Seongjae Cho

Bio: Seongjae Cho is an academic researcher from Gachon University. The author has contributed to research in topics: Transistor & Flash memory. The author has an hindex of 22, co-authored 241 publications receiving 2081 citations. Previous affiliations of Seongjae Cho include Stanford University & Seoul National University.

LiteBIRD: A Satellite for the Studies of B-Mode Polarization and Inflation from Cosmic Background Radiation Detection

Abstract: LiteBIRD is a candidate satellite for a strategic large mission of JAXA. With its expected launch in the middle of the 2020s with a H3 rocket, LiteBIRD plans to map the polarization of the cosmic microwave background radiation over the full sky with unprecedented precision. The full success of LiteBIRD is to achieve $\delta r < 0.001$ , where $\delta r$ is the total error on the tensor-to-scalar ratio r. The required angular coverage corresponds to $2 \le \ell \le 200$ , where $\ell $ is the multipole moment. This allows us to test well-motivated cosmic inflation models. Full-sky surveys for 3 years at a Lagrangian point L2 will be carried out for 15 frequency bands between 34 and 448 GHz with two telescopes to achieve the total sensitivity of 2.5 $\upmu $ K arcmin with a typical angular resolution of 0.5$^\circ $ at 150 GHz. Each telescope is equipped with a half-wave plate system for polarization signal modulation and a focal plane filled with polarization-sensitive TES bolometers. A cryogenic system provides a 100 mK base temperature for the focal planes and 2 K and 5 K stages for optical components.

RF Performance and Small-Signal Parameter Extraction of Junctionless Silicon Nanowire MOSFETs

Abstract: This paper presents a radio-frequency (RF) model and extracted model parameters for junctionless silicon nanowire (JLSNW) metal-oxide-semiconductor field-effect transistors (MOSFETs) using a 3-D device simulator. JLSNW MOSFETs are evaluated for various RF parameters such as cutoff frequency fT, gate input capacitance, distributed channel resistances, transport time delay, and capacitance by the drain-induced barrier lowering effect. Direct comparisons of high-frequency performances and extracted parameters are made with conventional silicon nanowire MOSFETs. A non-quasi-static RF model has been used, along with SPICE to simulate JLSNW MOSFETs with RF parameters extracted from 3-D-simulated Y-parameters. The results show excellent agreements with the 3-D-simulated results up to the high frequency of fT.

The LiteBIRD Satellite Mission - Sub-Kelvin Instrument

Abstract: Inflation is the leading theory of the first instant of the universe. Inflation, which postulates that the universe underwent a period of rapid expansion an instant after its birth, provides convincing explanation for cosmological observations. Recent advancements in detector technology have opened opportunities to explore primordial gravitational waves generated by the inflation through “B-mode” (divergent-free) polarization pattern embedded in the cosmic microwave background anisotropies. If detected, these signals would provide strong evidence for inflation, point to the correct model for inflation, and open a window to physics at ultra-high energies. LiteBIRD is a satellite mission with a goal of detecting degree-and-larger-angular-scale B-mode polarization. LiteBIRD will observe at the second Lagrange point with a 400 mm diameter telescope and 2622 detectors. It will survey the entire sky with 15 frequency bands from 40 to 400 GHz to measure and subtract foregrounds. The US LiteBIRD team is proposing to deliver sub-Kelvin instruments that include detectors and readout electronics. A lenslet-coupled sinuous antenna array will cover low-frequency bands (40–235 GHz) with four frequency arrangements of trichroic pixels. An orthomode-transducer-coupled corrugated horn array will cover high-frequency bands (280–402 GHz) with three types of single frequency detectors. The detectors will be made with transition edge sensor (TES) bolometers cooled to a 100 milli-Kelvin base temperature by an adiabatic demagnetization refrigerator. The TES bolometers will be read out using digital frequency multiplexing with Superconducting QUantum Interference Device (SQUID) amplifiers. Up to 78 bolometers will be multiplexed with a single SQUID amplifier. We report on the sub-Kelvin instrument design and ongoing developments for the LiteBIRD mission.

