Ho Young Cha

General Electric, Fairfield, California, United States

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Publications (10)8.68 Total impact

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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)
    physica status solidi (c) 02/2007; 4(2):528 - 530.
  • Materials Science Forum - MATER SCI FORUM. 01/2006;
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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
    Power Electronics Specialists Conference, 2005. PESC '05. IEEE 36th; 07/2005
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    ABSTRACT: Nanosecond-pulsed technique is used to study hot-electron transport in donor-doped 4H–SiC n =2 10 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 1.4 10 7 cm/ s at room temperature. © 2005 American Institute of Physics. [DOI: 10.1063/1.1851001] Silicon carbide 4H polytype (4H–SiC) is widely consid-ered for high-temperature and high-power applications in electronics. 1 Performance of donor-doped 4H–SiC channels designed for microwave field-effect transistors is mainly de-termined by electron drift velocity; in particular, its value at high electric fields decides the maximum frequency of chan-nel operation. There are two techniques for determining the drift velocity as a function of electric field: time-of-flight technique 2 and conductance technique. 3 The time-of-flight technique cannot be applied to donor-doped 4H–SiC chan-nels because it is applicable to semi-insulating samples only. Recently, the field-dependent electron drift velocity for 4H– SiC semiconductive channels has been measured in the di-rection parallel to the basal plane. 4 The reported room-temperature saturation value is 2.2 10 7 cm/ s. Channel self-heating is found to limit investigations at high electric fields. The main aim of this letter is to report on high-field electron transport experiments for 4H–SiC under conditions of well-controlled channel temperature. The effect of channel self-heating on current can be minimized when nanosecond-pulsed technique is used. 5 Nanosecond pulses of voltage were formed in an electri-cal circuit that consisted of a dc voltage source, a mercury-wetted relay, a charged coaxial line, a delay line, and the investigated sample. The pulse amplitude was changed through adjusting the dc source. When the relay was switched on and the line was discharged, the electric pulse reached the sample through the delay line, and the transmit-ted signal was fed into a 0 – 5 GHz bandwidth sampling os-cilloscope through a wide-band switch, a tee, and a fixed attenuator. In this technique, the drift velocity of electrons is estimated from the measured current and the low-field mo-bility data. The voltage pulse length ranged from 1.8 s to 1 ns. Channel self-heating was controlled through variation of voltage pulse length. The experiments were carried out at room temperature on passivated donor-doped 4H–SiC two-electrode samples processed together with field-effect transistors. The wafer consisted of a semi-insulating 4H–SiC substrate, a p-type buffer layer 5 10 15 cm −3 , and an n-type channel layer d = 0.175 m. Two types of the channel layers were inves-tigated: group A had square sheet resistance of 5940 / and group B had 6360 / . The estimated low-field elec-tron mobilities in these groups were 300 and 250 cm 2 V −1 s −1 , respectively; these values were lower as compared with the reported room temperature mobility data for 4H–SiC samples: 730 cm 2 V −1 s −1 , 1 450 cm 2 V −1 s −1 , 4 and 360 cm 2 V −1 s −1 . 6 Other authors 7 reported similar values. The channel thickness and the electron density n =2 10 17 cm −3 were estimated from capacitance–voltage
    Applied Physics Letters 01/2005; 86:022107. · 3.79 Impact Factor
  • Materials Science Forum - MATER SCI FORUM. 01/2005;
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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.
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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.
    Journal of Applied Physics 12/2004; 96(11). · 2.21 Impact Factor
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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.
    Electronics Letters 03/2004; · 1.04 Impact Factor
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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.
    Journal of Electronic Materials 01/2003; 32(5):437-443. · 1.64 Impact Factor
  • Materials Science Forum - MATER SCI FORUM. 01/2003;