Suppressed surface-recombination structure and surface passivation for improving current gain of 4H-SiC BJTs

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 s(p) . L(s) value, a product of a surface-recombination velocity (s(p)) and a surface-diffusion length (L(s)), is derived from an analysis of forward I-V characteristics of SiC pn diodes. We have experimentally demonstrated that the s(p) . L(s) 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 H(2) ambient, and an anneal in NH(3) 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 m Omega cm(2) and a blocking voltage V(CEO) of 950 V. The SSR-BJT kept a current gain of 60 degrees C at 250 degrees C with a specific on-resistance of 8 m Omega cm(2). These results have demonstrated the outstanding current-gain capability of the SSR-BJTs compared to conventional BJTs. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim