Seok-Gyu Choi

Dongguk University, Sŏul, Seoul, South Korea

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Publications (17)9.32 Total impact

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    ABSTRACT: InP is considered as the most promising material for millimeter-wave laser-diode applications owing to its superior noise performance and wide operating frequency range of 75–110 GHz. In this study, we demonstrate the fabrication of InP Gunn diodes with a current-limiting structure using rapid thermal annealing to modulate the potential height formed between an n-type InP active layer and a cathode contact. We also explore the reverse current characteristics of the InP Gunn diodes. Experimental results indicate a maximum anode current and an oscillation frequency of 200 mA and 93.53 GHz, respectively. The current–voltage characteristics are modeled by considering the Schottky and ohmic contacts, work function variations, negative differential resistance (NDR), and tunneling effect. Although no direct indication of the NDR is observed, the simulation results match the measured data well. The modeling results show that the NDR effect is always present but is masked because of electron emission across the shallow Schottky barrier.
    Journal of Nanoscience and Nanotechnology 07/2015; 15(7):5148-5150. DOI:10.1166/jnn.2015.10370
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    ABSTRACT: We have proposed a new tuning method of 94 GHz waveguide voltage controlled oscillator (VCO). We have designed and fabricated a varactor-tuned 94 GHz second harmonic VCO. We discuss a new method that a simple and effective tuning method increases greatly the bandwidth of the VCO with high output power and operating frequency simultaneously for the FMCW radar system by applying a controlled impact to the Gunn diode. The VCO employs a commercially available GaAs Gunn diode in packaged form and a GaAs Gunn diode mounted in a waveguide cavity. Also, we adapted MINT's GaAs Gunn diode.
    Millimeter Waves (GSMM), 2012 5th Global Symposium on; 01/2012
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    ABSTRACT: We present a 94 GHz MMIC single balanced mixer using the branch line couplers and 0.1 μm GaAs-based metamorphic high electron mobility transistors (MHEMTs) for FMCW radar sensor application. The mixer was designed in a resistive structure because of no drain bias and low DC currents. Two single ended mixers in the fabricated MMIC single balanced mixer share gate bias circuits. The fabricated mixer shows a conversion loss of 14.7 dB at 94 GHz, and the LO to RF isolation of 34.2 ~ 35.2 dB in LO frequency range of 93.675 ~ 94.275 GHz.
    Millimeter Waves (GSMM), 2012 5th Global Symposium on; 01/2012
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    ABSTRACT: We propose a simple, yet highly effective tuning method to increase the bandwidth of a waveguide voltage controlled oscillator (VCO) by applying controlled mechanical pressures to the packaged Gunn diode mounted in the waveguide cavity resonator. When we applied this method to a varactor-tuned, second-harmonic 94 GHz VCO, the bandwidth was doubled to 1 GHz with an output power over 14.8 dBm, which is suitable for frequency-modulated continuous-wave radar sensor applications.
    IEEE Microwave and Wireless Components Letters 04/2011; 21(3-21):154 - 156. DOI:10.1109/LMWC.2010.2103933
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    ABSTRACT: In this study, we fabricated a 94-GHz transceiver module for a millimeter-wave (MMW) frequency modulation continuous wave (FMCW) radar sensor. The transceiver modules consist of a waveguide voltage-controlled oscillator (VCO) and Rx module using a single balanced mixer. We designed a mixer with a conversion loss of 6.4 dB, without using an amplifier. Also, the waveguide VCO consisted of an InP Gunn diode, a varactor diode, two bias posts with LPF, and a Magic Tee for the MMW radar transceiver. The fabricated VCO has a tuning range of 1280 MHz by a varactor bias of 0~20 V, 1.69% linearity range of 680 MHz, and current consumption of 154 to 157 mA. The completed module has a good conversion loss of 10.6 dB with an LO power of 11.4 dBm at 94 GHz. With this RF and LO input power, the conversion loss was maintained between 10.2-11.5 dB in the RF frequency range of 93.67-94.95 GHz.
    IEEE Sensors Journal 03/2011; 11(2-11):370 - 376. DOI:10.1109/JSEN.2010.2057419
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    ABSTRACT: In this paper, we present a transceiver module, which operates at the center frequency of 94 GHz, for a frequency modulated continues wave (FMCW) radar. A transmitter part consists of a waveguide voltage controlled oscillator (VCO) and a magic tee for 94 GHz, and a receiver part is essentially a Schottky diode mixer chip mounted on an Al2O3 sapphire substrate. The fabricated transceiver module has good RF characteristics such as wide linear band with low error late (480 MHz at 1.45 %), high output power through a magic tee (over 11.43 dBm), and good conversion loss at the IF frequency of 500 MHz (10.8 to 11.8 dB).
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    ABSTRACT: In this paper, a 94 GHz microwave monolithic integrated circuit (MMIC) single balanced resistive mixer affording high LO-to-RF isolation was designed without an IF balun. The single balanced resistive mixer, which does not require an external IF balun, was designed using a 0.1 μm InGaAs/InAlAs/GaAs metamorphic high electron mobility transistor (HEMT). The designed MMIC single balanced resistive mixer was fabricated using the 0.1 μm MHEMT MMIC process. From the measurement, conversion loss of the single balanced resistive mixer was 14.7 dB at an LO power of 10 dBm. The P1 dB (1 dB compression point) values of the input and output were 10 dBm and −5.3 dBm, respectively. The LO-to-RF isolation of the single balanced resistive mixer was −35.2 dB at 94.03 GHz. The single balanced resistive mixer in this work provided high LO-to-RF isolation without an IF balun.
