Jung-Hee Lee

Electrical characteristics of TMAH-surface treated Ni/Au/Al2O3/GaN MIS Schottky structures

The electrical characteristics and reverse leakage mechanisms of tetramethylammonium hydroxide (TMAH) surface-treated Ni/Au/Al2O3/GaN metal-insulator-semiconductor (MIS) diodes were investigated by using the current-voltage (I–V) and capacitance-voltage (C–V) characteristics. The MIS diode was formed on n-GaN after etching the AlGaN in the AlGaN/GaN heterostructures. The TMAH-treated MIS diode showed better Schottky characteristics with a lower ideality factor, higher barrier height and lower reverse leakage current compared to the TMAH-free MIS diode. In addition, the TMAH-free MIS diodes exhibited a transition from Poole-Frenkel emission at low voltages to Schottky emission at high voltages, whereas the TMAH-treated MIS diodes showed Schottky emission over the entire voltage range. Reasonable mechanisms for the improved device-performance characteristics in the TMAH-treated MIS diode are discussed in terms of the decreased interface state density or traps associated with an oxide material and the reduced tunneling probability.

Design and Analysis of Sub-10 nm Junctionless Fin-Shaped Field-Effect Transistors

We design and analyze the n-channel junctionless fin-shaped field-effect transistor (JL FinFET) with 10-nm gate length and compare its performances with those of the conventional bulktype fin-shaped FET (conventional bulk FinFET). A three-dimensional (3-D) device simulations were performed to optimize the device design parameters including the width (Wfin) and height (Hfin) of the fin as well as the channel doping concentration (Nch). Based on the design optimization, the two devices were compared in terms of direct-current (DC) and radio-frequency (RF) characteristics. The results reveal that the JL FinFET has better subthreshold swing, and more effectively suppresses short-channel effects (SCEs) than the conventional bulk FinFET.

Effect of substrate temperature on structural, optical and electrical properties of sputtered NiO-Ag nanocrystalline thin films

NiO-Ag thin films were deposited on Corning 7059 glass substrates by DC reactive magnetron sputtering technique and investigated the substrate temperature (Ts) dependent properties of NiO-Ag thin films. X-ray diffraction results showed that crystalline films can be obtained at high Ts and all films have a preferred crystal growth texture with face centered cubic (fcc) structure and was also confirmed by Raman studies. The grain size, transmittance, band gap, mobility and carrier concentrations were increased with Ts. Room temperature deposited films have an average roughness around 6.9 nm where as increment of Ts resulted in increased roughness up to 14 nm with nanocrystalline morphology. The optimum substrate temperature to obtain NiO-Ag films was found to be 200°C. It was found that with increasing the Ts, resistivity of the films was significantly decreased.

Investigation of InAs/InGaAs/InP Heterojunction Tunneling Field-Effect Transistors

Tunneling field-effect transistors (TFETs) are very applicable to low standby-power application by their virtues of low off-current (Ioff) and small subthreshold swing (S). However, low on-current (Ion) of silicon-based TFETs has been pointed out as a drawback. To improve Ion of TFET, a gate-all-around (GAA) TFET based on III-V compound semiconductor with InAs/InGaAs/InP multiple-heterojunction structure is proposed and investigated. Its performances have been evaluated with the gallium (Ga) composition (x) for In1-xGaxAs in the channel region. According to the simulation results for Ion, Ioff, S, and on/off current ratio (Ion/Ioff), the device adopting In0.53Ga0.47As channel showed the optimum direct-current (DC) performance, as a result of controlling the Ga fraction. By introducing an n-type InGaAs thin layer near the source end, improved DC characteristics and radio-frequency (RF) performances were obtained due to boosted band-to-band (BTB) tunneling efficiency.

