4H‐SiC bipolar junction transistors (BJTs) are one of the promising candidates for next‐generation power devices. 4H‐SiC BJTs have the advantages of low on‐resistance and high temperature capability. On the other hand, high common emitter current gain is required from a practical use point of view because BJTs are current‐controlled devices. In order to improve the current gain of 4H‐SiC BJTs, we have concentrated on suppressing surface recombination on the SiC surface, which is a critical limiting factor of the current gain and can be suppressed by reducing carrier density and/or trap density. We have proposed novel 4H‐SiC bipolar junction transistors with suppressed surface‐recombination structure: SSR‐BJTs that have the SiC surface with low carrier density, characterized by a lightly doped n‐type layer (LDN layer) and a highly resistive p‐type region (HRP region). In addition, surface passivation suitable for reducing surface traps has been investigated by applying a new characterization method of surface‐recombination current in which the sp · Ls value, a product of a surface‐recombination velocity (sp) and a surface‐diffusion length (Ls), is derived from an analysis of forward I–V characteristics of SiC pn diodes. We have experimentally demonstrated that the sp · Ls value is a practical indicator to evaluate surface passivation. By using the characterization, an effective passivation method with the combination of a pyrogenic oxidation, a post‐oxidation anneal in H2 ambient, and an anneal in NH3 ambient has been developed. SSR‐BJTs with a variety of device structures and process conditions have been fabricated to investigate their characteristics. A fabricated SSR‐BJT showed a recorded maximum current gain of 134 at room temperature with a specific on‐resistance of 3.2 mΩ cm2 and a blocking voltage VCEO of 950 V. The SSR‐BJT kept a current gain of 60 °C at 250 °C with a specific on‐resistance of 8 mΩ cm2. These results have demonstrated the outstanding current‐gain capability of the SSR‐BJTs compared to conventional BJTs. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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 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.

No abstract