Kenji Hiratsuka

A Novel Truncated V-Groove 4H-SiC MOSFET with High Avalanche Breakdown Voltage and Low Specific on-Resistance

A breakdown of a conventional trench SiC-MOSFET is caused by oxide breakdown at the bottom of the trench. We have fabricated a novel trench SiC-MOSFET with buried p+ regions and demonstrated the high breakdown voltage of 1700 V and the specific on-resistance of 3.5 mΩcm2.

Novel Designed SiC Devices for High Power and High Efficiency Systems

Two types of 4H-silicon carbide (SiC) MOSFETs are proposed in this paper. One is the novel designed V-groove trench MOSFET that utilizes the 4H-SiC (0-33-8) face for the channel region. The MOS interface using this face shows the extremely low interface state density (Dit) of 3 × 1011 cm2 eV-1, which causes the high channel mobility of 80 cm2 V-1 s-1 results in very low channel resistance. The buried p+ regions located close to the trench bottom can effectively alleviate the electric field crowding without the significant sacrifice of the increase of the resistance. The low specific ON-state resistance of 3.5 mQ cm2 with sufficiently high blocking voltage of 1700 V is obtained. The other is the double implanted MOSFET with the carefully designed junction termination extension and field-limiting rings for the edge termination region, and the additional doping into the junction FET region. With a high-quality and high-uniformity epitaxial layer, 6 mm × 6 mm devices are fabricated. The well balanced specific ON-state resistance of 14.2 mQ cm2 and the blocking voltage of 3850 V are obtained for 3300 V application.

Application of Nanoimprint Lithography to Fabrication of Distributed Feedback Laser Diodes

We have succeeded in employing nanoimprint lithography (NIL) to form the diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. Uniform gratings and phase-shifted gratings with periods of 232 nm have been formed by using a reversal-tone NIL with a step-and-repeat imprint tool. Line edge roughness has been sufficiently low with the fabricated gratings. DFB LDs fabricated by NIL have indicated comparable characteristics with LDs fabricated by electron beam lithography. We have also demonstrated that phase-shifted DFB LDs show better uniformity in characteristics than uniform-grating DFB LDs. The results of this study indicate that NIL has high potential for the fabrication of DFB LDs.

26 Gbit/s Direct Modulation of AlGaInAs/InP Lasers with Ridge-Waveguide Structure Buried by Benzocyclobutene Polymer

26 Gbit/s direct modulation of 1.3 °m wavelength AlGaInAs/InP distributed feedback lasers with the ridge-waveguide structure (ridge width of 1.0 °m) buried by the benzocyclobutene polymer was achieved. The high electrical bandwidth of more than 20 GHz was acquired with this ridge-waveguide structure. Consequently, a clear eye-opening with extinction ratio of 6 dB was confirmed in the measurement temperature of 25 °C.

Fast switching 4H-SiC V-groove trench MOSFETs with buried P + structure

4H-SiC trench MOSFETs with novel V-groove structures have been investigated. We have fabricated trench MOSFETs with the inclined 4H-SiC{0-33-8} face [1, 2] as trench sidewalls for the channel region, resulting in a low specific on-resistance owing to the superior MOS interface properties. In addition, by using buried p+ regions inside the drift layer, a high voltage avalanche breakdown without oxide break was realized as well. The specific on-resistance and breakdown voltage were 3.5 mΩ cm2 (VGS = 18 V, VDS = 1 V) and 1700 V, respectively. The switching capability of the trench MOSFET demonstrated fast dynamic characteristics without adverse effects in comparison to the trench MOSFET without buried p+ regions. Typical turn-on and turn-off time for the switching were estimated to be 92 ns and 27 ns, respectively from the resistive load switching measurements at a drain voltage of 600V.

600 V -Class V-Groove SiC MOSFETs

The authors applied a thick gate oxide layer at the trench bottoms to 600 V class truncated V-groove MOSFETs of which MOS channels were formed on 4H-SiC (0-33-8) facets and validated the static and switching characteristics. The specific on-resistance and the threshold voltage were 3.6 mΩ cm2 (VGS=18 V, VDS=1 V) and about 1 V (normally-off), respectively. The breakdown voltage of the MOSFET with a thick oxide layer was 1,125 V (IDS=1 μA). The switching losses during turn-on and turn-off operations were estimated to be 105.8 μJ and 82.5 μJ (300 V, 10 A) at room temperature. The switching characteristics exhibited low temperature dependence for turn-on/off time.

Blocking Characteristics of 2.2 kV and 3.3 kV-Class 4H-SiC MOSFETs with Improved Doping Control for Edge Termination

Blocking characteristics of 2.2 kV and 3.3 kV -class 4H-SiC MOSFETs with various doping conditions for the edge termination region have been investigated. By optimizing the implanted dose into the edge termination structure consisting of junction termination extension (JTE) and field limiting ring (FLR), a breakdown voltage of 3,850 V for 3.3 kV -class MOSFET has been attained. This result corresponds to about 95% of the approximate parallel-plane breakdown voltage estimated from the doping concentration and the thickness of the epitaxial layer. Implanted doping for the JFET region is effective in reducing JFET resistance, resulting in the specific on-resistance of 14.2 mΩcm2 for 3.3 kV SiC MOSFETs. Switching characteristics at the high drain voltage of 2.0 kV are also discussed.

Study of reactive ion etching for reverse tone nanoimprint process

We have used reverse nanoimprint for fabricating diffraction gratings of distributed feedback laser diodes. Generation of residues in the etching process of resin is a serious issue leading to poor line edge roughness of the grating patterns. We have found that the residues are composed of oxide products from Si-containing resin. We have successfully suppressed the generation of the residues by optimizing oxygen partial pressure of reactive ion etching (RIE). We have also succeeded in effectively removing the residues by utilizing sputtering effect of RIE.

The optimised design and characterization of 1200 V / 2.0 mΩ cm 2 4H-SiC V-groove trench MOSFETs

V-groove trench MOSFETs with the 4H-SiC{0-33-8} face as the trench sidewall for the channel region have been investigated. The on-resistance and breakdown voltage strongly depend on the aperture ratio of the buried p+ regions. The VMOSFETs with the buried p+ regions of 71% on a 6-inch wafer exhibited a low specific on-resistance of 2.0 mΩ cm2 with 1200 V blocking voltage. The threshold voltage is 2.3 V at 175°C, which shows the VMOSFETs have tolerability for an erroneous ignition under high temperature. The switching capability showed low switching losses over DMOSFETs on 4° off 4H-SiC(0001) face and normal operation under fast switching repetitive test (40 Vns-1). The stability of the threshold voltage was demonstrated by HTGB tests.

Evaluation of nanoimprint lithography as a fabrication process of phase-shifted diffraction gratings of distributed feedback laser diodes

The authors have succeeded in employing nanoimprint lithography (NIL) to form diffraction gratings of distributed feedback laser diodes (DFB LDs) used in optical communication. Uniform gratings and phase-shifted gratings with periods of 232 nm have been formed by using a reversal NIL with a step-and-repeat imprint tool. Line edge roughness has been sufficiently low with the fabricated gratings. DFB LDs fabricated by NIL have indicated comparable characteristics with LDs fabricated by electron beam lithography. LDs show high long-term stability in threshold current. The authors have also demonstrated that phase-shifted DFB LDs show better uniformity in characteristics than uniform-grating DFB LDs. The results of this study indicate that NIL has high potential for fabricating DFB LDs.