Jung-Hun Oh

Dongguk University, Seoul, Seoul, South Korea

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Publications (29)28.9 Total impact

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    ABSTRACT: Si photomultipliers with three different guard-ring structures are fabricated, and a detailed comparative study on their device performances is performed. The virtual guard-ring structure shows a high-resolution full width at half maximum in the gamma spectrum and a high breakdown voltage of ~ 66 V but the lowest fill factor of 46.6%-59.8% among the examined structures. The best charge conversion performance, gain, and fill factor (67.1%) are achieved with the trench guard-ring structure. However, this structure shows a low energy resolution, which is supposed to be due to the trench-associated defects. The performance of the N-implantation guard-ring structure is intermediate in most aspects of the device performance compared to the other structures.
    IEEE Electron Device Letters 02/2010; · 2.79 Impact Factor
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    ABSTRACT: An efficient and practical technique for the small-signal modelling of the GaAs-based 0.1-μm metamorphic high electron mobility transistors (MHEMTs) is proposed and applied to realize the W-band (75 ~ 110 GHz) millimeter-wave monolithic integrated circuit (MMIC) amplifiers. The modelling technique adopts the gradient optimizer with the initial values of parameter set, which are determined from the measurements under the cold FET condition avoiding the forward gate-biasing, to improve the modelling accuracy at W-band frequencies. MMIC amplifiers designed and fabricated based on the proposed small-signal modelling method show the gain of ~10 dB at 94 GHz, and the measurements show an excellent agreement with simulation data in our design frequency range.
    01/2010;
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    ABSTRACT: In this study, we have performed both the channel modification of the conventional MHEMT (Metamorphic High Electron Mobility Transistor) and the variation of gate recess width to improve the breakdown and RF characteristics. The modified channel consists of the In x Ga 1− x As and the InP layers. Since InP has lower impact ionization coefficient than In 0.53 Ga 0.47 As, we have adopted the InP-composite channel in the modified MHEMT. Also, the gate recess width is both functions of breakdown and RF characteristic of a HEMT structure. Therefore, we have studied the breakdown and RF characteristic for various gate recess widths in MHEMT. We have compared breakdown characteristic of the InP-composite channel with that of conventional MHEMT. It is shown that on and off state breakdown voltages of the InP-composite channel MHEMT were increased by about 20 and 27%, respectively, compared with the conventional structure. Also, breakdown voltage of the InP-composite channel MHEMT was increased with increasing gate recess width. The f T was increased with decreasing the gate recess width, whereas f max was increased with increasing the gate recess width. Also, we extracted small-signal parameters. It was shown that G d of the InP-composite channel MHEMT is decreased about by 30% compared with the conventional MHEMT. Therefore, the suppression of the impact ionization in the InP-composite channel increases the breakdown voltage and decreases the output conductance.
    IEICE Transactions on Electronics 01/2010; · 0.33 Impact Factor
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    ABSTRACT: We investigated low-temperature SiOx conversion methods for the spin-coated perhydropolysilazane (a diluted dibutyl-ether solution) films prepared on (100) Si substrates under different curing schemes. From the Fourier transform-infrared (FTIR) spectroscopy and refractive index (RI) measurements, conversion to high-density SiOx was observed for the curing methods of dipping the coatings into various aqueous solutions such as H2O2, NH4OH, and deionized water with or without 405-nm ultraviolet irradiation at near room temperature. The SiOx films cured in H2O2 solution at 80 °C for 10 min exhibited a high conversion efficiency for the SiOx network, as observed from FTIR spectra, RI measurement (˜1.46), O/Si stoichiometry (˜1.5), surface smoothness (roughness
    Japanese Journal of Applied Physics 01/2010; 49. · 1.07 Impact Factor
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    ABSTRACT: This paper presents a 94GHz monolithic down-converter with low conversion loss and high local oscillator (LO)-to-RF isolation using the 0.1μm T-gate metamorphic high electron-mobility transistor (MHEMT) technology. The down-converter consists of a one-stage amplifier and a single-balanced mixer based on the high-directivity tandem coupler structure using the air-bridge crossovers, thereby amplifying the RF signals and maximizing the LO-to-RF isolation by using an inherent S12 isolation characteristic of the amplifier and good phase balance of the tandem coupler. The fabricated one-stage amplifier using a 30μm×2 MHEMT shows a small signal gain of 7dB at 94GHz. The single-balanced mixer comprising two 20μm×2 MHEMT Schottky diodes and the tandem coupler with an additional λ/4-length line exhibits the conversion loss less than 7.8dB and the LO-to-RF isolation higher than 30dB in a RF frequency range of 91–96GHz. Two circuits designed both for a 50Ω impedance system are integrated into the down-converter of a 2.6×2.5mm2 chip size, and it shows a low conversion loss of ∼1dB at 94GHz and excellent LO-to-RF isolation above 40dB in a frequency range of 90–100GHz. This is the best isolation among the W-band monolithic down-converters reported to date.
