Abstract: With characteristics of 4H-SiC,a double base epilayer 4H-SiC bipolar junction transistors(BJTs) is investigated.By analyzing build-in electric field in the base region and the base transit time,the device is numerically calculated in two-dimensional models according to orthogonal experiments.A mean range analysis is made to find optimized structure of 4H-SiC BJTs.It shows that common emitter current gain with negative temperature coefficient is about 72 and it remains high in a wide range of collector current.