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IEICE Transactions. 01/2010; 93-C:648-650.
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ABSTRACT: The noble synthesis method for hydroxyapatite (HAp) nanoparticles was exploited using a fairly simple reaction of Ca(OH)(2) and H(3)PO(4), which does not generate residual harmful anions and consequently does not need an additional washing process. HAp nanoparticles were found to yield from dicalcium phosphate dehydrate (DCPD) as the only intermediate phase, which was monitored by in situ observation study using X-ray diffraction (XRD), Fourier transform infrared (FT-IR), (1)H and (31)P magic-angle spinning (MAS) NMR. Furthermore, we found that the phase evolution of HAp was preceded by heteronucleation of HAp onto the DCPD surface. The combination of scanning electron microscopy (SEM) and inductively coupled plasma atomic emission spectroscopy (ICP-ES) analysis gave more information on the HAp crystallization process, which was found to be retarded by the residual Ca(OH)(2) and slow diffusion process of Ca ions into the interface between HAp and DCPD. These results demonstrate that the synthesis of pure HAp nanoparticles with high throughput can be achieved by controlling the residual Ca(OH)(2) and diffusion process of Ca ions.
Langmuir 10/2009; 26(1):384-8. · 4.19 Impact Factor
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Changhyun Cho,
Sangho Song,
Sangho Kim,
Sungho Jang,
Sungsam Lee, Hyungtak Kim,
Yangsoo Sung,
Sangmin Jeon,
Gisung Yeo,
Youngsun Kim,
Yungi Kim,
Gyoyoung Jin,
Kinam Kim
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ABSTRACT: A novel process technology for 6F<sup>2</sup> DRAM cell at 68nm design rule was for the first time developed. The cell size is 0.028μm<sup>2</sup>, which is the smallest cell size ever reported. ArF lithography with double expose technology and highly selective etching process were used for patterning of critical layers. This 6F cell was made of simple line/space patterns for easy patterning and self-aligned etches to improve process margins. MIM cell capacitor was developed with multi-layer high-k dielectric materials and 11A equivalent Tox and sub-lfA leakage was confirmed.
VLSI Technology, 2005. Digest of Technical Papers. 2005 Symposium on; 07/2005
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Hyungtak Kim,
Sangho Kim,
Sungsam Lee,
Sungho Jang,
Ji-hoon Kim,
Yangsoo Sung,
Junwoong Park,
Saehan Kwon,
Sangmin Jun,
Wontae Park,
Daehan Han,
Changhyun Cho,
Yungi Kim,
Kinam Kim,
Byungil Ryu
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ABSTRACT: For the first time, we developed 70nm DRAM technology applicable to a manufacturing level. This technology is aimed at DDR-3 application, which requires low-voltage operation and high speed performance. Fully working 70nm DRAMs were realized combining W-gate dual poly process, recess-channel-array-transistors (RCATs), and MIM cell capacitor module. In this paper, we present performance of 70nm node DRAMs which qualifies DDR-3 application requirement.
VLSI Technology, 2005. (VLSI-TSA-Tech). 2005 IEEE VLSI-TSA International Symposium on; 05/2005
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Changhyun Cho,
Sangho Song,
Sangho Kim,
Sungho Jang,
Seongsam Lee, Hyungtak Kim,
Junwoong Park,
Junshik Bae,
Yongsuk Ahn,
Yungi Kim,
Kinam Kim
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ABSTRACT: A novel process technology for 70nm DRAM was for the first time developed. ArF lithography with lithography friendly layout and highly selective etching process were used for patterning of critical layers. A novel gap-fill technology using spin coating oxide was used for STI and ILD processes. Metal tungsten on dual poly gate and dual gate oxide with plasma nitridation process was used for the performance of peripheral transistors. Bar type bit line contact was used to increase the transistor current about 10%. MIM cell capacitor was developed with buried-OCS scheme and 15Å equivalent Tox and 1fA leakage was confirmed.
