In this paper super-junction structures (SJS), which realise very low on-resistance, R(on), for a given breakdown voltage, V(br), are studied and comparisons made between 4H-SiC and Si. Simplified expressions are derived which describe the electric field distribution in the SJS under conditions where a charge imbalance exists in the drift region. These expressions are employed in determining the structure parameters which minimise the on-resistance of the SJS for a given breakdown voltage. The results show that SiC has more than 140 times lower Ran and smaller effects of pillar charge imbalance than Si, for the same V(br). In addition, the design margin for the pillar doping is defined and derived. It is shown that SiC has a 3.3 times higher margin than Si and it is concluded that the SJS is well suited to wide-bad-gap semiconductor materials.