Bok-Hyung Lee

Dongguk University, Seoul, Seoul, South Korea

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Publications (19)9.21 Total impact

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    ABSTRACT: AlGaN/GaN heterostructure was grown on semi-insulating 6H–SiC substrate. The effect of the thickness of the initial AlN buffer layer on the crystalline quality and the stress of the grown GaN layer were investigated. The semi-insulating characteristic of the undoped GaN layer, which is very important for obtaining low device leakage current, was analyzed by photoluminescence measurement. Modulation doping of Si during the growth of AlGaN barrier layer was also introduced to increase the concentration of the two-dimensional electron gas density and hence to improve the device performance. The fabricated AlGaN/GaN heterostructure field effect transistor with gate length of 0.2 μm and SiO2 passiviation layer exhibited good small-signal characteristics such as current gain cut-off frequency of 47 GHz and the maximum oscillation frequency of 121 GHz.
    Journal of Crystal Growth 01/2014; 395:5–8. · 1.55 Impact Factor
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    ABSTRACT: In this paper, a passive load-pull using a GaN HEMT (Gallium Nitride High Electron Mobility Transistor) bare-chip in X-band is presented. To obtain operation conditions that characteristic change by self-heating was minimized, pulsed drain bias voltage and pulsed-RF signal is employed. An accuracy impedance matching circuits considered parasitic components such as wire-bonding effect at the boundary of the drain is accomplished through the use of a electro-magnetic simulation and a circuit simulation. The microstrip line length-tunable matching circuit is employed to adjust the impedance. The measured maximum output power and drain efficiency of the pulsed load-pull are 42.46 dBm and 58.7%, respectively, across the 8.5-9.2 GHz band.
    The Journal of The Korea Institute of Intelligent Transport Systems. 01/2011; 10(1).
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    ABSTRACT: In this paper, we present a high performance 94 GHz MMIC diode mixer using a 100-nm metamorphic high electron mobility transistor (MHEMT) diode and coplanar waveguide (CPW) 3-dB tandem coupler. A novel single-balanced structure of MHEMT diode mixer was proposed in this work, and 3-dB tandem coupler with the air-bridge structure was used for broadband LO-RF isolation. The fabricated mixer has LO-RF isolation, greater than 19 dB, in 15 GHz bandwidth of 82-97 GHz. The good conversion loss of 14.8 dB was measured at 94 GHz.
    Microwave Conference, 2007. APMC 2007. Asia-Pacific; 01/2008
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    ABSTRACT: We report a high conversion gain millimeter-wave monolithic IC (MMIC) quadruple subharmonic mixer adopting a novel cascode harmonic generator for an improved conversion gain. The proposed cascode harmonic generator produces 2.9 dB higher fourth harmonic output powers on the average compared with those of the conventional multiplier. The fabricated mixer shows a high conversion gain of 3.4 dB at a local oscillator (LO) power of 13 dBm. High isolations were obtained the LO-to-intermediate frequency (IF) of -53.6 dB and the LO-to-radio frequency (RF) of -46.2 dB, at a frequency of 14.5 GHz. The conversion gain achieved in this work is the best performance among the millimeter-wave MMIC subharmonic mixers reported to date.
    Japanese Journal of Applied Physics 04/2007; 46(4A):1452-. · 1.07 Impact Factor
  • Journal- Korean Physical Society 01/2007; 51(96). · 0.51 Impact Factor
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    ABSTRACT: In this paper, we report our recent research works on the W-band waveguide-to-CPW transitions for various MIMIC applications. For this, the transitions operating in a frequency range of 85 ~ 100 GHz are designed, fabricated and characterized. The designed waveguide-to-CPW transition is optimized to achieve low loss by using an EM field solver of HFSS. From the measurement, an insertion of -2.2 dB and a return loss of -23.86 dB, respectively, were obtained at 94 GHz. The average insertion loss of waveguide-to-CPW back-to-back transition is -2.5 dB in a frequency range of 85 ~ 100 GHz.
