have an electrical resistivity that is in between those of good
conductors and those of good insulators. Both silicon and germanium,
which are the two basic semiconductors, have four electrons in the
outermost electron subshell. In formation of the lattice structure of
the silicon or germanium, all the valence electrons are involved in
the bonding, so the material should be an insulator. However, an
unusually small amount of energy is needed to break one of the bonds
and set an electron free to roam around the lattice. This energy is
approximately 1ev. This energy corresponds to the energy gap between
the valence and conduction bands. In an insulator, this energy gap is
very high to approximately 5ev. No electron can naturally attain 5ev.
At room temperature, a substantial number of electrons are dislocated
from their parent atom in a semiconductor. This number increases with
increasing temperature so we can say that semiconductors have higher
conductivity at higher temperatures.
When an electron
is removed from a covalent bond, it leaves a hole and this hole can
travel through the lattice and serve as an additional current
carrier. The current mostly comes from the electrons that are out of
the lattice structure. A hole behaves like a positively charged
particle. In a pure semiconductor, holes and electrons are always
present in equal numbers.
Devices such as
transistors and diodes are fabricated using impurity semiconductors
prepared by adding small quantities of foreign atoms, such as arsenic
or gallium, to an intrinsic semiconductor. The added foreign atom
only accounts for a few part per million. The process of this is
known as doping. This process produces two distinct kinds of systems.
When silicon is doped with a five valence electron atom as arsenic,
the fifth electron is not locked in place so it does not fit and can
move around freely within the crystal. These electrons stay in an
energy level just below the conduction band, into which can easily be
made to jump. Because these charge carriers are negative, the system
is referred to as an n-type semiconductor. When the silicon is doped
with a three valence electron atom like gallium, there will be a
missing electron in the lattice structure. In effect, there would be
a hole in the negative distribution of electrons. An outer electron
from a nearby silicon atom can drop out of its cloud and drop into
the hole, but this created another new hole in the place where the
electron originally was. Because the carriers are positive, this
system is referred to as a p-type semiconductor. When the n-type
semiconductor is formed, there is a new level formed in between the
valence band and the conduction band called the donor level. In a
p-type semiconductor, this level is called the acceptor level.
Silicon Essay, ResearchPaper Silicon Silicon is the raw ... . Semiconductors are usually materials which have energy-band gaps smaller than 2eV. An ... . It is used for diodes, transistors, integrated circuits, memories, infrared detection ...
... Report Project Essay, ResearchPaper 1. Company Information ... to mass-produce semiconductors .It used low ... cost plastic encapsulated transistors. The design ... environment. Motorola is an equal opportunity employer ... Infrastructure Group (CIG) have marked a 14 year ...
... Economics and Business Department ResearchPaper: „The Development ... of export have for an object encouragement ... office equipment components, transistors and semiconductors is very insignificant ... and light industry. Within each sub-industry a desirable ...