Woojun Lee

Bio: Woojun Lee is an academic researcher from Sogang University. The author has contributed to research in topics: Medicine & Semiconductor. The author has an hindex of 3, co-authored 5 publications receiving 400 citations.

Hetero-Gate-Dielectric Tunneling Field-Effect Transistors

Abstract: A tunneling field-effect transistor (TFET) is considered one of the most promising alternatives to a metal-oxide-semiconductor field-effect transistor due to its immunity to short-channel effects. However, TFETs have suffered from low on-current, severe ambipolar behavior, and gradual transition between on- and off -states. To address those issues, the authors have proposed hetero-gate-dielectric TFETs. The proposed device enhances on-current, suppresses ambipolar behavior, and makes abrupt on-off transition by replacing the source-side gate insulator with a high-k material, which induces a local minimum of the conduction band edge at the tunneling junction.

Influence of Inversion Layer on Tunneling Field-Effect Transistors

Abstract: The influence of inversion layer on tunneling field-effect transistors (TFETs) has been investigated. Simulation results show that drain current (ID) saturation is related to inversion layer formation. Surface channel potential (Ψ) pinning due to the inversion layer formation makes ID less sensitive to the gate voltage. Also, it has been shown that most of inversion carriers of TFETs are thermally injected from the drain. Inversion carriers supplied from the source by band-to-band tunneling are negligible.

A Novel Capacitorless 1T DRAM Cell for Data Retention Time Improvement

Abstract: This paper proposes a silicon-with-partially-insulating-layer-on-silicon-on-insulator (SISOI) one-transistor dynamic random access memory (1T DRAM) cell to increase data retention time. A conventional 1T DRAM cell has a data retention problem because it stores holes in an SOI layer, which is not separated from the source/drain region. However, the proposed SISOI 1T DRAM cell can keep holes electrically separated from the source/drain region, which leads to the increase of data retention time.

Morphological Control of 2D Hybrid Organic-Inorganic Semiconductor AgSePh.

Abstract: Silver phenylselenolate (AgSePh) is a hybrid organic-inorganic two-dimensional (2D) semiconductor exhibiting narrow blue emission, in-plane anisotropy, and large exciton binding energy. Here, we show that the addition of carefully chosen solvent vapors during the chemical transformation of metallic silver to AgSePh allows for control over the size and orientation of AgSePh crystals. By testing 28 solvent vapors (with different polarities, boiling points, and functional groups), we controlled the resulting crystal size from <200 nm up to a few μm. Furthermore, choice of solvent vapor can substantially improve the orientational homogeneity of 2D crystals with respect to the substrate. In particular, solvents known to form complexes with silver ions, such as dimethyl sulfoxide (DMSO), led to the largest lateral crystal dimensions and parallel crystal orientation. We perform systematic optical and electrical characterizations on DMSO vapor-grown AgSePh films demonstrating improved crystalline quality, lower defect densities, higher photoconductivity, lower dark conductivity, suppression of ionic migration, and reduced midgap photoluminescence at low temperature. Overall, this work provides a strategy for realizing AgSePh films with improved optical properties and reveals the roles of solvent vapors on the chemical transformation of metallic silver.

Light Emission in 2D Silver Phenylchalcogenolates.

Abstract: Silver phenylselenolate (AgSePh, also known as "mithrene") and silver phenyltellurolate (AgTePh, also known as "tethrene") are two-dimensional (2D) van der Waals semiconductors belonging to an emerging class of hybrid organic-inorganic materials called metal-organic chalcogenolates. Despite having the same crystal structure, AgSePh and AgTePh exhibit a strikingly different excitonic behavior. Whereas AgSePh exhibits narrow, fast luminescence with a minimal Stokes shift, AgTePh exhibits comparatively slow luminescence that is significantly broadened and red-shifted from its absorption minimum. Using time-resolved and temperature-dependent absorption and emission microspectroscopy, combined with subgap photoexcitation studies, we show that exciton dynamics in AgTePh films are dominated by an intrinsic self-trapping behavior, whereas dynamics in AgSePh films are dominated by the interaction of band-edge excitons with a finite number of extrinsic defect/trap states. Density functional theory calculations reveal that AgSePh has simple parabolic band edges with a direct gap at Γ, whereas AgTePh has a saddle point at Γ with a horizontal splitting along the Γ-N1 direction. The correlation between the unique band structure of AgTePh and exciton self-trapping behavior is unclear, prompting further exploration of excitonic phenomena in this emerging class of hybrid 2D semiconductors.

