Based on the material data of 4H-SiC, we present a breakdown model for 4H-SiC pn junction diode, which includes both the avalanche impact ionization and the ban d-to-band tunneling. With this model, the influence of tunneling on the breakdow n characteristics of the devices is analyzed. The breakdown mechanism at differe nt temperatures and doping concentrations is explained, and the simulation resul ts can give a nice description for the breakdown characteristics of real devices .