Consider the electrons distribution in the top bandstructure with no electric field $($left$)$ and when a contant positive electric field is applied to a uniform sample $($right$).$ Fill the electron states on the right assuming the maximum energy the electrons can reach before being scattered is indicated by the dashed line:

$-1$: Electrons contributing to the conduction band current:

$-2$: Electrons not contributing to the conduction band current:

$-3$: Electrons contributing to the valence band current:

$-4$: Electrons not contributing to the valence band current:

$-5$: Holes contributing to the valence band current:

$-6$: Holes not contributing to the valence band current: