group 14 leads to C being an insulator, while for a-Sn, the
band structure approaches that of a single, partially occupied
band and this allotrope of Sn tends towards being metallic.
Each of Si, Ge and a-Sn is classed as an intrinsic semiconductor,
the extent of occupation of the upper band
increasing with increasing temperature. Electrons present
in the upper conduction band act as charge carriers and
result in the semiconductor being able to conduct electricity.
Additionally, the removal of electrons from the lower valence
band creates positive holes into which electrons can move,
again leading to the ability to conduct charge.