Shuhei Nakata

4H-SiC Trench MOSFET with Bottom Oxide Protection

Ensuring gate oxide reliability and low switching loss is required for a trench gate SiC-MOSFET. We developed a trench gate SiC-MOSFET with a p-type region, named Bottom P-Well (BPW), formed at the bottom of the trench gate for bottom oxide protection. We can see an effective reduction in the maximum bottom oxide electric field (Eox) and a significant improvement in dynamic characteristics with a grounded BPW, whose dV/dt is 76 % larger than that with a floating BPW due to reduction in gate-drain capacitance (Cgd). The grounded BPW is found to be an effective means of both suppressing Eox and reducing switching loss.

3.3 kV/1500 A power modules for the world’s first all-SiC traction inverter

We have successfully developed 4H-SiC devices including metal–oxide–semiconductor field-effect transistors (MOSFETs) and Schottky barrier diodes (SBDs) with a rated voltage of 3.3 kV. The conduction loss of the SiC-MOSFET was reduced to as low as that of the Si-insulated gate bipolar transistor (IGBT) by the n-type doping of the junction field-effect transistor region (JFET doping). The JFET doping technique is effective in reducing the temperature coefficient of resistance in the JFET region, leading to the decreased on-resistance of the SiC-MOSFET at high temperatures. These devices have been applied to 3.3 kV/1500 A modules for the worlds first all-SiC traction inverter. The switching loss of the new traction inverter system is approximately 55% less than that of a conventional inverter system incorporating Si modules.

Emission characteristics of printed carbon nanotube cathodes after laser treatment

Recently, field emission displays using printed carbon nanotubes (CNTs) as an emitter have been researched eagerly. By laser irradiation, some CNTs in the printed CNT layer stand out, and they work as the emission sites. We found that the higher CNTs that are easier to be the emission sites tend to exist at the boundary of the laser irradiation pattern. We used the irradiation patterns that consist of the arrays of microirradiation pattern to increase the total length of boundary. Both emission characteristics and emission uniformity were improved by the increase of the boundary length. We examined several lasers, and the irradiation by the second harmonic generation yttrium aluminum garnet laser of which wavelength was greatest in our experiment provided the best emission characteristics (turn-on electric field <2V∕μm) and emission uniformity (turn-on electric field deviation <5%).

Development of 3.3 kV SiC-MOSFET: Suppression of Forward Voltage Degradation of the Body Diode

In order to achieve cost reduction or shrinkage of power devices, an internal body diode, which forms in a MOSFET parasitically, can be designed as a free-wheeling diode in substitution for an external Schottky barrier diode (SBD). However, in a SiC p-i-n diode, forward current stress causes reliability degradation due to expansion of the electron-hole recombination-induced stacking faults. Applying the process optimization of the epitaxial layer for the reduction of recombination-induced stacking faults and the body diode screening method to 3.3 kV SiC-MOSFETs, we obtained more stable devices under forward current operation.

25.2: Fabrication of CNT Electron Source by Simple Stacking for Obtaining Uniform Emission Distributions

We have developed the technique of fabricating triode structure with simple stacking method by forming an extremely smooth CNT layer and by activating CNT with a unique laser irradiation method. A test panel proves good emission property and uniformity.

Stacking fault expansion from basal plane dislocations converted into threading edge dislocations in 4H-SiC epilayers under high current stress

We evaluate the stacking faults (SFs) expansion from basal plane dislocations (BPDs) converted into threading edge dislocations (TEDs) under the current stress to the pn devices and analyzed the nucleation site of the SF by combined polishing, chemical etching in molten KOH, photoluminescence imaging, Focus ion beam, transmission electron microscopy, and Time-of-Flight secondary ion mass spectrometer techniques. It was found that the formation of SFs occurs upon the current stress levels of 400 A/cm2 where the diode area is not including BPDs in the drift layer after the high current stress, and the high current stress increases the SFs expansion density. It was also found the dependence of the junction temperature. The estimated activation energy for the expansion of SFs is Ea = 0.46 eV. The SF extends from the conversion point of the BPD into the TED within buffer layer. Even though BPDs converted into TEDs within the high doped buffer layer, SFs expand under high current stress.

Fabrication of carbon-nanotube field-emitter array using polymer insulator

This article reports the fabrication process of a carbon-nanotube (CNT) field-emitter array with the silicon-ladder polymer insulator polyphenylsilsesquioxane (PPSQ), whose feature is heat resistance, high breakdown voltage, and low outgassing. CNT islands are formed with a screen-printing method, polymer-insulator coating is carried out (8 μm in thickness), and the gate electrodes are deposited, followed by patterning of the electrodes. PPSQ insulator is applied reactive ion etching (RIE) to reveal CNT emitters. Because of using mixed gases of CF4 and O2, the etching rate for CNT is half that for PPSQ, there is a margin to stop etching with enough CNT left. After reactive ion etching, emission-current density from the revealed CNT is degraded, so laser activation treatment is applied and the emission current density is boosted by a hundredfold. In the case of the triode mode, the laser condition was chosen to prevent gate damage and to improve emission characteristics. Current density of 3 mA/cm2 is obta...

Design of a compact storage ring for industrial research

An 800‐MeV compact storage ring is under construction for development of the technology of a compact ring and for research of x‐ray lithography and material analysis. Synchrotron radiation with a critical wavelength of 0.65 nm is emitted from a pair of superconducting bending magnets. A straight section of the ring has only one quadrupole magnet to make the smaller ring. An injection efficiency with a single kicker magnet is optimized by computer simulation. The kicker field strength and an injection angle are determined. A beam tracking with a numerical integration method shows this ring has a sufficiently wide dynamic aperture of ‖x‖≂30 mm and ‖y‖≂50 mm at the center of the bending magnet, which has βxmin and βymax. A 1‐GeV synchrotron is used as an injector, which has accelerated a beam up to 1 GeV.

Improvement of emission characteristics uniformity of carbon nanotube field emission display by surface treatment

Carbon nanotubes (CNTs) have been investigated as an emitter for field emission display (FED). In our FED fabrication process, the printed CNTs are raised by a laser irradiation for the purpose to improve the emission characteristics. Because the emitters have to be controlled in each pixel individually on FED, the uniformity of the emission characteristics is necessary. We could improve the emission characteristics uniformity successfully by two methods. In the first method, the distribution of CNT raisings was homogenized by the modification of the irradiation pattern that consisted of an array of small areas. The emission deviation was improved to approximately half by the method. In the second method, the impurities of CNT raisings were cleaned by post-laser-irradiation of which fluence was lower than that of first irradiation. The emission characteristics improved and the turn-on electric field reduced from 2.1 to 1.4V∕μm. The emission deviation also improved to approximately half.

Optimization of the coil distribution of the deflection yoke for CRT

We have developed a computer program to optimize the coil distribution of the deflection yoke for CRTs. This program calculates changes of misconvergence and raster distortion by one additional winding. Then it modifies the coil distribution by minimization of the misconvergence. This program enables us to optimize deflection yokes in a short time.