VLSI Technology, 2004. Digest of Technical Papers. 2004 Symposium on; 07/2004
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ABSTRACT: A monolithic X-band oscillator based on an AlGaN/GaN high electron mobility transistor (HEMT) has been designed, fabricated, and characterized. A common-gate HEMT with 1.5 mm of gate width in conjunction with inductive feedback is used to generate negative resistance. A high Q resonator is implemented with a short-circuit low-loss coplanar waveguide transmission line. The oscillator delivers 1.7 W at 9.556 GHz into 50-Ω load when biased at V<sub>ds</sub>=30 V and V<sub>gs</sub>=-5 V, with dc-to-RF efficiency of 16%. Phase noise was estimated to be -87 dBc/Hz at 100-kHz offset. Low-frequency noise, pushing and pulling figures, and time-domain characterization have been performed. Experimental results show great promise for AlGaN/GaN HEMT MMIC technology to be used in future high-power microwave source applications.
IEEE Journal of Solid-State Circuits 10/2003; · 3.23 Impact Factor
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ABSTRACT: The authors report on the effects of silicon nitride (SiN) surface passivation and high-electric field stress (hot electron stress) on the degradation of undoped AlGaN-GaN power HFETs. Stressed devices demonstrated a decrease in the drain current and maximum transconductance and an increase in the parasitic drain series resistance, gate leakage, and subthreshold current. The unpassivated devices showed more significant degradation than SiN passivated devices. Gate lag phenomenon was observed from unpassivated devices and removed by SiN passivation. However, SiN passivated devices also showed gate lag phenomena after high-electric field stress, which suggests possible changes in surface trap profiles occurred during high-electric field stress test.
IEEE Electron Device Letters 08/2003; · 2.85 Impact Factor
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ABSTRACT: This paper presents a detailed study of high-field effects in GaN MODFETs. Degradation of DC characteristics and change of flicker noise due to hot electron and high-reverse current stresses in Si<sub>3</sub>N<sub>4</sub> passivated GaN MODFETs have been investigated. The authors observe that during hot electron stress, electron trapping in the barrier layer and interface state creation occur. These cause a positive shift of V<sub>t</sub>, reduce I<sub>D</sub>, skew the transfer characteristics, and degrade g<sub>m</sub>. Flicker noise (1/f) measurements show that after hot electron stress, the scaled drain current noise spectrum (S<sub>I(D)</sub>/I<sub>D</sub><sup>2</sup>) decreases in depletion, but increases only slightly in strong accumulation, corroborating the creation of interface states but only a small creation of transition-layer tunnel traps that contribute to 1/f noise. During high-reverse current stress, electron trapping dominates for the first 50-60 s and then hole trapping and trap creation begin to manifest. However, there still is net electron trapping under the gate after one hour of stress. The degradation processes bring about a positive shift of V<sub>t</sub>, degrade I<sub>D</sub> and g<sub>m</sub>, and increase reverse leakage. After high-reverse current stress, S<sub>I(D)</sub>/I<sub>D</sub><sup>2</sup> increases substantially in strong accumulation, indicating the creation of transition layer tunnel traps.
IEEE Transactions on Electron Devices 06/2003; · 2.32 Impact Factor
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ABSTRACT: This work presents the effects of hot electron stress on the degradation of undoped Al0.3GaN0.7/GaN power HFET’s with SiN passivation. Typical degradation characteristics consist of a decrease in the drain current and maximum transconductance, an increase in the drain series resistance, gate leakage and a subthreshold current. Degradation mechanism has been investigated by means of gate lag measurements (pulsed I–V) and current-mode deep level transient spectroscopy (DLTS). Stressed devices suffered from aggravated drain current slump (DC to RF dispersion) which indicates possible changes in surface charge profiles occurred during hot electron stress test. The DLTS was used to identify the trap creation by hot electron stress. The DLTS spectra of stressed device revealed the evidence of trap creation due to hot electron stress.
Microelectronics Reliability. 01/2003;
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ABSTRACT: Channel recessed 4H-SiC MESFETs have demonstrated excellent small signal characteristics and the effect of Si<sub>3</sub>N<sub>4</sub> 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 V<sub>ds</sub> 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 F<sub>t</sub> of 14.5 GHz and F<sub>max</sub> of 40 GHz. After Si<sub>3</sub>N<sub>4</sub> 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.
