Ho Young Cha

General Electric, Fairfield, California, United States

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Publications (14)8.21 Total impact

  • No preview · Article · Jan 2009 · Materials Science Forum
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    ABSTRACT: The hole dominated avalanche multiplication characteristics of 4H-SiC Separate Absorption and Multiplication avalanche photodiodes (SAM-APDs) were determined experimentally and modeled using a local multiplication model. The 0.5×0.5mm2 diodes had very low dark current and exhibited sharp, uniform breakdown at about 580V. The data agree with modeling result using extrapolated impact ionization coefficients reported by Ng et al. and is probably valid for electric fields as low as ∼0.9MV/cm at room temperature provided that both the C-V measurements and electric field determination in this work are correct. The packaged devices demonstrate a positive temperature coefficient of breakdown voltage for temperatures ranging from 100K to 300K which is a desired feature for extreme environment applications.
    No preview · Article · Jan 2009 · Materials Science Forum
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    ABSTRACT: We present details of the development of a GaN ballistic diode designed with THz applications in mind. Parallel theoretical and experimental analysis has shown that negative-differential-resistance (NDR) can only be achieved when hot electrons are injected into the drift region, contact and spreading resistances must be minimized in order to maximize the field in the drift layer. NDR remains achievable even in the presence of collision dominated transport or when transport is space-charge limited. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    No preview · Article · Feb 2007 · physica status solidi (c)
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    ABSTRACT: Avalanche photodiodes (APDs) based on 4H-SiC are excellent candidates to replace PMTs in the UV, particularly for harsh environment applications. Here, we report on dark current analysis of 4H-SiC APDs with separate absorption and multiplication regions. Detailed analysis of the leakage current as a function of device size showed that for a given device design, the bulk leakage component is dominant at U>600V, while surface leakage is dominant at U<600V. Electron beam induced current was also used to establish a correlation between leakage current and major types of defects in the substrate. There were two types of dislocations that could be easily distinguished in the images, including threading (spots) and basal plane (comet-like) dislocations. Using image processing software, densities of threading dislocations as well as basal plane dislocations were obtained and correlated with leakage currents of the corresponding APDs. The results suggest a strong effect of threading dislocations on dark current. Densities of basal plane dislocations were very similar in all devices tested suggesting that a role of basal plane dislocations was not dominant in leakage current of the APDs.
    No preview · Article · Jan 2007 · Materials Science Forum
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    ABSTRACT: A New silicon carbide (SiC) enhancement-mode lateral channel vertical junction fieldeffect transistor (LC-VJFET), namely "source inserted double-gate structure (SID-gate) with a supplementary highly doped region (SHDR)", was proposed for achieving extremely low power losses in high power switching applications. The proposed architecture was based on the combination of an additional source electrode inserted between two adjacent surface gate electrodes and a unique SHDR in the vertical channel region. Two-dimensional numerical simulations for the static and resistive switching characteristics were performed to analyze and optimize the SiC LC-VJFET structures for this purpose. Based on the simulation results, the excellent performance of the proposed structure was compared with optimized conventional structures with regard to total power losses. Finally, the proposed structure showed about a 20 % reduction in on-state loss (Pon) compared to the conventional structures, due to the effective suppression of the JFET effect. Furthermore, the switching loss (Psw) of the proposed structure was found to be much lower than the results of the conventional structures, about a 75 % - 95 % reduction, by significantly reducing both input capacitance (C iss) and reverse transfer capacitance (Crss) of the device.
    No preview · Article · Jan 2006 · Materials Science Forum
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    ABSTRACT: Steady‐state characteristics and low‐frequency noise spectra of SiC‐based metal‐semiconductor field‐effect transistors (MESFETs) before and after small doses (1×106 rad) of gamma radiation treatment are studied. The structural ordering of non‐controllable impurities with radiation leads to an increase in threshold voltage, decrease of the channel’s resistance and reduces the number of G‐R components observed in the total noise spectra of the devices.
    No preview · Article · Aug 2005
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    ABSTRACT: For a recessed gate SiC SIT, a new approach to solve a breakdown voltage specific on-resistance trade-off problem and consequently achieve an extremely high power capability was presented. Simulations were fully performed to analyze the influence of critical design parameters on the device power performance. They showed that the basic trade-off was successfully eliminated if the depth of a JFET diffusion layer was greater than 1.35 times the gate trench depth, due to the effective suppression of the JFET resistance. Consequently, the proposed device architecture showed much higher power capability than the conventional structure, suggesting that device power performance be maximized by implementing a JFET diffusion layer in a conventional SiC SIT, with a narrow half-width of a source region
