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As the number of atoms in the chain increases, so does the degree of level splitting
and ultimately two energy bands arise which are called the valence band and the
conduction band. They are continuous but contain a finite number of electronic
states. For a number of elements these zones overlap (Fig.
2.6
). The electrons of the
atoms occupy the lower levels of the valence band. The other levels remain vacant
and the electrons can move from the valence to the conduction band. Electronic
conductivity arises in the system. Such electronic structure corresponds to conduc-
tors of electric current. In general the properties of solids are determined by the
distance between the valence and conduction bands, i.e., by the band gap between
them, and by the degree to which the valence band is filled by electrons. If the band
gap is wide and the valence band is completely filled, the solid is an insulator. In the
most interesting case, corresponding to a semiconductor, the valence band is nearly
completely filled and the band gap is relatively narrow.
Semiconductors that practically do not contain dopants are called intrinsic, or
undoped. In this case, when an excited electron is promoted from the valence to the
conduction band, a positively charged vacancy is produced in the valence band.
Of course the neighboring electrons can neutralize this vacancy, but while doing so
they will form a new vacancy elsewhere. Thus, a positively charged moving entity
appears in the semiconductor that is called a hole. In intrinsic semiconductors
charge carriers must appear in pairs (electron-hole pair). The situation changes
significantly if a certain amount of dopants—alloying additions—is introduced into
the semiconductor. We will consider silicon whose electronic structure and proper-
ties correspond to a semiconductor. Tetravalent silicon forms four covalent bonds
with neighboring atoms. If trivalent boron is introduced into the structure of the
silicon crystal, one of the bonds remains unfilled (Fig.
2.7
). It can be filled by an
electron of any other neighboring silicon atom, leading to the formation of a hole.
Dopants of this kind are called acceptors, and the resulting holes are situated just
above the valence band. Such semiconductors are called p-type semiconductors.
A different situation arises when a pentavalent atom of a dopant (phosphorus or
antimony) is introduced into a silicon semiconductor. These atoms have five
valence electrons, one more than silicon. The fifth electron is easily detached
from the atom containing it. As a result a static ionic charge as well as an energy
state corresponding to the fifth electron, situated slightly below the conduction
band, arise. Such dopants are called donor dopants and the semiconductors n-type
semiconductors.
Remarkable properties arise upon contact of p- and n-type semiconductors.
In this case, due to large difference of concentrations—a whole sea of holes on
one side and a sea of electrons on another side—strong diffusion currents of holes
and electrons arise. As a result minority carriers appear in the system—electrons in
p-type semiconductors and holes in n-type semiconductors. At the same time ions
of the electron-hole pairs will be approaching the junction causing electric field
around it (Fig.
2.8
). In turn, this field will cause drift currents of electrons toward
the n-type material and of holes toward the p-type material. If the external voltage is
absent, the diffusion and drift currents will be equal in magnitude and opposite in
direction. Therefore, the total current will be zero. Suppose that electric field is
