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.