The electronic level structure of a type-I semiconductor superlattice such as GaAs/Al/sub x/Ga/sub 1-//sub x/As has been calculated self-consistently in a tight-binding envelope approximation. Fermi-level pinning due to defects at the ends of the superlattice gives rise to novel surfacelike states lying in electronic subband gaps and having localized eigenstates. Varying barrier transparency and modulation-doping density are shown to affect the location of these surface levels within the subband gap. Application of a gate voltage across the superlattice produces drastic changes in the level structure, including transitions from extended to localized behavior in some eigenstates concurrent to shifts in their energies. Sensitivity to gate voltage should yield a unique electrooptical response function in these systems.