    Communications and Electronics, 2006. ICCE '06. First International Conference on; 11/2006
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    ABSTRACT: We report a low conversion loss and high local oscillator (LO)-to-RF isolation 94-GHz monolithic-microwave integrated-circuit (MMIC) active down converter using 0.1-mum InGaAs/InAlAs/GaAs metamorphic high electron-mobility transistor (MHEMT). The fabricated MMIC active down converter employs a one-stage MHEMT amplifier in the RF port of the active down converter, thereby amplifying the RF signal and improving the LO-to-RF isolation by using an inherent S<sub>12</sub> isolation characteristic. The fabricated MMIC active down converter shows an excellent conversion loss of 6.7 dB at an LO power of 10 dBm and high LO-to-RF isolations of 21 plusmn 0.5 dB in a frequency range from 93.7 to 94.3 GHz. High dc and RF performances of the MHEMT used for the active down converter are due to the optimized epitaxial and device structure, and a maximum transconductance of 760 mS/mm, a current gain cutoff frequency of 195 GHz, and a maximum oscillation frequency of 391 GHz were measured. A active down-converter module is assembled by mounting the active down-converter chip on a jig with low-loss transition structure between the coplanar waveguide and waveguide. The fabricated active down-converter module shows a good conversion loss of 10.9 dB and a very high LO-to-RF isolation of 27.5 dB at 94.03 GHz
    IEEE Transactions on Microwave Theory and Techniques 07/2006; · 2.23 Impact Factor
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    ABSTRACT: For low LO power and broadband characteristics of the mixer, an antipodal fin-line to microstrip transition for operation in the 94 GHz frequency has been designed and characterized. Back to back transitions fabricated on soft substrates have been measured and simulated to verify their behavior. A single balanced fin-line mixer was designed and fabricated. In the mixer, a wideband fin-line to coplanar waveguide 180° balun and low pass filter were used. Conversion loss is less than 10 dB at LO power of 6 dBm.
    Microwave Conference Proceedings, 2005. APMC 2005. Asia-Pacific Conference Proceedings; 01/2006
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    ABSTRACT: We fabricate single-ended resistive W-band millimeter-wave monolithic IC (MIMIC) mixers based on 0.1μm InGaAs/InAlAs/GaAs metamorphic HEMT technology. The mixers show good characteristics in linearity and LO-RF isolation, and the mixers with IF amplifiers exhibit a conversion loss of ∼0.4dB, which is an improvement of ∼7.8dB compared to that of the mixers without IF amplifiers. We obtain P-1dB of 10 and 9dBm from the mixers with and without the IF amplifiers, respectively.
    Current Applied Physics - CURR APPL PHYS. 01/2006; 6(5):821-826.
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    ABSTRACT: We present the high performance 94 GHz resistive mixer including IF amplifier using 0.1 mum metamorphic HEMT. For 94 GHz resistive mixer, the metamorphic HEMT of excellent characteristics was developed. The circuit performance of resistive mixer with IF amplifier shows conversion gain of 1.3 dB at LO power of 7 dBm. In this work, we fabricated the resistive mixer which has high performance using metamorphic HEMT. The fabricated resistive mixer shows the superior conversion gain than that of previous reported results
    01/2006;
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    ABSTRACT: To perform a comparative study, we experimented on two differential epitaxial structures, the conventional metamorphic high-electron-mobility-transistor (MHEMT) using the InAlAs&sol;InGaAs&sol;InAlAs structure and the InP-composite-channel MHEMT adopting the InAlAs&sol;InGaAs&sol;InP structure. Compared with the conventional MHEMT, the InP-composite-channel MHEMT shows improved breakdown performance; more than approximately 3.8 V. This increased breakdown voltage can be explained by the lower impact ionization coefficient of the InP-composite-channel MHEMT than that of the conventional MHEMT. The InP-composite-channel MHEMT also shows improved Radio Frequency characteristics of S 21 gain of approximately 4.35 dB at 50 GHz, and a cutoff frequency ( f T ) and a maximum frequency of oscillation ( f max ) of approximately 124 GHz and 240 GHz, respectively, were obtained. These are due to decreases in g o and g m
    01/2006;
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    ABSTRACT: In this paper, millimeter-wave coplanar high gain and wideband cascode amplifiers based on metamorphic high electron mobility transistor (MHEMT) were designed and fabricated. The fabricated 100 nm gate length MHEMT devices exhibit DC characteristics with a drain current density of 471 mA/mm and an extrinsic transconductance of 845 mS/mm. The current gain cutoff frequency (fT) and the maximum oscillation frequency (fmax) are 193 GHz and 325 GHz, respectively. The matching circuit of cascode amplifier was designed for wideband characteristics using CPW (coplanar waveguide) transmission line. The one-stage amplifier showed a very wide 3 dB bandwidth of 37 GHz from 31.3 GHz to 68.3 GHz. The average S21 gain was 9.7 dB in band, with the maximum gain of 11.3 dB at 40 GHz. The two-stage amplifier had a 3 dB bandwidth of 29.5 GHz from 32.5 GHz to 62.0 GHz. The two-stage amplifier showed an excellent gain characteristic with average S21 gain of 20.4 dB in band and the maximum gain of 22.3 dB at 36.5 GHz. To our knowledge, these results have higher gain-per-stage with wider bandwidth than some other millimeter-wave amplifiers
    01/2006;
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    ABSTRACT: We report high switching performance of 0.1-μm metamorphic high-electron mobility transistors (HEMTs) for microwave/millimeter-wave monolithic integrated circuit (MMIC) resistive mixer applications. Very low source/drain resistances and gate capacitances, which are 56 and 31% lower than those of conventional pseudomorphic HEMTs, are due to the optimized epitaxial and device structure. Based on these high-performance metamorphic HEMTs, a 94-GHz MMIC resistive mixer was designed and fabricated, and a very low conversion loss of 8.2 dB at a local oscillator power of 7 dBm was obtained. This is the best performing W-band resistive field-effect transistor mixer in terms of conversion loss utilizing GaAs-based HEMTs reported to date.
    IEEE Electron Device Letters 11/2005; · 2.79 Impact Factor
  • Bok-Hyung Lee, Sam-Dong Kim, Jin-Koo Rhee
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    ABSTRACT: We examined the effects of gate recess process conditions on the electrical characteristics of 0.1-mum-gate-length metamorphic high-electron-mobility transistors (MHEMTs) by the comparative analysis of small-signal parameters. When the wide-gate-recess method is adopted, significant reductions in gate-to-drain conductance and gate-to-drain capacitance were obtained compared with those obtained by the of narrow-gate-recess method. These differences in small-signal parameters are due to the removal of the entire n+ cap layer and corresponding dissimilarity in gate structure when the wide-gate-recess method is used. The wide-gate-recess method produced ˜1/2 drain-source saturation current and extrinsic transconductance compared with the narrow-gate-recess method. In contract to the DC performances, a markedly enhanced S21 gain of 3.5 dB and an fmax of 447 GHz were obtained from the MHEMTs processed by the wide-gate-recess method. This high fmax is responsible for the proper selection of the gate recess method for what and is one of the best data thus far reported for 0.1-mum-gate-length MHEMTs.
    Japanese Journal of Applied Physics 01/2004; 43:1914-1918. · 1.07 Impact Factor
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    ABSTRACT: We examine the effects of device scaling in both vertical and lateral dimensions for the metamorphic high electron mobility transistors (MHEMTs) on the DC and millimeter-wave electrical performances by using a hydrodynamic transport model. The well-calibrated hydrodynamic simulation for the sub-0.1-μm offset Γ-gate In0.53Ga0.47As/In0.52Al0.48As MHEMTs shows a reasonable agreement with the electrical characteristics measured from the fabricated 0.1 μm devices. We have calibrated all the parameters using the measurement data with various physical considerations to take into account the sophisticated carrier transport physics in sub-0.1-μm devices. Being simulated with these calibrated parameters, the optimum device performance is obtained at a source-drain spacing of 2 μm, a gate length of 0.05 μm, a barrier thickness of 10 nm and a channel thickness of 12 nm.
