development of electric field in a MOS capacitor
Potential difference is the integral of the electric field along a path between two points:

$$V_{ba} = -\int_a^b \vec{E}\cdot\rm{d}\vec{\mathcal{l}}$$

So whenever you see a potential difference between two points, that's really just a simplified way of looking at the fact that there's an electric field between them.

You shouldn't expect insulators to prevent electric fields. In fact, conductors tend to "block" electric fields, since they allow charge to move around to counteract whatever fields are imposed on them. Dielectrics tend to have minimal effect on static fields.