element method analysis results are presented for the effect of size, depth and magnetic saturation of the ferromagnetic structure on the stimulation coil performance. The experimental results show that the stimulation coil performance is improved by up to 40% by the attaching of a ferromagnetic structure on the coil.

High-Performance Homojunction a-IGZO TFTs With Selectively Defined Low-Resistive a-IGZO Source/Drain Electrodes

We report high-performance homojunction amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) with low-resistive a-IGZO source/drain (S/D) electrodes. The a-IGZO S/D electrodes are selectively treated with high-power NF3 plasma, which reduces their resistivity from ~16 to 5.5×10-3 Ω · cm. X-ray photoelectron spectroscopy indicates an increase in weakly bonded oxygen and a substantial amount of indium-fluorine and zinc-fluorine bonds at the a-IGZO top surface (extending to ~7 nm into the bulk) after plasma treatment. Temperature-dependent conductivity measurements show metallic behavior of the a-IGZO after treatment. It is concluded that fluorine atoms substitute for oxygen atoms-generating free electrons in the process and/or occupy oxygen vacancy sites-eliminating electron trap sites. As a result, the homojunction TFTs show good ON-state characteristics with typical field-effect mobility, subthreshold gate-voltage swing, and turn-ON voltage of 19 ± 1 cm2/V·s, 178 ± 30 mV/decade, and -3.2 ± 1.5 V, respectively. Good stability at high temperature and under bias and light stress are also exhibited by the homojunction TFTs, verifying a stable doping effect by the NF3 plasma treatment.

Intrinsic Channel Mobility of Amorphous, In–Ga–Zn–O Thin-Film Transistors by a Gated Four-Probe Method

Intrinsic mobility and intrinsic channel resistance (R_CH) of amorphous, In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) with varying channel length (L) are investigated using a gated four-probe back-channel-etched TFT design. The intrinsic R_CH is found to decrease from ~500 to ~250 kΩ per unit area by increasing V_GS from 10 to 20 V. The intrinsic mobility is ~17 cm²/V·s, which is about 20% higher than that derived from the normal two-point probe measurements. Source and drain parasitic resistance (R_PAR) of the a-IGZO TFTs is found to be of the same order of magnitude as the R_CH-which is different from hydrogenated amorphous-silicon (a-Si:H) TFTs, where TFT operation is dominated by R_PAR.

Lateral Grain Growth of Amorphous Silicon Films With Wide Thickness Range by Blue Laser Annealing and Application to High Performance Poly-Si TFTs

We report high performance p-type poly-Si thin-film transistors (TFTs) achieved by lateral grain growth of amorphous silicon (a-Si) by using a continuous-wave blue diode laser of wavelength 445 nm. The blue laser beam is efficiently absorbed into the a-Si film, such that full melting and lateral crystallization is achieved in all thicknesses investigated, 50-200 nm. TFTs fabricated with 75-, 100-, and 125-nm-thick poly-Si films laterally grown by the blue laser annealing exhibited field-effect mobility of 108 ± 7, 104 ± 9, and 134 ± 12 cm2/V · s, and subthreshold swing of 210±51, 191±16, and 193±53 mV/decade, respectively, and high ON/OFF current ratio of ~108.

Bulk-Accumulation Oxide Thin-Film Transistor Circuits With Zero Gate-to-Drain Overlap Capacitance for High Speed

The overlap between gate and source/drain electrodes gives rise to parasitic capacitance (Cgd), which causes RC signal delay in thin-film transistor (TFT) circuits. Here, we show that in amorphous-indium-gallium-zinc-oxide TFTs, offsets as large as 0.5 μm, result in only slight reductions in draincurrent, such that (compared with single-gate TFTs with 2.5-μm gate-to-source/drain overlaps) an overall three times increase in switching speed can be achieved in dual-gate TFTs with offset top-gates shorted to offset bottom-gates. The high switching speed (~18 ns/stage delay), which is a combined effect of the bulk-accumulation achieved by shorting the two gates and zero Cgd, results in high-speed amorphous oxide TFT-based circuits.