Pure bismuth is a semimetal, containing a small band gap, which leads to it having a relatively high conductivity ( at 20 °C). When the bismuth is doped with antimony, the conduction band decreases in energy and the valence band increases in energy. At an antimony concentration of 4%, the two bands intersect, forming a Dirac point (which is defined as a point where the conduction and valence bands intersect). Further increases in the concentration of antimony result in a band inversion, in which the energy of the valence band becomes greater than that of the conduction band at specific momenta.  Between Sb concentrations of 7 and 22%, the bands no longer intersect, and the BiSb becomes an inverted-band insulator. It is at these higher concentrations of Sb that the band gap in the surface states vanishes, and the material thus conducts at its surface.