Yongsik Kim

Impact of Oxygen Flow Rate on the Instability Under Positive Bias Stresses in DC-Sputtered Amorphous InGaZnO Thin-Film Transistors

The effect of O2 flow rate (OFR) during channel deposition is investigated on the electrical instability of the amorphous indium-gallium-zinc oxide (a-IGZO) thin-film transistors (TFTs) under positive gate bias stresses. From the transfer curves measured before and after bias stresses, we can observe that the high OFR degrades the electrical stability and causes the large threshold voltage shift (ΔVT) in a-IGZO TFTs. To elucidate the origin of the observed phenomenon, we extract and compare the subgap density of states (DOS) in devices with various OFRs. The extracted DOS shows that the subgap states become higher with the increase of OFR in a wide range of bandgap, and the enhanced electron trapping due to the increased number of trap states is considered as the cause of larger ΔVT in higher OFR devices.

Differential Ideality Factor Technique for Extraction of Subgap Density of States in Amorphous InGaZnO Thin-Film Transistors

We propose a differential ideality factor technique (DIFT) for extraction of subgap density of states (DOS) over the bandgap in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) by using the differential ideality factor dη/dV_GS on behalf of the ideality factor itself. Contrary to the sub threshold current method which requires an accurate threshold voltage (V_T), the DIFT is free from V_T itself and consider ably useful to TFTs with a nonuniform distribution of DOS over the bandgap. Through the DIFT applied to an a-IGZO TFT with W/L = 200 μm/30 μm, the subgap DOS is extracted to be a superposition of exponential deep and tail states with NUA = 7.1 × 10¹⁵ cm^-3 · eV^-1, kT_DA = 0.6 eV, N_TA=1.5 × 10¹⁶ cm^-3 · eV^-1, and kT_TA = 0.024 eV.

Analytical Models for Drain Current and Gate Capacitance in Amorphous InGaZnO Thin-Film Transistors With Effective Carrier Density

Analytical drain current and gate capacitance models for amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) over sub- and above-threshold regions are proposed by adopting an effective carrier density for the dominant carrier density. The effective carrier density fully considers the free carriers in the conduction band, the localized subgap deep states, and tail states over the bandgap for analytical I-V and C-V characteristics. The proposed analytical models are verified by comparing the measured I-V and C-V characteristics. The proposed models make a time-efficient simulation of a-IGZO TFT-based circuits possible due to their analytical form.

Amorphous InGaZnO Thin-Film Transistors—Part I: Complete Extraction of Density of States Over the Full Subband-Gap Energy Range

A combination of the multifrequency C- V and the generation-recombination current spectroscopy is proposed for a complete extraction of density of states (DOS) in amorphous InGaZnO thin-film transistors (a-IGZO TFTs) over the full subband-gap energy range (EV ≤ E ≤ EC) including the interface trap density between the gate oxide and the a-IGZO active layer. In particular, our result on the separate extraction of acceptor- and donor-like DOS is noticeable for a systematic design of amorphous oxide semiconductor TFTs because the former determines their dc characteristics and the latter does their threshold voltage (VT) instability under practical operation conditions. The proposed approach can be used to optimize the fabrication process of thin-film materials with high mobility and stability for mass-production-level amorphous oxide semiconductor TFTs.

Amorphous InGaZnO Thin-Film Transistors—Part II: Modeling and Simulation of Negative Bias Illumination Stress-Induced Instability

Based on the physical model of amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) and the extracted density of states described in Part I, a quantitative investigation of mechanisms on the negative bias illumination stress (NBIS)-induced threshold voltage VT instability of a-IGZO TFTs is presented. It is found that the shallow donor state-creation model explains the NBIS time evolution of the electrical characteristics very well. Furthermore, the semi-empirical rule of the NBIS-induced ΔVT is proposed and demonstrated based on the shallow donor state-creation model. The proposed approach can be used to optimize the fabrication process and to explore high-performance thin-film materials for mass-production-level amorphous oxide semiconductor TFTs to be innovatively used in the near future.

