Yusuke Maeyama

A New High Current Gain 4H-SiC Bipolar Junction Transistor with Suppressed Surface Recombination Structure: SSR-BJT

A new 4H-SiC Bipolar Junction Transistor with Suppressed Surface Recombination structure: SSR-BJT has been proposed to improve the common emitter current gain which is one of the main issues for 4H-SiC BJTs. A Lightly Doped N-type layer (LDN-layer) between the emitter and base layers, and a High Resistive P-type region (HRP-region) formed between the emitter mesa edge and the base contact region were employed in the SSR-BJT. A fabricated SSR-BJT showed a maximum current gain of 134 at room temperature with a specific on-resistance of 3.2 mΩcm2 and a blocking voltage VCEO of 950 V. The SSR-BJT kept a current gain of 60 at 250°C with a specific on-resistance of 8 mΩcm2. To our knowledge, these current gains are the highest among 4H-SiC BJTs with a blocking voltage VCEO more than about 1000 V which have been ever reported.

Surface Passivation of 4H-SiC for High Current Gain Bipolar Junction Transistors

Surface passivation of 4H-SiC has been investigated for high current-gain bipolar junction transistors (BJTs). For the characterization of surface passivation, we have introduced the product “sp•Ls” of a surface recombination velocity (sp) and a surface diffusion length (Ls). The sp•Ls value was obtained by analyzing the I-V characteristics of pn diodes. Both BJTs and pn diodes were fabricated with several passivation methods. We have found clear correlation between the sp•Ls value and the current gain of the fabricated BJTs. Optimizing the surface passivation, we realized high performance BJTs with a current gain of 107 and a blocking voltage VCEO of 950 V.

Device Simulation Model for Transient Analysis of SiC-SBD

We report that it seems to be necessary to select Bologna University mobility model for accurate transient phenomenon analysis of SiC-SBD under the condition of forward surge current because the maximum of the temperature inside SiC-SBD arises up to above 425 K. Other mobility models seem to be mostly inadequate because they are corresponding to the experimental condition under the temperature below 425 K.

Reverse Biased Electrochemical Etching of SiC-SBD

With development of very-low-micropipe-density substrate, reduction of other device killer defects becomes important for large size power devices. We employed reverse biased electrochemical etching (RECE) method in order to elucidate where the current leaks out in Schottky barrier diode. Low concentration 4H-SiC epi-layer with Al implanted guard rings was electrochemically etched under reverse bias voltage up to 400V in HF-based electrolyte. The surface of the substrate was observed with Nomarski microscopy before and after RECE. In guard ring area, holes appeared which are aligned toward off-direction of the substrate. The length of the aligned holes is about 25μm. The guard ring surrounding the holes were etched uniformly but limited as though there exists a boundary parallel to the steps. In Schottky contact area, not all but some of carrot defects showed an etched feature after RECE. Such etching features are also observed at different position without carrot. We consider that threading screw dislocation is one cause of leakage current in SBD. An etching feature like tangled strings also appeared. Its outer dimension is more than 1mm with thickness of about 50μm. The origin of tangled-string-like feature is not clear yet.

Ohmic Contact for C-face n-Type 4H-SiC with Reduced Graphite Precipitation

The characteristics of Ni, Monel (Ni-Cu alloy, Ni55mol%-Cu45mol%), Monel/Si, Ni/Ti/Ni and Mo electrodes were studied for ohmic contact to C-face N-type 4H-SiC. Low contact resistivity (ρC) was not compatible with reduction of graphite precipitation in the case of Ni, Monel, Ni/Ti/Ni, and Mo electrodes. Monel/Si achieved less graphite precipitation and low ρC, which is enough to apply for actual rectifier, because a Monel/Si electrode forms a silicide without reaction between the deposits and the substrate.