As a thyristor-like device, IGCT is more applicable to the high-voltage and high-power fields due to the lower on-state voltage drop, and a combination of transparent anode and hard drive enables IGCT to turn off faster and more reliably. However, with an increase in power capacity of IGCT, the reliability of IGCT is becoming an increasing concern.
Based on the multi-cell structure model, the turn-off characteristics and robustness of IGCT under over-stress conditions are studied in this paper. The results show that during GCT turn-off the modulation of free carrier to the electric field in the space charge region makes the dynamic avalanche effect occur at the anode-cathode voltage much lower than the rated blocking voltage of the device, and the avalanche-induced current filament effect may occur due to the distortion of electric field and resulting negative differential resistance effect at strong dynamic avalanche. In comparison, the behavioral characteristics of current filament at different stages of turn-off behave differently.
During the voltage rise period of IGCT turn-off, the avalanche-induced current filament can move rapidly, which will not cause too much temperature rise and has a less impact on the robustness of the device. In contrast, once the anode-cathode voltage rises close to the static avalanche breakdown voltage, the SSCM will occur, the device will operate in its static avalanche breakdown mode. If the device operates on the NDR branch of its static avalanche breakdown characteristic curve, a very slow moving current filament driven only by temperature rise will arise. This makes the power consumption that is required to be borne by the entire device only by the area where the current filament is located, resulting in a very high local current density and a large local temperature rise, and the device is easy to re-trigger or thermal breakdown.
The static avalanche breakdown characteristics of IGCT determine the nature of the current filament under SSCM. The larger the common-base current gain α_pnp of the parasitic pnp transistor of IGCT, the stronger the avalanche-induced current filament under SSCM, and the slower its movement speed, thereby significantly reducing the robustness of the device. Therefore, in order to improve the robustness of the device under SSCM, α_pnp needs to be controlled more precisely during the design of GCT chip.