The distribution of potential in the space charge region of a semiconductor results in a change in the electron energy levels with distance from the interface. This is usually described as 'bending of the energy bands'. Thus the bands are bent, upwards if $\sigma >0$ and downwards if $\sigma <0$, where $\sigma$ is the free charge density. When $\sigma =0$ the condition of flat bands is met, provided no surface states are present.