Joong-Ho Song

Improving the Gate Stability of ZnO Thin-Film Transistors with Aluminum Oxide Dielectric Layers

We report on the fabrication of gate-stable ZnO thin-film transistors (TFTs) with aluminum oxide dielectric. When an off-stoichiometric AlO x was deposited at room temperature, the ZnO-TFT revealed unreliable transfer characteristics: a large drain current-gate bias (I D -V G ) hysteresis and a large amount of threshold voltage (V T ) shift under gate-bias stress. As rapid thermal annealing (RTA) in O 2 ambient was applied onto AIO X at 300°C prior to ZnO channel deposition, the gate-bias reliability of the ZnO device was improved. The RTA might cause our AlO x surface to be more stoichiometric and thus to be resistant against ZnO sputter-induced damage. When the bottom-gate ZnO-TFT was fabricated with a stoichiometric Al 2 O 3 dielectric grown by atomic layer deposition (ALD), our device showed much more stable electrical characteristics than with the sputter-deposited off-stoichiometric AlO x . Last, as an ultimate effort to improve the gate reliability, we fabricated a top-gate ZnO-TFT device adopting the same thick ALD-grown stoichiometric Al 2 O 3 as in the bottom-gate device. Our top-gate device with the Al 2 O 3 dielectric then showed no hysteresis and no V T shift after several times of gate bias sweep. We conclude that both the high quality dielectric and optimized device structure are necessary to realize electrically stable ZnO-TFTs.

Torque-Ripple Minimization and Fast Dynamic Scheme for Torque Predictive Control of Permanent-Magnet Synchronous Motors

This paper proposes a simple and effective method to reduce torque ripples for the torque predictive control (TPC) of permanent-magnet synchronous motors (PMSMs). The conventional TPC analyzes the relationship among the electrical torque, stator flux, and stator voltage using the magnitude of the stator voltage vector of PMSMs to obtain the angle of the reference voltage vector and accurately control the torque. In addition, the stator-flux control uses the hysteresis method. However, the voltage vector that can be chosen in an inverter is limited because the conventional TPC fixes the magnitude of the reference voltage vector, and thus, a large torque ripple is generated in the low-speed region. The proposed TPC does not fix but varies the magnitude of the reference voltage vector using both the torque and flux error information. Therefore, it not only has the fast dynamic of a direct torque control but also can reduce effectively the torque ripple. The proposed method is proven by the simulation and experimental results, and the proposed algorithm provides an excellent steady-state response and fast dynamics.

Improvement of low-speed operation performance of DTC for three-level inverter-fed induction motors

A direct torque control algorithm for three-level inverter-fed induction motors is presented. Basic voltage selection methods similar to a two-level inverter provoke some problems such as stator-flux drooping phenomenon and undesirable torque control deterioration appeared, especially at low-speed operation. To overcome these problems, an algorithm with the basic switching sectors subdivided and intermediate voltage vectors applied is proposed in this paper. This algorithm basically considers applications in which direct torque-controlled induction motors are fed by three-level inverters with maximum switching frequency lowered around 1 kHz. An adaptive observer is also employed to bring better responses at the low-speed operation, by estimating some state variables and motor parameters which take a deep effect on the performance of the low-speed operation. Simulation and experiment results verify effectiveness of the proposed algorithm.

Torque ripple reduction in DTC of induction motor driven by 3-level inverter with low switching frequency

A torque ripple reduction technique of direct torque control (DTC) for high power induction motors driven by 3-level inverters with the inverter switching frequency limited around 0.5-l kHz level is presented. It is noted that conventional DTC algorithms to reduce torque ripple are devised for applications with relatively high switching frequency above 2-3 MHz. Such conventional algorithms cannot accomplish satisfactory torque ripple reduction for 3-level inverter systems with lower switching frequency. A new DTC algorithm, especially for low switching frequency inverter system, illustrates relatively reduced torque ripple characteristics all over the operating speed region. Simulation and experimental results show effectiveness of the proposed control algorithm.