    Microelectronics Journal 10/2010; 41(10-41):627-631. DOI:10.1016/j.mejo.2010.06.012
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    ABSTRACT: In this work, we have designed and fabricated the GaAs Gunn diodes for a 94 GHz waveguide voltage controlled oscillator (VCO) which is one of the important parts in a frequency modulated continuous wave (FMCW) radar application. For fabrication of the high power GaAs Gunn diodes, we adopted a graded gap injector which enhances the output power and conversion efficiency by effectively removing the dead-zone. We have measured RF characteristics of the fabricated GaAs Gunn diodes. The operating current, oscillation frequency, and output power of the fabricated GaAs Gunn diodes are presented as a function of the anode diameters. The operating current increases with anode diameters, whereas the oscillation frequency decreases. The higher oscillation frequency was obtained from 60 μm anode diameters of the fabricated Gunn GaAs diodes and higher power was obtained from 68 μm. Also, for application of the 94 GHz FMCW radar system, we have fabricated the 94 GHz waveguide VCO. From the fabricated GaAs Gunn diodes of anode diameter of 60 μm, we have obtained the improved VCO performance.
    Japanese Journal of Applied Physics 09/2010; 49(11):111202. DOI:10.1143/JJAP.49.111202
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    ABSTRACT: We present design and fabrication of a 94 GHz radar sensor module. The 94 GHz sensor module has four components including 94 GHz single balanced diode mixer part, waveguide VCO (voltage controlled oscillator) part, magic tee part and bias PCB part. The 94 GHz single balanced diode mixer is developed on Duroid RT 5880 substrate with DC 1346 Schottky diode. The mixer has advantage of good conversion loss at high LO power and isolation characteristic. 94 GHz single balanced diode mixer has advantage of comparatively easy fabrication. The waveguide VCO consist of GaAs Gunn diode, varactor diode, two-bias post and cavity. The waveguide VCO is operated at 94 GHz. The magic tee has four waveguide arm regions with standard WR-10 and divides output power from VCO. Transmission frequency of 94 GHz MMW sensor module is 93.607 ~ 94.727 GHz. Bandwidth is 1.12 GHz. 2% linearity range is 680 MHz. Power is 11.03 ~ 11.47 dBm. Conversion loss about -7 dB at IF 500 MHz.
    Microwave Conference, 2009. APMC 2009. Asia Pacific; 01/2010
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    ABSTRACT: We have designed and fabricated the passive millimetre-wave imaging system for receiving radiation energy from the an object and a human body. The lens and front-end of the receiver are important in the system to detect input thermal noise signal. Passive millimetre-wave imaging system required of high sensitivity and wide broadband to detect input thermal noise. The LNA module of the imaging system has gain of 65.8 dB in average linear gain and 11 GHz in bandwidth to enhance sensitivity for thermal noise and to receive it in wide-band width as well. The zero-bias Schottky diode has been used for the detector circuit to convert amplified millimeter-wave signals to DC output.
  • Journal- Korean Physical Society 05/2009; 54(51). DOI:10.3938/jkps.54.1868
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    ABSTRACT: In this work, we fabricated a hyperabrupt varactor diode and W-band waveguide VCO using fabricated varactor diode. With the anode diameter of 90 ¿m, a maximum reverse breakdown voltage of 40 V at a leakage current of 30 ¿A, a maximum capacitance of 5.82 pF, and a minimum capacitance of 0.7 pF were obtained, resulting in a Cmax/Cmin ratio of 8.31. Fabricated VCO showed an excellent linearity of 1.6 % within 800 MHz. The bandwidth of the VCO was 1.165 GHz from 93.305 GHz to 94.47 GHz, and the output power was from 14.6 dBm to 15.42 dBm.
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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.
  • Journal- Korean Physical Society 12/2007; 51(96). DOI:10.3938/jkps.51.253
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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 10/2007; 46(10A):6503-6508. DOI:10.1143/JJAP.46.6503
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    ABSTRACT: Effects of the gate recess structures on the DC performances were investigated in 0.1-μm metamorphic high-electron-mobility transistors. Narrow gate recess structure showed significantly enhanced DC characteristics compared to wide gate recess structure in terms of drain-source saturation current increasing from 440 to 710 mA/mm and extrinsic transconductance increasing from 420 to 910 mS/mm. We propose that the observed variations in DC characteristics are due to the deep-level acceptor-type interface defects formed between the silicon-nitride passivation layer and the Schottky barrier layer. We performed hydrodynamic model simulation to verify the proposed mechanism, and the calculated result showed a good agreement with the experimental observation.
    ECS Transactions 04/2006; 2(5). DOI:10.1149/1.2204880
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    ABSTRACT: In this work, balanced medium power amplifiers for 60 GHz wireless LAN application were designed and fabricated. The single-ended and the balanced medium power amplifier on MIMIC technology were designed using 0.1μm Γ-gate GaAs PHEMT and CPW library. We compared the single-ended medium power amplifier on the balanced medium power amplifier of S-parameter and 1 dB gain compression point (P<sub>1dB</sub>). From measurement, the single-ended and the balanced medium power amplifiers show S<sub>21</sub> gains of 13.14 dB and 12.8 dB, respectively, at 60 GHz. Also, we obtain P<sub>1dB</sub> of 5.9 dBm and 7.5 dBm at 60 GHz, respectively. The balanced medium power amplifier has better return losses and P<sub>1dB</sub> than those of the single-ended medium power amplifier within V-band region.
    Microwave Conference Proceedings, 2005. APMC 2005. Asia-Pacific Conference Proceedings; 01/2006