bulk
solid produces
an electrondeficient
site.
This introduces
a discrete,
unoccupied level
into
the band structure (Figure 5.13a). The band gap that
separates this level from the lower-lying occupied band is
small (10 kJ mol1) and thermal population of the acceptor
level is possible. The acceptor levels remain discrete if the
concentration of Ga atoms is low, and in these circumstances
the electrons in them do not contribute directly to the
electrical conductance of the semiconductor. However, the
positive holes
left behind
in the valence
band act
as charge
carriers;
one can
think
either
in terms of an electron
moving into the hole, thereby leaving another hole into
which another electron can move and so on, or in terms of
the movement of positive holes (in the opposite direction
to the electron migration). This gives rise to a p-type ( p
stands for positive) semiconductor. Other group 13 dopants
for Si are B and Al.