H.-Y. Kim

Effects of proton irradiation on dc characteristics of InAlN/GaN high electron mobility transistors

The effects of proton irradiation on the dc characteristics of InAlN/GaN high electron mobility transistors were investigated. In this study we used 5 MeV protons with doses varying from 2u2009×u20091011 to 2u2009×u20091015u2009cm−2. The transfer resistance and contact resistivity suffered more degradation as compared to the sheet resistance. With irradiation at the highest dose of 2u2009×u20091015u2009cm−2, both forward- and reverse-bias gate currents were increased after proton irradiation. A negative threshold-shift and reduction of the saturation drain current were also observed as a result of radiation-induced carrier scattering and carrier removal. Devices irradiated with doses of 2u2009×u20091011 to 2u2009×u20091015u2009cm−2 exhibited minimal degradation of the saturation drain current and extrinsic transconductance. These results show that InAlN/GaN high electron mobility transistors are attractive for space-based applications when high-energy proton fluxes are present.

Proton-irradiated InAlN/GaN high electron mobility transistors at 5, 10, and 15 MeV energies

InAlN/GaN high electron mobility transistors (HEMTs) grown on SiC substrates were subjected to 5-15 MeV high energy protons with a fixed 5u2009×u20091015u2009cm−2 fluence. The saturation currents and gate leakage currents of all the proton-irradiated InAlN/GaN HEMTs were degraded. Proton irradiation at lower energy was found to degrade the direct current (DC) current-voltage (I-V) characteristics more severely than higher-energy irradiation, because the energy loss component of the lower energy protons was larger than those of higher-energy protons in the vicinity of the 2-dimensional electron gas conducting channel. Our experimental results were consistent with stopping and range of ions in matter simulation results of the energy deposition profile by the protons.

Electrical Characterization of 5 MeV Proton-Irradiated Few Layer Graphene

Few layer graphene (FLG) samples contacted were irradiated with protons at an energy of 5 MeV and doses up to 2 × 10 15 /cm 2 . The electrical properties of ungated FLG sheets contacted by Pd/Au in a source (S)-drain (D) configuration, including V DS -I DS , V G -I DS , and the hole mobility, were compared before and after proton irradiation. After irradiation, it is observed that the ambipolar conduction of the FLG sheets was changed to a p-type conduction. The field-effect mobility of the hole carriers and the resistance in the graphene sheets greatly decreased because the proton irradiation increased the number of the surface states.

Enhanced voltage-current characteristics of GaN nanowires treated by a selective reactive ion etching

In characterizing the electrical properties of individual NWs (nanowires), the amorphous oxide layer on the surface of NWs is known to limit the electrical conductivity owing to the contact barriers between metal electrodes and NWs. To remove the native oxide layer, a systematic reactive ion etching (RIE) was performed, resulting in a gradual decrease of the diameters of NWs. Voltage-current characteristics of the GaN NW devices treated by tuning the RIE process were improved as reflected by a 1000 times increase in conductance, which was in turn attributed to the removal of the thick (d∼3.5nm) contact barrier formed by the native oxide layer.

Characterization of chemical doping of graphene by in-situ Raman spectroscopy

We explored single-layer graphene and graphene field-effect transistors immersed in nitric acid using in-situ Raman spectroscopy. Two distinct stages were observed in the chemical doping process. The first stage involved blue shifts of the G and 2D peaks, whose saturation occurred rapidly with a time constant in the range of 10–25u2009s depending on the molar concentration of the acid. In the second stage, the intensity of the D peak, which was associated with structural defect formation, increased for a relatively long period of time. Since the major doping effects appeared during the first stage, the optimal doping conditions under which no noticeable structural defect formation occurred can be determined by monitoring the frequency shift. Transient doping concentrations along with structural defect densities were obtained from the Raman peak positions and intensities. We found that the doping-induced shift in the Dirac point in graphene field-effect transistors exhibited a fast response with respect to fre...

Numerical studies on the mixing characteristics of exhaust gas recirculation gases with air, and their dependence on system geometries in four-cylinder engine applications

Abstract To achieve low nitrogen oxide (NO x ) emissions and combustion reliability, the variation in the geometry of the exhaust gas recirculation (EGR) system should be optimized. However, the layout process essentially fixes the position of a conventional EGR system, and modifying the intake manifold geometry tends to be economically unjustifiable for a commercial engine. In the present study, two types of EGR system, namely ‘linked single’ and ‘individual’ EGR systems, are proposed and their performances are evaluated numerically. Parametric studies were performed using a three-dimensional numerical model to assess the mixing uniformity of the intake air and EGR gases. The distribution of the mixed gases into the four runners, which force the mixed gases into the four combustion cylinders, is predicted. Three different geometries of the EGR system are considered: single, side-feed individual, and top-feed individual configurations. These geometries reflect the means by which the EGR gases are supplied into the four cylinders. These three configurations are called cases A, B, and C respectively, and their corresponding detailed geometries are identified as A1 to A9, B1 to B17, and C1 to C10, which give a total of 36 computational runs. It was found that case A5 with a hole angle of 60° yielded the most optimal operating mixing and distribution condition. In general, the single systems were found to be superior to the individual systems, among which the top-feed system tended to be slightly more advantageous for mixing than the side-feed individual system owing to the symmetric supply of the top-feed system. The flow of the throttle-valve system, which controls the mass flowrate of the intake air, was used as the validation case; Hyundai-Kia Motor Co. provided the experimental data, which were compared with the computational results.

Characterization of 5MeV proton-irradiated gallium nitride nanowires

GaN nanowires were irradiated using a cyclotron at 5MeV energy with a fluency of up to 3.38×1015∕cm2 protons. The resistance of the GaN was increased by 95% at a dose of 1.69×1015∕cm2 protons and then 116% at a dose of 3.38×1015∕cm2 protons because of the damage induced by the high energy protons. Cathodoluminescence of the GaN nanowires found a slight broadening of near band-edge emission and a dramatic decrease in the intensity of midgap transitions. These GaN-based nanomaterials have a potential in space technology because of their strong bonding energy compared to other material systems such as silicon and GaAs. Furthermore, the relatively small decrease in resistivity confirms the predicted robustness to proton irradiation of GaN nanowires compared to a GaN thin film.