High Performance Devices, 2002. Proceedings. IEEE Lester Eastman Conference on; 09/2002
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L.F. Eastman,
V. Tilak,
J. Smart,
B.M. Green,
E.M. Chumbes,
R. Dimitrov, Hyungtak Kim,
O.S. Ambacher,
N. Weimann,
T. Prunty,
M. Murphy,
W.J. Schaff,
J.R. Shealy
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ABSTRACT: Undoped AlGaN/GaN structures are used to fabricate high electron
mobility transistors (HEMTs). Using the strong spontaneous and
piezoelectric polarization inherent in this crystal structure a
two-dimensional electron gas (2DEG) is induced. Three-dimensional (3-D)
nonlinear thermal simulations are made to determine the temperature rise
from heat dissipation in various geometries. Epitaxial growth by MBE and
OMVPE are described, reaching electron mobilities of 1500 and 1700 cm
<sup>2</sup>/Ns, respectively, For electron sheet density near
1×10<sup>13</sup>/cm<sup>2</sup>, Device fabrication is described,
including surface passivation used to sharply reduce the problematic
current slump (dc to rf dispersion) in these HEMTs. The frequency
response, reaching an intrinsic f<sub>t</sub> of 106 GHz for 0.15 μm
gates, and drain-source breakdown voltage dependence on gate length are
presented. Small periphery devices on sapphire substrates have
normalized microwave output power of ~4 W/mm, while large periphery
devices have ~2 W/mm, both thermally limited. Performance, without and
with Si<sub>3</sub>N<sub>4</sub> passivation are presented. On SiC
substrates, large periphery devices have electrical limits of 4 W/mm,
due in part to the limited development of the substrates
IEEE Transactions on Electron Devices 04/2001; · 2.32 Impact Factor
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ABSTRACT: AlGaN-GaN high electron mobility transistors (HEMTs) have shown
great potential for high temperature/high power electronics. However,
the study on the reliability of GaN-based devices is still at the
initial stages. In this work, we report the degradation characteristics
of AlGaN HEMTs under various stress conditions such as DC stress (gate
current extraction and hot electron cycles) and RF input drive stress at
room temperature. While AlGaN-GaN HEMTs have shown robust reliabilities
under gate current extraction stress, degradation due to hot carriers
was observed under hot electron cycles and RF input drive stress.
Si<sub>3</sub>N<sub>4</sub> passivation was found to provide better
reliability than SiO<sub>2</sub> passivation under DC stress and RF
input drive stress
Reliability Physics Symposium, 2001. Proceedings. 39th Annual. 2001 IEEE International; 02/2001
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ABSTRACT: Broadband, high power cascode AlGaN/GaN HEMT MMIC amplifiers with high gain and power-added efficiency (PAE) have been fabricated on high-thermal conductivity SiC substrates. A cascode gain cell exhibiting 5 W of power at 8 GHz with a small signal gain of 19 dB was realized. A broadband amplifier MMIC using these cascode cells in conjunction with a lossy-match input matching network was designed, fabricated, and evaluated, showing a useful operating range of DC-8 GHz with an output power of 5-7.5 W and a PAE of 20-33% respectively. A nonuniform distributed amplifier (NDA) based on this same process yielded an output power of 3-6 W over a DC-8 GHz bandwidth with an associated PAE of 13-31%
Microwave Symposium Digest, 2001 IEEE MTT-S International; 02/2001
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ABSTRACT: We report on the fabrication and characteristics of
cascode-connected AlGaN/GaN HEMTs. The HEMTs were realized using
Al<sub>0.3</sub>Ga<sub>0.7</sub>N/GaN heterostructures grown on 6-N
semi-insulating SIC substrates. The circuit reported here employs a
common source device having a gate length of 0.25 μm cascode
connected to a 0.35 μm common gate device. The gate width of each
device is 250 μm. The fabricated circuit exhibited a current density
of 800 mA/mm and yielded an f<sub>T</sub> and f<sub>max</sub> of 24.5
and 56 (extrapolated) GHz, respectively. Large signal measurements taken
at 4 GHz produced 4 W/mm saturated output power at 36% power-added
efficiency Comparisons to the performance of a 250×0.35
μm<sup>3</sup> common source device taken from the same wafer show
that the cascode configuration has 7 dB more linear gain and 3 db more
compressed gain than the common source device at 4 GHz
IEEE Microwave and Guided Wave Letters 09/2000;
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ABSTRACT: An analytical nonlinear model describing AlGaN/GaN HEMTs grown on
sapphire substrates has been extracted based on measured device data.