APPLICATION OF FLOCKING MECHANISM TO THE MODELING OF STOCHASTIC VOLATILITY

In this study, we present a new stochastic volatility model incorporating a flocking mechanism between individual volatilities of assets. Collective phenomena of asset pricing and volatilities in financial markets are often observed; these phenomena are more apparent when the market is in critical situations (market crashes). In the classical Heston model, the constant theoretical mean of the square of the volatility was employed, which can be assumed a priori. Our proposed model does not assume this mean value a priori, we instead use the flocking effect to continuously update the theoretical mean value using the local weighted average of individual volatility values. To perform this function, we use the Cucker–Smale flocking mechanism to calculate the local mean. For some classes of interaction weights such as all-to-all and symmetric coupling with a positive lower bound, we show that the fluctuations of the square process of volatility are uniformly bounded, such that the overall dynamics are mainly dictated by the averaged process. We also provide several numerical examples showing the dynamics of volatility.

Extraction of Subgap Donor States in a-IGZO TFTs by Generation–Recombination Current Spectroscopy

A physics-based generation-recombination current (JG r) spectroscopy is proposed for the extraction of the sub gap donorlike density of states (DOS) of amorphous InGaZnO thin-film transistors. Physics-based Shockley-Read-Hall recombination through the subgap DOS over the bandgap is fully considered, and the potential for the carrier concentration is calculated through the DeAOTS model. The extracted parameters for the exponential deep donorlike states are N_DD = 5.5 × 10²¹ [cm^-3 · eV^-1] and kT_DD = 0.115 [eV].

A Miniaturized Electron Beam Column Simulation by the Fast Moving Least Square Reproducing Kernel Point Collocation Method

A miniaturized electron beam column, microcolumn, operated at a low electron energy of 1–2 keV has been simulated by the fast moving least square reproducing kernel point collocation method (FCM) which is a new concept of point collocation calculations. The salient feature of the method here is the use of the dilation function instead of constant dilation parameter. The simulation results of FCM for microcolumn configuration show good agreement with previous calculation and experimental results. Typically, the electron beam column design has been simulated by the finite difference method (FDM) with the grid and finite element method (FEM) based on mesh generation. However, The FCM method dilation function can readily calculate high-aspect-ratio structures employing only nodes instead of grid or mesh generation. The accuracy of this method will be proved through careful analysis of the error between numerical and analytic solutions. The full microcolumn structure consisting of electron emitter, source lens, and Einzel lens parts can be readily calculated by FCM. We will discuss the basic concept of FCM and its applications in this paper.

The Effect of the Active Layer Thickness on the Negative Bias Stress-Induced Instability in Amorphous InGaZnO Thin-Film Transistors

The effect of the active layer thickness (<i>T</i>_IGZO) on the negative bias stress (NBS)-induced threshold voltage shift (Δ<i>VT</i>) in amorphous InGaZnO (a-IGZO) thin-film transistors (TFTs) is investigated and explained by using a subgap density-of-states (DOS) model. The NBS-induced negative Δ<i>VT</i> in a-IGZO TFT with a thinner <i>T</i>_IGZO is larger than that with a thicker <i>T</i>_IGZO. Based on the simulation result with the subgap DOS model, it is concluded that the <i>T</i>_IGZO-dependent Δ<i>VT</i> is originated from the accelerated creation of shallow donor states due to a higher surface electric field in a-IGZO TFTs with a thinner <i>T</i>_IGZO.

Physical Parameter-Based SPICE Models for InGaZnO Thin-Film Transistors Applicable to Process Optimization and Robust Circuit Design

In this letter, we show that the physics-based equation that was derived for amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) in our previous work can be successfully incorporated into the SPICE model via Verilog-A. The proposed model and extracted SPICE parameters successfully reproduce the measured current-voltage characteristics of amorphous indium-gallium-zinc-oxide (a-IGZO) TFTs and the load line diagram of a-IGZO TFT inverters. The main advantage of our model is that each parameter has its physical meaning and most of them can be related with the fabrication conditions of AOS TFTs. To show the advantage of the proposed models and extracted SPICE parameters more clearly, we investigate the effect of ionized donor concentration (ND+) on the inverter circuit operation and determine the optimum value of ND+ and device dimensions considering the tradeoff between the power consumption and the output swing in a-IGZO inverters. The proposed physics-based SPICE model via Verilog-A is expected to play a significant role in the process optimization and circuit design with AOS TFTs.