Keun Heo

Characterization and Wideband Modeling of Miniaturized LTCC Helical Inductors

We report fabrication of miniaturized low temperature co-fired ceramic helical (3-D) inductors with more than seven layers of helices and the conductor linewidth of 100mum. We achieve the QmaxLeff/A product ranging from 200 to 370. Such high values are possible thanks to the drastic reduction of the active area and still acceptably large Qmax and self-resonant frequency. High frequency characterization of the inductors is performed up to 12GHz, and a new equivalent circuit model considering vertical electromagnetic coupling between conductors is shown to reproduce all the measured characteristics

Engineering of band gap states of amorphous SiZnSnO semiconductor as a function of Si doping concentration.

We investigated the band gap of SiZnSnO (SZTO) with different Si contents. Band gap engineering of SZTO is explained by the evolution of the electronic structure, such as changes in the band edge states and band gap. Using ultraviolet photoelectron spectroscopy (UPS), it was verified that Si atoms can modify the band gap of SZTO thin films. Carrier generation originating from oxygen vacancies can modify the band-gap states of oxide films with the addition of Si. Since it is not easy to directly derive changes in the band gap states of amorphous oxide semiconductors, no reports of the relationship between the Fermi energy level of oxide semiconductor and the device stability of oxide thin film transistors (TFTs) have been presented. The addition of Si can reduce the total density of trap states and change the band-gap properties. When 0.5 wt% Si was used to fabricate SZTO TFTs, they showed superior stability under negative bias temperature stress. We derived the band gap and Fermi energy level directly using data from UPS, Kelvin probe, and high-resolution electron energy loss spectroscopy analyses.

Reliability enhancement of germanium nanowires using graphene as a protective layer: aspect of thermal stability.

We synthesized thermally stable graphene-covered Ge (Ge@G) nanowires and applied them in field emission devices. Vertically aligned Ge@G nanowires were prepared by sequential growth of the Ge nanowires and graphene shells in a single chamber. As a result of the thermal treatment experiments, Ge@G nanowires were much more stable than pure Ge nanowires, maintaining their shape at high temperatures up to 850 °C. In addition, field emission devices based on the Ge@G nanowires clearly exhibited enhanced thermal reliability. Moreover, field emission characteristics yielded the highest field enhancement factor (∼2298) yet reported for this type of device, and also had low turn-on voltage. Our proposed approach for the application of graphene as a protective layer for a semiconductor nanowire is an efficient way to enhance the thermal reliability of nanomaterials.

Quantitative Extraction of Temperature-Dependent Barrier Height and Channel Resistance of a-SIZO/OMO and a-SIZO/IZO Thin-Film Transistors

Temperature (T)-dependent electrical characteristics of thin-film transistors fabricated using oxide-metal-oxide (OMO) and indium-zinc-oxide (IZO) as electrodes and amorphous silicon-doped IZO (a-SIZO) as channel material were studied. The measured data were fit, using a Schottky diode/resistor/Schottky-diode-equivalent circuit model, to obtain the barrier height and the channel resistance. The barrier height coefficients α of the IZO and OMO electrode devices were found to be 1.59 and 1.61 meV/K, respectively. The T-dependent resistivity of the a-SIZO channel material was consistent with the variable range hopping conduction mechanism.

Temperature-Dependent Electrical Characterization of Amorphous Indium Zinc Oxide Thin-Film Transistors

In this study, N-type amorphous indium zinc oxide thin-film transistors are fabricated and temperature-dependent electrical characteristics in the range of 170–295 K are analyzed through experimental measurements and using an equivalent-circuit model. In this model, thermionic field emission for reverse bias and a thermionic emission mechanism for forward bias are applied. The barrier height coefficient of a contact region between the channel and the Ti/Au electrode is 1.26 meV/K, and the resistance of the channel material decreases at a rate of

Stable and Reversible Triphenylphosphine-Based n-Type Doping Technique for Molybdenum Disulfide (MoS2)

- 0.39 \Omega \cdot {\mathrm{ K}}^{-1}

Correction to “Quantitative Extraction of Temperature-Dependent Barrier Height and Channel Resistance of a-SIZO/OMO and a-SIZO/IZO Thin-Film Transistors” [Feb 13 247-249]

at various temperatures. The obtained energy level is experimentally confirmed through a Kelvin probe measurement. In addition, the simulation results of the channel resistance successfully describe the Arrhenius behavior of the drain current and the Mott variable-range hopping conduction mechanism in a low-temperature regime below 230 K.

Large-scale fabrication of 2-D nanoporous graphene using a thin anodic aluminum oxide etching mask.

A highly stable and reversible n-type doping technique for molybdenum disulfide (MoS2) transistors and photodetectors is developed in this study. This doping technique is based on triphenylphosphine (PPh3) and significantly improves the performance of MoS2 transistor and photodetector devices in terms of the on/off-current ratio (8.72 × 104 → 8.70 × 105), mobility (12.1 → 241 cm2/V·s), and photoresponsivity ( R) (2.77 × 103 → 3.92 × 105 A/W). The range of doping concentrations is broadly distributed between 1.56 × 1011 and 9.75 × 1012 cm-2 and is easily controlled by adjusting the temperature at which the PPh3 layer is formed. In addition, this doping technique provides two interesting properties that have not been reported for previous molecular doping techniques: (i) high stability leading to small variations in device performance after exposure to air for 14 days (on-current: 1.34% and photoresponsivity: 1.58%) and (ii) reversibility enabling the repetitive formation and removal of PPh3 molecules (doping and dedoping).

LTCC bandpass filter using 3D coupled helical inductors

In the above paper (ibid., vol. 34, no. 2, pp. 247-249, Feb. 2013), the information for corresponding authors is not correctly indicated. Authors S. Kim, S. Y. Lee, and S. W. Hwang are the corresponding authors for this paper. Their information can be found at the bottom of this page.