    No preview · Conference Paper · Jul 2005
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    ABSTRACT: Nanosecond-pulsed technique is used to study hot-electron transport in donor-doped 4H-SiC (n=2x10(17) cm(-3)) biased parallel to the basal plane. The measurements of current with 1 ns voltage pulses are carried out at average electric fields up to 570 kV/cm. A region with a negative differential conductance is observed for the range of fields exceeding 280 kV/cm, followed by a sharp increase in the current at fields over 345 kV/cm. The dependence of drift velocity on electric field is deduced for the field range below the onset of the negative differential conductance to appear: the value of the saturation velocity is estimated as 1.4x10(7) cm/s at room temperature. (C) 2005 American Institute of Physics.
    Full-text · Article · Jan 2005 · Applied Physics Letters
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    ABSTRACT: Because SiC does not have velocity overshooting behaviour, the current density of SiC metal-semiconductor field-effect transistors (MESFETs) is restricted by low drift velocity in the parasitic region between source and gate where the applied electric field is low. In addition, the extension of the depletion region toward the drain side at high drain voltages increases the effective channel length and, as a result, lowers the cut-off frequency due to the increased transit time.
    No preview · Article · Jan 2005 · Materials Science Forum
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    ABSTRACT: Experimental results on electron transport and microwave noise are used to investigate, in the electron temperature approximation, dissipation of hot-electron energy in a biased donor-doped 4H-SiC channel (n=2⋅1017 cm−3). The electron energy relaxation time is estimated to be 2.8±1 ps in the electric-field range below 25 kV∕cm at room temperature. Longitudinal-optical phonons are found responsible for electron energy dissipation when the supplied power ranges from 0.5 to 25 nW per electron. In this range, accumulation of nonequilibrium (hot) longitudinal-optical phonons takes place. Equivalent hot-phonon temperature reaches 3000 K near 30 kV∕cm, hot-phonon lifetime is comparable to the electron energy relaxation time. Dependence of the lifetime on electric field is weak in the investigated range of electric fields below 50 kV∕cm.
    Preview · Article · Dec 2004 · Journal of Applied Physics
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    ABSTRACT: A cascode model is proposed to analyse current-voltage and cutoff frequency characteristics of the buried-gate device. A high saturation current of the buried-gate device is associated with the short channel FET representing the buried region. Two unsaturated side FETs result in a relatively lower cutoff frequency of the buried-gate device compared to the gate-recessed one.
    No preview · Article · Mar 2004 · Electronics Letters
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    ABSTRACT: The performances of silicon carbide (SiC) metal-semiconductor field-effect transistors (MESFETs) fabricated on conventional V-doped semi-insulating substrates and new V-free semi-insulating substrates have been compared. The V-free 4H-SiC substrates were confirmed by secondary ion mass spectrometry (SIMS). X-ray topography revealed significantly fewer micropipes and low-angle boundaries in V-free semi-insulating substrates than in conventional V-compensated substrates. Deep-level transient spectroscopy (DLTS) indicated that the spectra signals observed in conventional V-doped substrates were either reduced or disappeared in V-free substrates. The intrinsic deep levels in V-free substrates to make semi-insulating properties were also observed in DLTS spectra. Under various DC and RF stresses, SiC MESFETs fabricated on new V-free semi-insulating substrates showed superior device performance and stability.
    No preview · Article · May 2003 · Journal of Electronic Materials
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    ABSTRACT: Channel recessed 4H-SiC MESFETs were fabricated and have demonstrated excellent small signal characteristics, such as, Ft of 14.5 GHz and Fmax of 40 GHz (for MAG = 1). The effect of Si3N 4 passivation on these devices has been studied in this work. Current instability and RF power performance were improved after passivation. From our measurements, we found out that the passivation of SiC MESFETs reduces the surface effects and improves the RF power performance by suppressing the instability in DC characteristics.
    No preview · Article · Jan 2003 · Materials Science Forum
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    ABSTRACT: Channel recessed 4H-SiC MESFETs have demonstrated excellent small signal characteristics and the effect of Si3N4 passivation on these devices has been studied in this work. A saturated current of 250-270 mA/mm and a maximum transconductance of 40-45 mS/mm were measured for these devices. The 3-terminal breakdown voltage Vds ranges from 120 V to more than 150 V, depending on gate-drain spacing. 2 × 200 μm devices with 0.45 μm gate length show high Ft of 14.5 GHz and F max of 40 GHz. After Si3N4 passivation, the output power and PAE were increased by 40% and 16%, respectively, for CW power measurement. Other measurements, such as, the change in surface potential and the dispersion of the drain current make it clear that the passivation of SiC MESFETs reduces the surface effects and enhances the RF power performance by suppressing the instability in DC characteristics.
    No preview · Article · Jan 2002

Publication Stats

49 Citations
8.21 Total Impact Points


  • 2007
    • General Electric
      Fairfield, California, United States
  • 2003-2005
    • Cornell University
      • Department of Electrical and Computer Engineering
      Ithaca, NY, United States