Commutation torque ripple reduction in brushless DC motor drives using a single DC current sensor

This paper presents a comprehensive study result on reducing the commutation torque ripples generated in brushless DC motor drives with only a single DC-link current sensor provided in the inverter DC-link. In brushless DC motor drives equipped with only a single current sensor, it seems that commutation torque ripple suppression which is practically effective in low speed as well as high speed regions has not been reported. A proposed commutation compensation technique combined with a deadbeat DC-link current controller is based on a strategy that the current slopes of the incoming and the outgoing phases during the commutation interval can be equalized by a proper duty-ratio control. The proposed control method is verified through simulations and experiments.

Ferromagnetism in 200-MeV Ag+15-ion-irradiated Co-implanted ZnO thin films

Structural, electrical resistivity, and magnetization properties of 200-MeV Ag+15-ion-irradiated Co-implanted ZnO thin films are presented. The structural studies show the presence of Co clusters whose size is found to increase with increase of Co implantation. The implanted films were irradiated with 200-MeV Ag+15 ions to fluence of 1×1012ions∕cm2. The Co clusters on irradiation dissolve in the ZnO matrix. The electrical resistivity of the irradiated samples is lowered to half. The magnetization hysteresis measurements show ferromagnetic behavior at 300K, and the coercive field increases with the Co implantation. The ferromagnetism at room temperature is confirmed by magnetic force microscopy measurements. The results are explained on the basis of the close interplay between the electrical and the magnetic properties.

System identification of power converters based on a black-box approach

This paper addresses the topic of a model-free system identification of highly nonlinear power electronics systems from the data either through a time-domain simulation or a hardware measurement. This system identification method based on the black-box approach using the data generated from well-known design tools such as PSpice and MATLAB is generally simpler and independent of type of converter. As an application of identifying an unknown plant in power electronics systems, a constructive black-box approach is presented which aims at generating discrete-time small-signal linear equivalent models for a general class of converters, which includes resonant converters, pulsewidth modulation (PWM) converters and zero-voltage-switched (ZVS) PWM converters. The resulting small-signal model describes the converter as a linear time-invariant system, and the knowledge of the identified linear system can be applied to the switching converters for constructing feedback controllers. The identification results are compared with the analytical model and experimental data.

Sensorless vector control of induction motor using a novel reduced-order extended Luenberger observer

A synthesis method of the reduced-order extended Luenberger observer (ROELO) and its design procedure for a nonlinear dynamic system are presented. This paper proposes a method to reduce the order of the observer and to select the observer gain matrix. The main features of the proposed observer are discussed and compared with the characteristics of the other observers. The proposed algorithm is applied for high-performance induction motor drives without a speed sensor. The simulation results show that the proposed ROELO provides both the rotor flux and rotor speed estimation with good transient and steady state performance.

Improving crest factor of electronic ballast-fed fluorescent lamp current using pulse frequency modulation

In case where electronic ballast employing a valley-fill passive power-factor correction (PFC) circuit is used for feeding fluorescent tamps, a new method to reduce crest factor of the lamp current is studied in this paper. It is known that a 50% valley-fill passive PFC provided for high input power factor results in undesirable value of crest factor of the fluorescent lamp current, In order to reduce crest factor to a lower value, a pulse frequency modulation technique based on the waveform of the DC-link voltage which is predetermined by the passive PFC circuit is taken into the switching control action of the electronic ballast. An equation-based analysis between the crest factor of lamp current and the effect of varying the inverter switching frequency is comprehensively performed. Several simulation and experiment results illustrate the Effectiveness of the proposed control scheme.

Enhancing defect-related photoluminescence by hot implantation into SiO2 layers

Visible photoluminescence around an orange band of 580 nm wavelength are observed from 300 nm thin SiO2 layers implanted by Si or Ge ions at both substrate temperatures of 25 °C [room temperature (RT)] and 400 °C (hot). Si implantations at an energy of 30 keV were performed with doses of 5×1015, 3×1016, and 1×1017 cm−2 while Ge implantations were done at 100 keV with a dose of 5×1015 cm−2. Samples implanted at 400 °C always show much higher intensities of luminescence than those implanted at room temperature. Electron spin resonance signals of the hot-implanted samples indicate relatively smaller amounts of nonradiative defects than those of RT-implanted samples. It is concluded that the hot-implantation effectively enhances the intensity of defect-related photoluminescence by reducing the density of the nonradiative defects and introducing the radiative defects, which contribute to the luminescence in SiO2 layers.