For 4H-SiC power (1800 V, 10 A) npn bipolar junction transistors, the dependence of base current gain beta on collector current I-C have been measured and properly calculated to determine the contribution of different factors limiting the current gain. At low current densities, the emitter injection coefficient gamma(E) is limited by carrier recombination in the emitter space charge region so that beta increases with increasing I-C. At high current densities, beta steeply falls with increasing I-C and the gamma(E) value decreases because of surface recombination and increasing carrier recombination in the emitter layer, which is enhanced by the current crowding effect.