    Microelectronics Journal. 01/2004;
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    ABSTRACT: A high conversion gain receiver chip set for Q-band millimeter-wave wireless communication systems is designed and fabricated by using 0.1 μm GaAs-based pseudomorphic high electron mobility transistors (PHEMTs) and the coplanar waveguide (CPW) library. The fabricated receiver chip set consists of a subharmonic mixer circuit and a low noise amplifier. From the device characterization, the subharmonic mixer shows a maximum conversion gain of ∼4.8 dB at an RF frequency of 40 GHz for a local oscillation (LO) power of 10 dBm at 17.5 GHz. The subharmonic mixer also exhibits a high degree of isolation characteristic of -35.8 dB for LO-to-IF and -40.5 dB for LO-to-RF, respectively, at a LO frequency of 17.5 GHz. The low noise amplifier shows a S<sub>21</sub> gain of ∼25.6 dB at a RF frequency of 40 GHz. Due to the high performances of the circuits, the fabricated receiver chip set produces a high conversion gain of 30.4 dB for the Q-band application purpose.
    TENCON 2003. Conference on Convergent Technologies for Asia-Pacific Region; 11/2003
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    ABSTRACT: In this paper, we have designed and fabricated high conversion gain Q-band active sub-harmonic mixers for a receiver of millimeter wave wireless communication systems. The fabricated active sub-harmonic mixer uses 2nd harmonic signals of a low local oscillator (LO) frequency. The fabricated mixer was successfully integrated by using 0.1 µm GaAs pseudomorphic high electron mobility transistors (PHEMTs) and coplanar waveguide (CPW) structures. From the measurement, it shows that maximum conversion gain of 4.8 dB has obtained at a RF frequency of 40 GHz for 10 dBm LO power of 17.5 GHz. Conversion gain from the fabricated sub-harmonic mixer is one of the best reported thus far. And a phase noise of the 2nd harmonic was obtained -90.23 dBc/Hz at 100 kHz offset. The active sub-harmonic mixer also ensure a high degree of isolations, which are -35.8 dB from LO-to-IF and -40.5 dB from LO-to-RF, respectively, at a LO frequency of 17.5 GHz.
    01/2003; 3.
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    ABSTRACT: We investigated the effects of recovery schemes on the characteristics of GaN Schottky diodes damaged by inductively coupled plasma etching. The recovery schemes included plasma exposure, plasma plus annealing at 600°C in and annealing at 700°C in Ga-deficient GaN surface layers were observed after etching the GaN, and the Ga/N stoichiometry at the surface was most strongly affected by the flow rate among the process variables. The samples annealed at 700°C showed a clear improvement in diode characteristics and a complete restoration of the Ga/N ratio from ca. 0.87 to ca. 1.0 at the surface layers, as measured by Auger electron spectroscopy. For all other damage-recovery schemes, the samples showed no significant enhancement in diode characteristics and incomplete restoration of Ga/N stoichiometry at the surface layers. © 2001 The Electrochemical Society. All rights reserved.
    Journal of The Electrochemical Society. 09/2001; 148(10):G592-G596.
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    ABSTRACT: In this work,,we present the characteristics of the 0.1 ໃ gate length InGaAs/InAlAs/GaAs metamorphic high electron mobility transistors (MHEMTs). The MHEMTs with ī-shaped off-set gates (70 ໃ width and 2 fingers)were fabricated using the double heterostructure epitaxial structure and characterized through the DC,Noise and RF measurements.Measured channel current density and transconductance (gm )were 442 mA/mm and 409 mS/mm,respectively.Noise characteristics were measured in the frequency range from 50 GHz to 61 GHz,and show 1.8 dB at 50GHz.From RF measurements,154 and 433 GHz were obtained for the cut- off frequency (fT )and maximum frequency of oscillation (fmax ),respectively.A superior fmax of 433 GHz achieved in the work is one of the first reports among the fabricated 0.1 ໃ gate length MHEMTs.