Kitae Lee

Bio: Kitae Lee is an academic researcher from Seoul National University. The author has contributed to research in topics: Laser & Field-effect transistor. The author has an hindex of 6, co-authored 73 publications receiving 169 citations. Previous affiliations of Kitae Lee include Inha University.

Investigation of Electrical Characteristic Behavior Induced by Channel-Release Process in Stacked Nanosheet Gate-All-Around MOSFETs

Abstract: In this brief, several issues attributed to the channel-release process in vertically stacked-gate-all-around MOSFETs (GAAFETs) having various nanosheet (NS) widths were rigorously investigated. Because of the finite selectivity of SiGe (sacrificial layer) etchant to Si (channel layer), Si channel is likely to be thinned during the channel-release step which is one of the key processes in stacked-GAA FET fabrication. Consequently, the thickness of channel and the interchannel space becomes variable depending on the NS width, since the etch time must be determined by the widest channel within a wafer. It results in a channel width dependence of gate work function, gate-to-drain capacitance, and channel interfacial property as well as the electrostatic gate controllability. The electrical characteristic behavior of stacked-GAAFETs induced by these effects was thoroughly investigated through process-based 3-D technology computer-aided design (TCAD) device simulation along with a transmission electron microscopy (TEM) and an energy-dispersive spectroscopy (EDS) analyses. The results confirm that width-dependent effects should be taken into account when fabricating and compact modeling the stacked-GAAFETs with various NS widths which are required for logic and static random access memory (SRAM) applications.

Wakeup-Free and Endurance-Robust Ferroelectric Field-Effect Transistor Memory Using High Pressure Annealing

Abstract: Wakeup-free and endurance-robust HfZrO2 (HZO) ferroelectric field-effect transistor (FeFET) was fabricated on a silicon-on-insulator substrate. After a high-pressure forming gas annealing as the last alloy step, the performance and endurance of the FeFETs were significantly improved by trap states reduction, polarization enhancement, and wake-up elimination. As the result, the FeFETs show superior endurance exceeding 1010 cycles and robust retention behavior at program/erase biases of ±3.5V and pulse width of 100 ns. These results indicate that appropriate thermal treatment for the interlayer and ferroelectric material could substantially improve FeFET performance and reliability.

Simulation of the effect of parasitic channel height on characteristics of stacked gate-all-around nanosheet FET

Abstract: By using technology computer aided design (TCAD) simulation, the aim of this paper is to investigate the effect of Si parasitic channel, which is placed under stacked nanosheet channels, on electrical characteristic of stacked nanosheet GAA FETs. We have controlled the parasitic channel height, and evaluated the effect on electrical performance of the device. Trade-off in performance of the nanosheet FET is observed: the increase in parasitic channel height results in improvement in subthreshold swing and on/off ratio, while the increase in capacitance brings worse RC delay and active power. The parasitic channel height control in devices with ground plane doping is also investigated.

Ferroelectric-Gate Field-Effect Transistor Memory With Recessed Channel

Abstract: We demonstrate a novel ferroelectric-gate field effect transistor with recessed channel (R-FeFET) to improve memory window (MW), program/erase speed, long-time retention, and endurance simultaneously. Based on technology computer-aided design (TCAD) simulations including calibrated ferroelectric material (FE) parameters, it is revealed that the polarization is enhanced by the larger electric field (e-field) across the FE compared to a conventional planar FeFET, resulting in the wider MW and the faster program/erase speed. Moreover, the endurance/retention of the R-FeFET is expected to be improved as the e-field across the SiO2 interlayer is significantly reduced.

