Study of the electrostatic potential of the floating-p region during the turn-on period of IGBT

In this work, the influence of the initial electrostatic potential value of the floating-p region, Vfp, on the controllability over the turn-on dIc/dt (Collector current increase rate) is studied using TCAD simulation. To investigate the influence of Vfp on the turn-on characteristics, a two-step calculation procedure is used. In the first step, the off state is calculated in which the floating-p region is connected to a fixed voltage source. In the second step, the turn-on characteristics are calculated in which a large resistance is connected between the floating-p region and the voltage source. In this manner, Vfp can be fixed at arbitrary value Vfix at the beginning of the turn-on period, and, then, becomes floating during the following period. The increase rate of the collector current becomes lower as Vfix increases because the high initial Vfp accumulates fewer holes in the floating-p region. And the mechanism that determines the initial Vfp is also analyzed. A hypothesis is set up that the off-state Vfp is equal to the threshold voltage of the P-channel MOSFET which consist of the floating-p, the n-drift, and the p-base region. To confirm the hypothesis, the relation between Vfp and major structural components such as the oxide permittivity, the oxide thickness, and the doping concentration are investigated. These results show a good agreement with the theoretical formula of threshold voltage of MOSFET.