In FACTS (Flexible AC Transmission Systems) and VSC-HVDC (Voltage Sourced Converter based High Voltage Direct Current) system, IGBT (Insulated Gate Bipolar Transistor) modules consisted of dozens of IGBT chips are required to achieve huge current capabilities. In order to have a highly reliable operation of the system, it is required that the threshold voltage $(\mathrm{V}_{\mathrm{TH}})$ variation of IGBT chips must be controlled within +/-0.1V in one module. To reduce the $\mathrm{V}_{\mathrm{TH}}$ variation of the 3300V IGBT platform, in this paper the P+ spacer implant and out-diffusion during P-well drive-in are investigated experimentally in detail. It is found that the P+ spacer implant has a significant impact on $\mathrm{V}_{\mathrm{TH}}$ . Being skipped it, the variation of $\mathrm{V}_{\mathrm{TH}}$ can be reduced as much as 80%. For the out-diffusion of phosphorus from highly doped polysilicon, the silicon experimental data reveals that it has little impact on $\mathrm{V}_{\mathrm{TH}}$ . Based on these data, more delicate experiments have been explored to improve the $\mathrm{V}_{\mathrm{TH}}$ variation of the IGBT platform.
