With the increase of IGBT voltage and current ratings, the avalanche effect has become an important factor limiting the safe operating area (SOA) of the device. The hole injection of the p+n junction on the back of the device when the avalanche effect occurs is the main feature that distinguishes the avalanche effect of the IGBT from other devices. In this paper, the avalanche breakdown characteristics of IGBT and the behavior of avalanche-generated current filaments are studied through theoretical analysis and numerical simulation, and the physical mechanism dominating the behavior of avalanche-generated current filaments is revealed. The results show that the hole injection on the backside of IGBT leads to an additional negative differential resistance branch on the avalanche breakdown curve, and the strength of the negative differential resistance effect depends on the common base current gain of IGBT α _pnp. With the increase of α _pnp, the negative differential resistance effect becomes stronger, the avalanche current at the valley point where the additional negative differential resistance branch transforms into the positive differential resistance branch also becomes higher. And the valley point at the avalanche breakdown curve of IGBT dominates the strength of the avalanche-generated filament. As a result, the strength of avalanche-generated filament depends on the α _pnp. With the lattice temperature increasing, the avalanche breakdown voltage of IGBT increases, leading to the shifting of the avalanche breakdown curve towards a higher voltage. And with the increase of α _pnp, the offset of the avalanche breakdown curves at high and low temperature becomes smaller, which dominates the lateral movement speed of the avalanche-generated filament. With the increase of the α _pnpand the decrease of the offset of avalanche breakdown curves at high and low temperature, the avalanche-generated filament laterally moves more slowly. To sum up, with the increase of the α _pnpof IGBT, the avalanche-generated filament becomes stronger and moves more slowly, which extremely causes the local overheating where the filaments exist and lowers the avalanche robustness of the device. Therefore, the α _pnpof IGBT must be controlled precisely in order to have a good trade-off between the characteristics and the reliability of IGBT.