Analyses on Small-Signal Parameters and Radio-Frequency Modeling of Gate-All-Around Tunneling Field-Effect Transistors

Abstract: The small-signal parameters of gate-all-around tunneling field-effect transistors (GAA TFETs) with different gate lengths were extracted and analyzed in terms of their gate capacitance, source-drain conductance, transconductance, distributed channel resistance, and inversion layer length. Because of the unique current drive and inversion layer formation mechanisms of a TFET compared to a conventional MOSFET, the gate-bias dependence values of the primary small-signal parameters of a GAA TFET also differ. Based on understanding these parameters, the high-frequency performances of GAA TFETs were investigated using a technology computer-aided design simulation. A nonquasistatic radio-frequency model was used to extract the small-signal parameters, which were verified up to 100 GHz. The modeling results showed excellent agreement with the Y-parameters up to the cutoff frequency fT.

Resistive switching characteristics of Si3N4-based resistive-switching random-access memory cell with tunnel barrier for high density integration and low-power applications

Abstract: In this letter, a bipolar resistive-switching random-access memory (RRAM) in Ni/Si3N4/SiO2/p+-Si structure and its fabrication process are demonstrated. The proposed device with double-layer dielectrics consisting of Si3N4 layer (5 nm) as a resistive switching and SiO2 (2.5 nm) layer for the tunnel barrier is investigated in comparison with that having a single layer of Si3N4. Double-layer cell shows ultra-low power operation under a compliance current (ICOMP) of 500 nA, which ensures the reset current (IRESET) of sub-1 μA much lower than that of the single-layer cell. Also, large on/off ratio (∼105) has been obtained since the SiO2 layer efficiently suppresses the current in the high-resistance state. Moreover, maximum selectivity in double-layer cell is 122 when 1/2 read bias scheme is applied to the crossbar array. Highly nonlinear I-V characteristics of the double-layer Si3N4-based RRAM cell warrant the realization of selector-free RRAM cell in the crossbar array pursuing higher integration density.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Early Dark Energy can Resolve the Hubble Tension.

Abstract: Early dark energy (EDE) that behaves like a cosmological constant at early times (redshifts z≳3000) and then dilutes away like radiation or faster at later times can solve the Hubble tension. In these models, the sound horizon at decoupling is reduced resulting in a larger value of the Hubble parameter H_{0} inferred from the cosmic microwave background (CMB). We consider two physical models for this EDE, one involving an oscillating scalar field and another a slowly rolling field. We perform a detailed calculation of the evolution of perturbations in these models. A Markov Chain Monte Carlo search of the parameter space for the EDE parameters, in conjunction with the standard cosmological parameters, identifies regions in which H_{0} inferred from Planck CMB data agrees with the SH0ES local measurement. In these cosmologies, current baryon acoustic oscillation and supernova data are described as successfully as in the cold dark matter model with a cosmological constant, while the fit to Planck data is slightly improved. Future CMB and large-scale-structure surveys will further probe this scenario.

A Survey of Neuromorphic Computing and Neural Networks in Hardware.

Abstract: Neuromorphic computing has come to refer to a variety of brain-inspired computers, devices, and models that contrast the pervasive von Neumann computer architecture This biologically inspired approach has created highly connected synthetic neurons and synapses that can be used to model neuroscience theories as well as solve challenging machine learning problems The promise of the technology is to create a brain-like ability to learn and adapt, but the technical challenges are significant, starting with an accurate neuroscience model of how the brain works, to finding materials and engineering breakthroughs to build devices to support these models, to creating a programming framework so the systems can learn, to creating applications with brain-like capabilities In this work, we provide a comprehensive survey of the research and motivations for neuromorphic computing over its history We begin with a 35-year review of the motivations and drivers of neuromorphic computing, then look at the major research areas of the field, which we define as neuro-inspired models, algorithms and learning approaches, hardware and devices, supporting systems, and finally applications We conclude with a broad discussion on the major research topics that need to be addressed in the coming years to see the promise of neuromorphic computing fulfilled The goals of this work are to provide an exhaustive review of the research conducted in neuromorphic computing since the inception of the term, and to motivate further work by illuminating gaps in the field where new research is needed