Novel Attributes of a Dual Material Gate Nanoscale Tunnel Field-Effect Transistor

Abstract: In this paper, we propose the application of a dual material gate (DMG) in a tunnel field-effect transistor (TFET) to simultaneously optimize the on-current, the off-current, and the threshold voltage and also improve the average subthreshold slope, the nature of the output characteristics, and immunity against the drain-induced barrier lowering effects. We demonstrate that, if appropriate work functions are chosen for the gate materials on the source side and the drain side, the TFET shows a significantly improved performance. We apply the technique of DMG in a strained double-gate TFET with a high-k gate dielectric to show an overall improvement in the characteristics of the device, along with achieving a good on-current and an excellent average subthreshold slope. The results show that the DMG technique can be applied to TFETs with different channel materials, channel lengths, gate-oxide materials, gate-oxide thicknesses, and power supply levels to achieve significant gains in the overall device characteristics.

Controlling Ambipolar Current in Tunneling FETs Using Overlapping Gate-on-Drain

Abstract: In this paper, we have demonstrated that overlapping the gate on the drain can suppress the ambipolar conduction, which is an inherent property of a tunnel field effect transistor (TFET). Unlike in the conventional TFET where the gate controls the tunneling barrier width at both source-channel and channel-drain interfaces for different polarity of gate voltage, overlapping the gate on the drain limits the gate to control only the tunneling barrier width at the source-channel interface irrespective of the polarity of the gate voltage. As a result, the proposed overlapping gate-on-drain TFET exhibits suppressed ambipolar conduction even when the drain doping is as high as \(1 \times 10^{19}\) cm \(^{-3}\) .

An analysis on the ambipolar current in Si double-gate tunnel FETs

Abstract: This work presents a study on the influence of the design parameters on the ambipolar current (IAMB) of the Tunnel Field Effect Transistors (TFETs). Using numerical device simulations, IAMB is reduced progressively by underlapping the gate and the drain, by using low-k spacers and by placing the contacts in the top and bottom configuration. It is explained that a structure with top and bottom contacts leads to the field distribution inside the drain spacer, limiting the ambipolar current through the device. A TFET structure with ultra-low ambipolar current, totally independent of the gate voltage, is obtained by combining the layout of top and bottom contacts with low-k spacers. The scaling of the Silicon (Si) TFET is limited by the length of the drain spacer that cannot be scaled beyond a minimal limit without increasing IAMB to undesired high values.

Interfacial Charge Analysis of Heterogeneous Gate Dielectric-Gate All Around-Tunnel FET for Improved Device Reliability

Abstract: In this paper, we have investigated device reliability by studying the impact of interface traps, both donor (positive interface charges) and acceptor (negative interface charges), present at the Si/SiO2 interface, on analog/RF performance and linearity distortion analysis of heterogeneous-gate-dielectric gate-all-around tunnel FET (HD-GAA-TFET), which is used to enhance the tunneling current of TFET. Various figures of merit such as cutoff frequency $f_{{T}}$ , maximum oscillation frequency $f_{\max}$ , transconductance frequency product, higher order transconductance coefficients $({g}_{{m}1}, {g}_{{m}3})$ , VIP2, VIP3, IIP3, IMD3, zero crossover point, and 1-dB compression point have been investigated, and the results obtained are simultaneously compared with a gate-all-around TFET (GAA-TFET). Simulation results indicate that HD-GAA-TFET is more immune toward the interface trap charges present at the Si/SiO2 interface than the GAA TFET and hence can act as a better candidate for low power switching applications. All simulations have been done on an ATLAS device simulator.

A Dielectric-Modulated Tunnel-FET-Based Biosensor for Label-Free Detection: Analytical Modeling Study and Sensitivity Analysis

Abstract: In this paper, an analytical model for a p-n-p-n tunnel field-effect transistor (TFET) working as a biosensor for label-free biomolecule detection purposes is developed and verified with device simulation results. The model provides a generalized solution for the device electrostatics and electrical characteristics of the p-n-p-n-TFET-based sensor and also incorporates the two important properties possessed by a biomolecule, i.e., its dielectric constant and charge. Furthermore, the sensitivity of the TFET-based biosensor has been compared with that of a conventional FET-based counterpart in terms of threshold voltage (Vth) shift, variation in the on-current (Ion) level, and Ion/Ioff ratio. It has been shown that the TFET-based sensor shows a large deviation in the current level, and thus, change in Ion can also be considered as a suitable sensing parameter. Moreover, the impacts of device parameters (channel thickness and cavity length), process variability, and process-induced damage on the sensitivity of the biosensor have also been discussed.