IGBT Basic (5)

3-3. Static Blocking Characteristics

Reverse Blocking Capability
When negative voltage is introduced to the collector as shown in Fig. 1, the P+ substrate / Ndrift junction (J1) is reverse biased, and the depletion layer generally expands to the N- drift region.

As such, securing an optimal design in resistivity and thickness for the N- drift region is essential in obtaining desirable reverse blocking capability. As a general guideline, the width of the N-drift region is equivalent to the sum of depletion width at maximum operating voltage and minority carrier diffusion length. It is important to optimize the breakdown voltage while maintaining a narrow N- drift region width, as the forward voltage drop increases with an increase in N- drift region width. The following is an equation for calculating the N- drift region width:

Forward Blocking Capability
When the gate is shorted at the emitter, and a positive voltage is introduced at the collector, the P base / N- drift region junction (J2) is reverse biased, and it is supported upto the rated voltage by the depletion region formed at the N- drift region.