The model accounts for dispersion in transconductance and output
conductance present in the devices. Model validations based on
comparisons with DC I-V, S-parameter, 4 GHz time-domain waveforms, and 7
GHz power sweep data show good agreement between the model predictions
and measurements
Microwave Symposium Digest. 2000 IEEE MTT-S International; 02/2000
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ABSTRACT: An analytical nonlinear model describing AlGaN/GaN HEMT's grown on
SiC substrates has been extracted based on measured device data. The
model accounts for dispersion in transconductance and output conductance
present in the devices. Model validations based on comparisons with 16
GHz power sweep data show good agreement between the model predictions
of a 3.3 W/mm power density (18% PAE) and the measurements for these
devices
High Performance Devices, 2000. Proceedings. 2000 IEEE/Cornell Conference on; 02/2000
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L.F. Eastman,
B Green,
J. Smart,
V. Tilak,
E. Chumbes, Hyungtak Kim,
T. Prunty,
N. Weimann,
R. Dimitrov,
O. Ambacher,
W.J. Schaff,
J.R. Shealy
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ABSTRACT: The present and predicted limits on microwave power performance of undoped AlGaN/GaN HEMTs are presented, based on measured frequency response and drain-source breakdown voltage, both as functions of gate length. The spontaneous and piezoelectric polarization that induce the 2DEG in these HEMTs are covered. Process methods, including Si<sub>3 </sub>N<sub>4</sub> passivation are included. Thermal simulation results are shown for heat dissipation that limits channel temperature to 300°C. Microwave cw power density limits of 12.5 W/mm at 10 GHz are predicted for class A operation on thick SiC substrates
High Performance Devices, 2000. Proceedings. 2000 IEEE/Cornell Conference on; 02/2000
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Hyungtak Kim
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ABSTRACT: Gallium nitride (GaN) has attracted a lot of attention as the next
generation of semiconductor material for microwave power application.
The unique and superior material properties of GaN and its
heterostructure, such as excellent transport property, high breakdown
voltage and sheet carrier concentration, and thermal and mechanical
stabilities, enable AlGaN/GaN heterostructure field effect transistors
(HFETs) to deliver unprecedented levels of microwave power performance.
Potential applications include ultra-wide bandwidth communications and
radar systems, wireless base stations, and communications satellites.
Tremendous efforts to realize the potential of Al-GaN/GaN HFETs have
been made over the last decade focusing on improving microwave power
performance via optimizing material growth and semiconductor processing
technologies. As the device performance is getting mature, the device's
reliability becomes a major concern for manufacturability of
commercially available AlGaN/GaN HFETs. However, comprehensive study on
the reliability of these devices is still lacking. This dissertation
describes the fabrication, performance and degradation characteristics
and mechanism of AlGaN/GaN HFETs. The devices were fabricated with
alloyed Ti/Al/Ti/Au ohmic contact and Ni/Au mushroom gate contact using
E-beam lithography. The device's microwave performance was significantly
improved after SiN passivation due to reduced surface effects. Several
degradation modes, primarily a decrease of the output current and
microwave output power density, were observed under various electrical
stress tests including high current stress, high field stress, and RF
overdrive. To further investigate the physical mechanism of observed
degradations, SiN passivation, pulsed IV (gate lag), low frequency noise
measurements, deep level transient spectroscopy (DLTS), and scanning
kelvin probe microscopy (SKPM) have all been employed with hot electron
stress testing. The results clearly demonstrated that charge
accumulation and trap creation at the semiconductor surface and
interface induced by hot electron effects are responsible for observed
degradation.
