Figure 6.3: The doping process illustrated using the bonding model. (a) A phosphorus (P) atom replaces a Si atom in the
lattice resulting in the positively-ionized P atom and a free electron. (b) A boron (B) atom replaces a Si atom resulting in
the negatively ionized B atom and a hole.
An atom with three valence electrons such as boron cannot form bonds with all four
neighbouring Si atoms when it replaces a Si atom in the lattice. However, it can readily
“accept” an electron from a nearby Si-Si bond. The thermal energy of the c-Si lattice at
room temperature is sufficient to enable an electron from a nearby Si-Si bond to attach
itself to the boron atom and complete the bonding to the four Si neighbours. In this
process a hole is created that can move around the lattice. The impurity atoms that
enhance the concentration of holes are called acceptors. We denote the concentration of
acceptors by N A .
Note that by substituting Si atoms with only one type of impurity atoms the
concentration of only one type of mobile charge carrier is increased. Charge neutrality of
the material is nevertheless maintained because the sites of the bonded and thus fixed
impurity atoms become charged. The donor atoms become positively ionized and the
acceptor atoms become negatively ionized.
The possibility to control the electrical conductivity of a semiconductor by doping is
one of the most important semiconductor features. The electrical conductivity in
semiconductors depends on the concentration of electrons and holes as well as their
mobility. The concentration of electrons and holes is influenced by the amount of the
doping atoms that are introduced into the atomic structure of the semiconductor. Figure
6.4 shows the range of doping that is used in case of c-Si. We denote a semiconductor as
p-type or n-type when holes or electrons, respectively, dominate its electrical conductivity.
When one type of charge carriers has a higher concentration than the other type, these
carriers are called majority carriers (holes in the p-type and electrons in the n-type), while
the other type with lower concentration are then called minority carriers (electrons in the
p-type and holes in the n-type).
lattice resulting in the positively-ionized P atom and a free electron. (b) A boron (B) atom replaces a Si atom resulting in
the negatively ionized B atom and a hole.
An atom with three valence electrons such as boron cannot form bonds with all four
neighbouring Si atoms when it replaces a Si atom in the lattice. However, it can readily
“accept” an electron from a nearby Si-Si bond. The thermal energy of the c-Si lattice at
room temperature is sufficient to enable an electron from a nearby Si-Si bond to attach
itself to the boron atom and complete the bonding to the four Si neighbours. In this
process a hole is created that can move around the lattice. The impurity atoms that
enhance the concentration of holes are called acceptors. We denote the concentration of
acceptors by N A .
Note that by substituting Si atoms with only one type of impurity atoms the
concentration of only one type of mobile charge carrier is increased. Charge neutrality of
the material is nevertheless maintained because the sites of the bonded and thus fixed
impurity atoms become charged. The donor atoms become positively ionized and the
acceptor atoms become negatively ionized.
The possibility to control the electrical conductivity of a semiconductor by doping is
one of the most important semiconductor features. The electrical conductivity in
semiconductors depends on the concentration of electrons and holes as well as their
mobility. The concentration of electrons and holes is influenced by the amount of the
doping atoms that are introduced into the atomic structure of the semiconductor. Figure
6.4 shows the range of doping that is used in case of c-Si. We denote a semiconductor as
p-type or n-type when holes or electrons, respectively, dominate its electrical conductivity.
When one type of charge carriers has a higher concentration than the other type, these
carriers are called majority carriers (holes in the p-type and electrons in the n-type), while
the other type with lower concentration are then called minority carriers (electrons in the
p-type and holes in the n-type).