A graded-gap superlattice has been investigated to study the formation of miniband states at finite electric fields. In contrast to conventional superlattices only one type of states, either the conduction band or the valence band states, form a miniband. The transition from extended miniband states at finite electric fields back to localized states is clearly observed with increasing electric field. Calculations of the miniband structure using different methods support the formation of a miniband in the conduction band at finite electric fields.