    Current Applied Physics 01/2010; 10(2):395-400. · 2.03 Impact Factor
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    ABSTRACT: We investigate the effects of a multigate-feeding structure on the gate resistance (R<sub>g</sub>) and RF characteristics of the high electron-mobility transistors (HEMTs). In this structure, the increase of R<sub>g</sub> with the gatewidth (W) is minimized; therefore, high maximum frequency of oscillation (f<sub>max</sub>) is achieved. Various numbers of gate feedings (N<sub>gf</sub>) using the air-bridge interconnections are adopted for fabricating the 0.1-mum depletion-mode metamorphic HEMTs. From these structures, we observe great reduction in R<sub>g</sub> with the increase of N<sub>gf</sub>, and their relationship is given by R<sub>g</sub>prop 1/[2middot(N<sub>gf</sub>-1)]<sup>2</sup>, where N<sub>gf</sub>=2,3,4,...; on the other hand, the effects of N<sub>gf</sub> on other small-signal parameters are negligible. Calculated cutoff frequency (f<sub>T</sub>) and f<sub>max</sub> from the extracted small-signal parameters all show good agreement with the measurement results. f<sub>T</sub> is slightly decreased with the increase of N<sub>gf</sub> due to the increase of gate-to-source capacitance. f<sub>max</sub> is, however, greatly increased with N<sub>gf</sub>, and this effect becomes greater at longer total gatewidth (W times number of gate fingers) . This is due to the smaller R<sub>g</sub> at greater N<sub>gf</sub> in the multigate-feeding structure. We propose that this gate-feeding structure provides a very effective way to suppress R<sub>g</sub> and maximize f<sub>max</sub> for the applications of the HEMTs with long W.
    IEEE Transactions on Microwave Theory and Techniques 07/2009; · 2.23 Impact Factor
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    ABSTRACT: We present a high-performance 94-GHz single-balanced monolithic millimeter-wave integrated-circuit (MMIC) mixer using the disk-shaped GaAs Schottky diodes grown on an n/n+ epitaxial structure. Due to the superior characteristics of the GaAs diodes with high diode-to-diode uniformity, the mixer shows a conversion loss of 5.5 dB at 94 GHz, a 1-dB compression point ( P <sub>1</sub> <sub>-dB</sub>) of 5 dBm, and high local-oscillator to radio-frequency isolation above 30 dB in an RF frequency range of 91-97 GHz. To our knowledge, the fabricated mixer shows the best performance in terms of conversion loss at 94 GHz and P <sub>1</sub> <sub>-dB</sub> among the W-band MMIC mixers without amplifier circuits.
    IEEE Electron Device Letters 04/2009; · 2.79 Impact Factor
  • Journal- Korean Physical Society 01/2009; 54(51). · 0.51 Impact Factor
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    ABSTRACT: From a hydrodynamic device simulation for the pseudomorphic high electron mobility transistors (pHEMTs), we observe an increase of maximum extrinsic transconductance and a decrease of source-drain capacitances. This gives rise to an enhancement of the switching speed and isolation characteristics as the upper-to-lower planar-doping ratios (UTLPDR) increase. On the basis of simulation results, we fabricate single-pole-double-throw transmitter/receiver monolithic microwave integrated circuit (MMIC) switches with the pHEMTs of two different UTLPDRs (4:1 and 1:2). The MMIC switch with a 4:1 UTLPDR shows about 2.9 dB higher isolation and approximately 2.5 times faster switching speed than those with a 1:2 UTLPDR.
    Etri Journal 01/2009; 31(3):342-344. · 0.74 Impact Factor
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    ABSTRACT: We present a gate-recess structure for metamorphic high-electron-mobility transistors to enhance the maximum frequency of oscillation . Among the established gate-recess structures, the narrow gate-recess structure shows a degraded , despite superior dc characteristics due to a large gate-to-drain capacitance caused by a small effective gate-to-drain spacing, while the wide gate-recess structure exhibits lower dc characteristics due to the surface effects. To minimize and maintain the dc characteristics of the narrow gate-recess structure, an additional gate-recess is performed for an electrical isolation between the drain side cap layer and drain electrode. We obtain almost the same extrinsic transconductance of from this, while we achieve enhancement of due to reduction of by the increase of effective gate-to-drain spacing compared to the narrow gate-recess structure.
    Journal of The Electrochemical Society 11/2008; 155(12):H987-H990. · 2.59 Impact Factor
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    ABSTRACT: In this paper, millimeter-wave 94 GHz drive amplifiers based on metamorphic high electron mobility transistors (MHEMTs) were designed and fabricated. The fabricated 100 nm gate length MHEMT devices exhibit DC characteristics with a drain current density of 690 mA/mm and an extrinsic transconductance of 770mS/mm. The current gain cutoff frequency (f<sub>T</sub>) and the maximum oscillation frequency (f<sub>max</sub>) are 185 GHz and 230 GHz, respectively. The matching circuit of amplifier was designed using CPW (coplanar wave-guide) transmission line. The fabricated amplifier shows a good S<sub>21</sub> gain of 7.79 dB, an input return loss (S<sub>11</sub>) of -16.5 dB and an output return loss (S<sub>22</sub>) of-15.9 dB.
    Millimeter Waves, 2008. GSMM 2008. Global Symposium on; 05/2008
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    ABSTRACT: In this paper, millimeter-wave broadband cascode amplifiers were designed and fabricated. The 0.1 mum InGaAs/InAlAs/GaAs MHEMT (metamorphic high electron mobility transistor) was fabricated for the cascode amplifier. The DC characteristics of MHEMT are 670 mA/mm of drain current density, 688 mS/mm of maximum transconductance. The current gain cut-off frequency (f<sub>T</sub>) is 139 GHz and the maximum oscillation frequency (f<sub>max</sub>) is 266 GHz. To prevent oscillation of the designed cascode amplifiers, a parallel resistor and capacitor were connected to the drain of common gate device. By using the CPW (coplanar wave guide) transmission line, the cascode amplifier was designed and matched for the broadband characteristics. The designed amplifier was fabricated by the standard MHEMT MMIC (millimeter-wave monolithic IC) process that was developed through this research. As the results of measurement, the amplifier was obtained 3 dB bandwidth of 50.37 GHz from 20.76 to 71.13 GHz. Also, the amplifier represents the S<sub>21</sub> gain with the average 7.07 dB in bandwidth and the maximum gain of 10.3 dB at 30 GHz.
    Millimeter Waves, 2008. GSMM 2008. Global Symposium on; 05/2008
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    ABSTRACT: In this paper, we successfully demonstrated the D-band MMIC amplifiers based on 0.1 mum InGaAs/InAlAs/GaAs MHEMT which has two fingers of 30 mum gate width. The device exhibited a cut-off frequency (fT) of 189 GHz, and a maximum oscillation frequency (fmax) of 334 GHz. The D-band MMIC amplifier exhibited a good RF gains of 7.8 dB at a frequency of 110 GHz. Actually, the D-band MMIC amplifiers exhibited the S21 gains of at 140 GHz in Momentum simulation. We try to measure a frequency range of 110-140 GHz because our measurement equipment can measure in a frequency range of 0.1-110 GHz. Proceeding from these results, we expect satisfactory results in S21 gain performance at 140 GHz.