Numerical study on the dynamics of Z-pinch carbon plasma

Abstract: The dynamics of Z‐pinch carbon plasma has been investigated using one‐dimensional Lagrangian code. This code calculates the single‐fluid, two‐temperature magnetohydrodynamic (MHD) equations coupled with an ionization balance equation. The motion of plasma column and shock front is studied in comparison with the analytical models such as the snowplow and the slug model. The energy flow during the pinch is also studied. During the pinch phase, the temperature increases due to shock heating and adiabatic heating. After the pinch the plasma is cooled down rapidly due to adiabatic expansion which can lead to an adequate condition for recombination Extreme‐Ultra‐Violet (XUV) lasers. The effect of the radiative trapping of resonance line on hydrodynamics and population kinetics is also investigated. The calculation shows that there can exist a high gain on hydrogen‐like C VI Balmer‐α line (18.2 nm).

Fourier-transform method of fringe-pattern analysis for computer-based topography and interferometry

Abstract: A fast-Fourier-transform method of topography and interferometry is proposed. By computer processing of a noncontour type of fringe pattern, automatic discrimination is achieved between elevation and depression of the object or wave-front form, which has not been possible by the fringe-contour-generation techniques. The method has advantages over moire topography and conventional fringe-contour interferometry in both accuracy and sensitivity. Unlike fringe-scanning techniques, the method is easy to apply because it uses no moving components.

Shock Ignition of Thermonuclear Fuel with High Areal Density

Abstract: A novel method by C. Zhou and R. Betti [Bull. Am. Phys. Soc. 50, 140 (2005)] to assemble and ignite thermonuclear fuel is presented. Massive cryogenic shells are first imploded by direct laser light with a low implosion velocity and on a low adiabat leading to fuel assemblies with large areal densities. The assembled fuel is ignited from a central hot spot heated by the collision of a spherically convergent ignitor shock and the return shock. The resulting fuel assembly features a hot-spot pressure greater than the surrounding dense fuel pressure. Such a nonisobaric assembly requires a lower energy threshold for ignition than the conventional isobaric one. The ignitor shock can be launched by a spike in the laser power or by particle beams. The thermonuclear gain can be significantly larger than in conventional isobaric ignition for equal driver energy.

RF Compression of sub-relativistic space-charge-dominated electron bunches for single-shot femtosecond electron diffraction

Abstract: We demonstrate the compression of 95 keV, space-charge-dominated electron bunches to sub-100 fs durations. These bunches have sufficient charge (200 fC) and are of sufficient quality to capture a diffraction pattern with a single shot, which we demonstrate by a diffraction experiment on a polycrystalline gold foil. Compression is realized by means of velocity bunching by inverting the positive space-charge-induced velocity chirp. This inversion is induced by the oscillatory longitudinal electric field of a 3 GHz radio-frequency cavity. The arrival time jitter is measured to be 80 fs.

Energetic Proton Generation in Ultra-Intense Laser-solid Interactions

Abstract: An explanation for the energetic ions observed in the PetaWatt experiments is presented. In solid target experiments with focused intensities exceeding 1020 W/cm2, high-energy electron generation, hard bremsstrahlung, and energetic protons have been observed on the backside of the target. In this report, an attempt is made to explain the physical process present that will explain the presence of these energetic protons, as well as explain the number, energy, and angular spread of the protons observed in experiment. In particular, we hypothesize that hot electrons produced on the front of the target are sent through to the back off the target, where they ionize the hydrogen layer there. These ions are then accelerated by the hot electron cloud, to tens of MeV energies in distances of order tens of μm, whereupon they end up being detected in the radiographic and spectrographic detectors.

Memristive technologies for data storage, computation, encryption, and radio-frequency communication

