A p-SiC/n-GaN heterojunction double drift (DDR) IMPATT diode was fabricated while p-GaN was replaced by wide bandgap p-SiC. The output characteristics of AC large signal were simulated numerically. The results showed that compared with the traditional GaN single drift (SDR) IMAPTT diode, the breakdown voltage, optimal negative conductance, AC power density and DC-AC conversion efficiency of the new p-SiC/N-GaN DDR device were significantly improved, and the device had a wider oscillation frequency band. The new structure of the device had significant application potential in AC power density, which reached 1.97 MW/cm2. The diode was based on the wide gap semiconductor material, which provided reference value for the design and manufacture of GaN and SiC IMPATT device.