    01/2008;
  • Journal- Korean Physical Society 01/2008; 53(6). · 0.51 Impact Factor
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    ABSTRACT: We investigate effects of the gate-recess structure on the high-frequency characteristics of the depletion-mode metamorphic high electron mobility transistors (HEMTs). We characterize the dc and radio frequency performances of two different gate-recess structures and perform a comparative study using the hydrodynamic device model simulation and small-signal parameter analysis. The narrow gate-recess structure shows significantly higher dc performances than the wide gate-recess structure, and this phenomenon is due to the presence of the negatively charged surface states on the Schottky barrier layer surface in the wide gate-recess structure. Despite the superior dc characteristics, the narrow gate-recess structure shows more degraded maximum frequency of oscillation of than that of the wide gate-recess structure and almost the same cutoff frequency of . The degraded of the narrow gate-recess structure is attributed to a higher gate-to-drain capacitance . The significant increase of is caused by the reduction of effective gate-to-drain spacing due to the nonlinear electric potential distribution in the gate-to-drain region.
    Journal of The Electrochemical Society 06/2007; 154(7):H541-H546. · 2.59 Impact Factor
  • Journal- Korean Physical Society 01/2007; 51(96). · 0.51 Impact Factor
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    ABSTRACT: We present a uniplanar coplanar-waveguide 3-dB tandem coupler operating at V-band frequencies. The uniplanar structure is monolithically fabricated by using two-section parallel-coupled lines and air-bridge crossovers replacing the conventional multilayer or bonded structures. Due to an optimized tandem structure and non-bonded crossovers minimizing the parasitic components, a maximum coupling of 2.5dB is measured at 62GHz with a 2dB bandwidth of 83%, while a high directivity factor of 33dB is simultaneously obtained at 58–62GHz. Over the entire design frequency range of 30–90GHz, we achieve good phase unbalance of 90±6.0°, as well as return loss and isolation lower than −23 and −16dB, respectively.
    Current Applied Physics 01/2007; 7(5):463-468. · 2.03 Impact Factor
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    ABSTRACT: We investigate the effects of the number of gate fingers (N) and gate width (W) on the high-frequency characteristics of 0.1 mum depletion-mode metamorphic high-electron-mobility transistors (MHEMTs). The extracted gate-to-source capacitance (Cgs), gate-to-drain capacitance (Cgd), intrinsic transconductance (gm,int), and drain conductance (Gds) are proportional to total gate width (wt), whereas intrinsic resistance (Ri) and source resistance (Rs) are inversely proportional to wt. Gate resistance (Rg) linearly increases at various slopes with non-zero gate resistances at zero gate width depending on N. The cutoff frequency ( fT) and maximum frequency of oscillation ( fmax) are calculated using a small-signal model and curve-fitting equations extracted from each small-signal parameter. fT is almost constant; however, fmax is a strong function of Rg1/2 and is affected by both N and wt. A large wt produces a low fmax; however, at a given wt, increasing the number of gate fingers is more efficient than increasing single gate width for maximizing the fmax.
    Japanese Journal of Applied Physics 01/2007; 46:6503-6508. · 1.07 Impact Factor
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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: We present a uniplanar coplanar waveguide 3-dB tandem coupler operating at V-band frequencies. The uniplanar structure is monolithically fabricated by using two-section parallel-coupled lines and air-bridge crossovers replacing the conventional multilayer or the bonded structures. Due to an optimized tandem structure and nonbonded crossovers minimizing the parasitic components, a maximum coupling of 2.5dB is measured at 62GHz with a 2-dB bandwidth of 83%, while a high directivity factor of 33dB is simultaneously obtained at 58-62GHz. Over the entire design frequency range of 30-90GHz, we achieve good phase unbalance of 90±6.0°, return loss, and isolation lower than -23 and -16dB, respectively.
    IEEE Microwave and Wireless Components Letters 05/2006; · 1.78 Impact Factor