Semiconductor device: a
conductor made with semiconducting material. Semiconductors
are made up of a substance with electrical properties
intermediate between a good conductor and a good insulator.
A semiconductor device conducts electricity poorly at room
temperature, but has increasing conductivity at higher
temperatures. Metalloids are usually good semiconductors.
Material, such as germanium, gallium arsenide and silicon
are some of the good semiconductor substances.
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In order to understand how semiconductor devices work one needs a
more complete description of the nature of charge in the real world.
Charge does not exist
independently; it is carried by subatomic particles.
For this
discussion we will be concerned primarily with electrons, which
carry a negative charge of 1.6 × 10-19 C , the minimum
amount of charge that can exist in isolation. At least, no one has
found any smaller amount than this fundamental quantum of charge.
Electrons are one component of atoms and molecules. Atoms
are the building blocks out of which all matter is
constructed.
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Atoms bond with each other to form substances. Substances composed
of just one type of atom are called
elements.
For example, copper, gold and silver are all elements; that is, each
of them consists of only one type of atom. More complex substances
are made up of more than one atom and are known as
compounds. Water, which
has both hydrogen and oxygen atoms, is such a compound. The smallest
unit of a compound is a
molecule.
A water molecule, for example, contains two hydrogen atoms and one
oxygen atom.
Atoms themselves are made up of even smaller components:
protons,
neutrons and
electrons. Protons and
neutrons form the
nucleus
of an atom, while the electrons orbit the nucleus. Protons carry
positive charge and electrons carry negative charge; the magnitude
of the charge for both particles is the same, one quantum charge,
1.6 ×10-19 C . Neutrons are not charged. Normally, atoms
have the same number of protons and electrons and have no net
electrical charge.
Electrons that are far from the nucleus are relatively free to move
around under the influence of external fields because the force of
attraction from the positive charge in the nucleus is weak at large
distances. In fact, it takes little force in many
cases to completely remove an outer electron from an atom, leaving
an
ion with a net
positive charge. Once free, electrons can move at speeds approaching
the speed of light (roughly 670 million miles per hour) through
metals, gases and vacuum. They can also become attached to another
atom, forming an ion with net negative charge. |
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Fig - 1 : Structure of an
Atom
Fig - 2 : Simple Model of
Electron Flow
Electric current in metal conductors consists of a flow of free
electrons. Because electrons have negative charge, the flow of
electrons is in a direction
opposite to the positive current. Free electrons
traveling through a conductor drift until they hit other electrons
attached to atoms. These electrons are then dislodged from their
orbits and replaced by the formerly free electrons. The newly freed
electrons then start the process anew. At the microscopic level,
electron flow through a conductor is not a steady stream, like water
flowing from a faucet, but rather a series of short bursts.
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