Abstract: Memristive devices, which combine a resistor with memory functions such that voltage pulses can change their resistance (and hence their memory state) in a nonvolatile manner, are beginning to be implemented in integrated circuits for memory applications. However, memristive devices could have applications in many other technologies, such as non–von Neumann in-memory computing in crossbar arrays, random number generation for data security, and radio-frequency switches for mobile communications. Progress toward the integration of memristive devices in commercial solid-state electronic circuits and other potential applications will depend on performance and reliability challenges that still need to be addressed, as described here. Description Putting memristors to work Memristors, which are resistors that change conductivity and act as memories, are not only being used in commercial computing but have several application areas in computing and communications. Lanza et al. review how devices such as phase-change memories, resistive random-access memories, and magnetoresistive random-access memories are being integrated into silicon electronics. Memristors also are finding use in artificial intelligence when integrated in three-dimensional crossbar arrays for low-power, non–von Neuman architectures. Other applications include random-number generation for data encryption and radiofrequency switches for mobile communications. —PDS A review explains how resistors with memory functions are being integrated into electronics and new computer architectures. BACKGROUND Memristive devices exhibit an electrical resistance that can be adjusted to two or more nonvolatile levels by applying electrical stresses. The core of the most advanced memristive devices is a metal/insulator/metal nanocell made of phase-change, metal-oxide, magnetic, or ferroelectric materials, which is often placed in series with other circuit elements (resistor, selector, transistor) to enhance their performance in array configurations (i.e., avoid damage during state transition, minimize intercell disturbance). The memristive effect was discovered in 1969 and the first commercial product appeared in 2006, consisting of a 4-megabit nonvolatile memory based on magnetic materials. In the past few years, the switching endurance, data retention time, energy consumption, switching time, integration density, and price of memristive nonvolatile memories has been remarkably improved (depending on the materials used, values up to ~1015 cycles, >10 years, ~0.1 pJ, ~10 ns, 256 gigabits per die, and ≤$0.30 per gigabit have been achieved). ADVANCES As of 2021, memristive memories are being used as standalone memory and are also embedded in application-specific integrated circuits for the Internet of Things (smart watches and glasses, medical equipment, computers), and their market value exceeds $621 million. Recent studies have shown that memristive devices may also be exploited for advanced computation, data security, and mobile communication. Advanced computation refers to the hardware implementation of artificial neural networks by exploiting memristive attributes such as progressive conductance increase and decrease, vector matrix multiplication (in crossbar arrays), and spike timing–dependent plasticity; state-of-the-art developments have achieved >10 trillion operations per second per watt. Data encryption can be realized by exploiting the stochasticity inherent in the memristive effect, which manifests as random fluctuations (within a given range) of the switching voltages/times and state currents. For example, true random number generator and physical unclonable functions produce random codes when exposing a population of memristive devices to an electrical stress at 50% of switching probability (it is impossible to predict which devices will switch because that depends on their atomic structure). Mobile communication can also benefit from memristive devices because they could be employed as 5G and terahertz switches with low energy consumption owing to the nonvolatile nature of the resistive states; the current commercial technology is based on silicon transistors, but they are volatile and consume data both during switching and when idle. State-of-the-art developments have achieved cutoff frequencies of >100 THz with excellent insertion loss and isolation. OUTLOOK Consolidating memristive memories in the market and creating new commercial memristive technologies requires further enhancement of their performance, integration density, and cost, which may be achieved via materials and structure engineering. Market forecasts expect the memristive memories market to grow up to ~$5.6 billion by 2026, which will represent ~2% of the nearly $280 billion memory market. Phase-change and metal-oxide memristive memories should improve switching endurance and reduce energy consumption and variability, and the magnetic ones should offer improved integration density. Ferroelectric memristive memories still suffer low switching endurance, which is hindering commercialization. The figures of merit of memristive devices for advanced computation highly depend on the application, but maximizing endurance, retention, and conductance range while minimizing temporal conductance fluctuations are general goals. Memristive devices for data encryption and mobile communication require higher switching endurance, and two-dimensional materials prototypes are being investigated. Part of Science’s coverage of the 75th anniversary of the discovery of the transistor Fundamental memristive effects and their applications. Memristive devices, in which electrical resistance can be adjusted to two or more nonvolatile levels, can be fabricated using different materials (top row). This allows adjusting their performance to fulfill the requirements of different technologies. Memristive memories are a reality, and important progress is being achieved in advanced computation, security systems, and mobile communication (bottom row).