The band gap opens up at low temperatures due to hybridization of localized electrons (mostly f-electrons) with conduction electrons, a correlation effect known as the Kondo effect. As a consequence, a transition from metallic behavior to insulating behavior is seen in resistivity measurements. The band gap could be either direct or indirect. Most studied Kondo insulators are FeSi, Ce 3 Bi 4 Pt 3, SmB 6, YbB 12, and CeNiSn, although As of 2016 there are over a dozen known Kondo insulators.