GaN junctionless trigate field-effect transistor with deep-submicron gate length: Characterization and modeling in RF regime

Radio-frequency (RF) performances of a gallium nitride (GaN)-based junctionless (JL) trigate field-effect transistor (TGFET), or fin-shaped FET (FinFET), are demonstrated along with RF modeling for the first time. The fabricated GaN JL TGFET had a gate length of 350 nm and had no AlGaN/GaN heterojunction on which conventional high-mobility electron transistors (HEMTs) had been based to have a fully junctionless channel. The device with five fin channels exhibits a maximum drain current of 403 mA/mm and maximum transconductance of 123.6 mS/mm. The maximum cutoff frequency (fT) and maximum oscillation frequency (fmax) are 2.45 and 9.75 GHz, respectively. In order to confirm its potential for high-frequency applications, small-signal modeling has been carried out up to a frequency above the maximum fT.

Effect of spacer dielectrics on performance characteristics of Ge-based tunneling field-effect transistors

In this study, we investigated the effects of dielectric materials for spacers on the performance characteristics of germanium (Ge)-based tunneling field-effect transistors (TFETs). Direct current (DC), radio frequency (RF), and switching performance characteristics were analyzed using various spacer dielectric materials including recent interlayer dielectric (ILD) oxides, silicon dioxide (SiO2), silicon nitride (Si3N4), and hafnium oxide (HfO2). Since spacer dielectrics affect the band bending in the source and drain regions caused by fringing fields, a Ge-based TFET having a low-κ spacer dielectric showed a high current drivability owing to its steep energy-band bending on the source side. At the same time, outstanding switching and RF performance characteristics were achieved by reducing parasitic capacitances when a low-κ spacer dielectric was employed. On this basis, a Ge-based TFET with a low-κ spacer dielectric was designed with a drain underlap for satisfactory control of the ambipolar behaviors and for optimization of RF performance characteristics. It was proven that the drain underlap suppressed ambipolar current characteristics and reduced gate capacitance by minimizing the electric field induced by the gate electrode on the drain-side channel without degradation of current drivability.

Fabrication and Crystal Structure Analysis of a CIGS Solar Cell Absorber by Thermal Annealing of Sputtered CIG Precursors Deposited with a Se Layer

Various Cu(In,Ga)Se2 (CIGS) films were prepared by using in-line sputter for CIG precursor and evaporator for selenization. Deposition parameters are substrate temperature at sputtering process, post annealing temperature (400∼500°C) and annealing method (thermal furnace and high-temperature in-situ XRD). Temperature dependent XRD patterns of the samples were measured with high temperature option and this pattern shows that post annealing at 400∼500°C is necessary for CIGS absorber deposition with no second phase by in-line sputtering. The CIGS film which is annealed at 500°C shows good crystallographic property and there is no second phase like Cu2−xSe.

Fabrication and Characterization of GaN-based Light-emitting Diode (LED) with Triangle-type Structure

This study investigated characteristics of the fabricated triangle-type light-emitting diode (T-LED), conventional triangular LED (CT-LED), and conventional square LED (CS-LED). The T-LED is expected to provide uniform current spreading that leads to high output power, low current crowding, and high light extraction in the lateral direction of the LED device, compared to CT-LED and CS-LED. The light extraction of the T-LED in the lateral direction is much higher than that of the CT-LED and the CS-LED due to enhancement of light emission from the sidewalls of the T-LED.

Growth and Device Performance of AlGaN/GaN Heterostructure with AlSiC Precoverage on Silicon Substrate

A crack-free AlGaN/GaN heterostructure was grown on 4-inch Si (111) substrate with initial dot-like AlSiC precoverage layer. It is believed that introducing the AlSiC layer between AlN wetting layer and Si substrate is more effective in obtaining a compressively stressed film growth than conventional Al precoverage on Si surface. The metal semiconductor field effect transistor (MESFET), fabricated on the AlGaN/GaN heterostructure grown with the AlSiC layer, exhibited normally on characteristics, such as threshold voltage of −2.3u2009V, maximum drain current of 370u2009mA/mm, and transconductance of 124u2009mS/mm.