The on-resistance which limits the maximum drive current, has to be optimized. This value preliminarily depends on the geometry and the doping of the n-epi-layer. A thinner layer reduces the on-resistance, but also reduces the breakdown voltage. On the other hand, increasing the dopant concentration in the epi-layer also reduces the on-resistance, but because of the higher electric field the breakdown voltage is limited. The optimization has to find a compromise between a small RDS on and an acceptable breakdown voltage.