In As-doped Si, replacing an Si (group 14) by an As (group
15) atom introduces an electron-rich site. The extra electrons
occupy
a discrete
level below
the conduction
band (Figure
5.13b), and,
because of the small band gap (10 kJ mol1),
electrons from the donor level can thermally populate the
conduction band where they are free to move. Electrical
conduction can be described in terms of the movement of
negatively charged electrons and this generates an n-type (n
stands for negative) semiconductor. Phosphorus atoms can
similarly be used as dopants in silicon.