  Topological phases of matter are often characterized by interface states,
which were already known to occur at the boundary of a band-inverted junction
in semiconductor heterostructures. In IV-VI compounds such interface states are
properly described by a two-band model, predicting the appearance of a Dirac
cone in single junctions. We study the quantum-confined Stark effect of
interface states due to an electric field perpendicular to a band-inverted
junction. We find a closed expression to obtain the interface dispersion
relation at any field strength and show that the Dirac cone widens under an
applied bias. Thus, the Fermi velocity can be substantially lowered even at
moderate fields, paving the way for tunable band-engineered devices based on
band-inverted junctions.
