Osamu Kusumoto

Novel SiC power MOSFET with integrated unipolar internal inverse MOS-channel diode

A novel SiC power MOSFET with an integrated unipolar internal inverse diode has been developed for the first time. Our novel SiC MOSFET has two specific features. One is that the growth of the SiC crystal defects caused by the continuous bipolar forward current of the internal diode with pn junction is completely eliminated because the unipolar diode current passes through the MOS channel region. The other is that the very small-size power modules and/or power systems are successfully designed because the external inverse diode chips paired with the transistor chips are not necessary.

Electronic Properties of Nitrogen Delta-Doped Silicon Carbide Layers

Nitrogen delta-doped silicon carbide (SiC) layers were grown by a new pulse doping method in a chemical vapor deposition. Doping distribution with high peak concentration (1.×10 18 cm −3 ) and narrow distribution width (12 nm) was fabricated in the nitrogen delta-doped structure of SiC. Mobility enhancement due to spatial separation of electrons and their ionized parent donors was observed for the delta-doped structure. Metal-semiconductor field-effect transistors with a nitrogen delta-doped channel and a recess gate structure were fabricated. The devices had large source-drain breakdown voltages, high drain current capability and easy control of the threshold voltage with a good pinch-off characteristics.

Normally-Off 4H-SiC Power MOSFET with Submicron Gate

In order for SiC-MOSFET to be practical in various power electronics applications, low specific on-resistance Ron,sp, high breakdown voltage and “normally-off” characteristics have to be fulfilled even at high temperature. We fabricated a SiC-MOSFET employing a submicron gate with channel length Lg of 0.5μm by a self-aligned implantation and aδ-doped epitaxial channel layer to successfully demonstrate the following features. The normally-off characteristics was confirmed from room temperature to 200°C where the therethold voltages Vth were 2.9V at room temperature and 1.6V at 200°C, respectively. The Ron,sp were 4.6mΩcm2 at room temperature and 9.2mΩcm2 at 200°C, respectively, while the breakdown voltage was greater than 1400V .

Nitrogen delta doping in 6H silicon carbide layers

Nitrogen delta-doped silicon carbide (SiC) layers were grown by a pulse doping method in a chemical vapor deposition. Doping distribution with high peak concentration (1×1018 cm−3) and narrow distribution width (12 nm) was fabricated in the nitrogen delta-doped structure of SiC. Mobility enhancement due to spatial separation of electrons and their ionized parent donors was observed for the delta-doped structure. Metal-semiconductor field-effect transistors with a nitrogen delta-doped channel and a recess gate structure were fabricated. The devices had large source-drain (200 V) and source-gate (140 V) breakdown voltages, high drain current capability and easy control of the threshold voltage with a good pinch-off characteristics.

Normally-Off 1400V/30A 4H-SiC DACFET and its Application to DC-DC Converter

Large (3.6 x 3.6 mm2) chips of the SiC DACFET were fabricated and mounted in TO220 packages. The drain-source avalanche breakdown voltage without any gate bias (Vgs=0V) is measured to be >1400V. The SiC DACFET keeps the normally-off characteristics even at 150°C. Ron and specific Ron of the SiC DACFET is measured to be 62mΩ and 6.7 mΩcm2 at RT while those at 150°C change to 107 mΩ and 11.6 mΩcm2. The 400V / 3 kW DC-DC switched-mode power-conversion circuit with 100kHz switching was fabricated using the SiC DACFET and the SiC SBD. The turn-off switching loss reduces dramatically using the SiC-DACFET down to 77μJ/pulse which is less than 1/10 of that using the Si-IGBT.

Formation of SiC Delta-Doped-Layer Structures by CVD

Ultranarrow n-type delta-doped-layers of 4H-SiC are grown utilizing the vertical hot-wall-type CVD system. A single N2 gas pulse is injected within the ON period of the pulse valve during the epitaxial growth. The peak carrier concentration and full-width at half-maximum (FWHM) of the doped layer grown with the ON period of 700ms are 9x10 18 cm -3 and less than 2nm, respectively. The peak concentration increases as the ON period increased from 0 to 700ms, while the FWHM decreases with increased ON period. The uniformities of the peak position and the peak concentration of n-type delta-doped-layer are evaluated to be around 8% and 12%, respectively, excluding 5mm at 2-inch wafer edge. Furthermore, formations of p-type delta-doped-layers with growth rate of 5μm/hr are investigated by utilizing single pulse-doping procedure. Secondary ion mass spectroscopy profile indicates that very narrow p-type delta-doped-layers are also successfully grown. The well-defined delta-doped-layer structure is confirmed by using cross-sectional transmission electron microscopy.

SiC Delta-Doped-Layer Structures and DACFET

Formation of SiC delta-doped layers during epitaxial growth and appli c tions of the delta-doped-layer structures to the SiC DACFET have been investiga t d. The delta-doped layers were grown in the SiC epitaxial films with a single N 2 gas pulse supplied within the ON period of the pulse valve during the CVD growth. The observed peak concentration i s increased with increasing the ON period from 100ms to 300ms. The full-width at hal f-m ximum (FWHM) of the delta-doped layers becomes narrower as lengthening the ON pe riod within this range. It is observed that the dopant-concentration profile of the delta-doped layer grown with ON period of 1000ms is almost identical with that of 300ms. 2nm-thick delta-doped layer s r successfully grown on the whole 2-inch wafer with good uniformity using the pulse-dopi ng rocedure with a single gas pulse with the ON period of 300ms. The electrical chara cteristics of the DACFET can be finely tuned by controlling the delta-doped-layer structures. The low st on resistance of the fabricated 5μm-gate DACFET is measured to be 14mOHMcm . The normally-off DACFET is also achieved with Ron = 27mOHMcm .

Influence of the Metal Film of Micro-cantilever Fabricated by Photolithographical Technique on the Mechanical Resonant Frequency.

マイクロカンチレバー上に形成した金属膜の機械的共振周波数への影響に関して検討した.片持組合せ梁のモデルを用いて導出した式の計算値と試作した複合構造のマイクロカンチレバーの測定共振周波数がほぼ一致し, 最高1.07MHzの共振周波数を実現した.