Chapter 01
Electrical Conductors, Semiconductors and Insulators
Materials can be divided
into three categories depending on their ability to conduct electricity. That
is as,
1. Conductors
2. Semiconductors
3. Insulators
Conductors
Materials that conduct
electricity are known as electrical conductors.
All the metals such as copper, aluminium, and iron
and the metal alloys such as brass, nichrome, and manganin are electrical
conductors.
The reason for
electricity to flow easily through metals is the existence of free electrons.
The valence electrons (electrons in the outermost shell of an atom) of different materials possess different energies. The higher the energy of a valence electron, the lesser it is bound to the nucleus.
In certain materials, particularly metals, the valence electrons
possess very high energy that they are loosely attached to the nucleus.
Therefore, the valence electrons of metallic atoms can be easily detached from
the atom by applying a small amount of external energy. This makes them good
conductors of electricity.
A large number of such detached free electrons from the outermost shell of metal atoms are in random motion in the regions between metal atoms as shown below.
These loosely attached
valence electrons that move randomly within the material are called free
electrons.
Insulators
Materials that do not
conduct electricity are known as electrical insulators.
Ebonite, polythene,
plastic, dry wood, asbestos, rubber, glass air etc. are examples of electrical
insulators.
The reason for no flow of
electricity exists in these materials is because virtually there are no
effective free electrons present in them. That is, in insulators valence
electrons are bound very tightly to their parent atoms, thus requiring very
large energy to remove them from the attraction of their nuclei.
Semiconductors
Materials that conduct a
smaller amount of electricity are known as semiconductors.
Semiconductors are
neither very good conductors nor very good insulators. Its electrical properties lie between insulators and good
conductors.
Materials such as silicon
(Si) and germanium (Ge) in their crystalline form show such properties. These
elements belong to the fourth group in the periodic table and have four
electrons in their outermost shell. Such elements form crystal lattice
structures by sharing the four electrons in their outermost shell to make
covalent bonds with four nearby atoms, thereby acquiring a stable electronic
configuration having eight electrons in the outermost shell.
However, these bonds are
rather weak and can be broken from the thermal energy available even at room
temperature, releasing electrons.
The following figure shows the covalent
bonds of the silicon lattice at 0 K. All the bonds are complete at this
temperature.
The following figure shows that some bonds have been broken releasing some free electrons at a temperature higher than 0 K.
An electron deficiency can be observed at the
positions that the free electrons occupied previously. Such positions with an
electron deficiency are known as holes.
Due to the positively charged
protons in the nucleus, a hole gives rise to a positive charge that has not been
neutralized (In a neutral atom, the number of protons in the nucleus is equal to
the number of electrons). Therefore, a hole is equivalent to a positive
charge.
In semiconductors, not
only free electrons contribute to the conduction of electricity. When an
electron in an adjacent atom jumps to an atom with a hole having a positive
charge, the position of the hole can change. By changing the position of a hole
from one atom to another in this manner, holes can move around in the lattice and
contribute to conducting a current. Free electrons act as negative
charge carriers while holes act as positive charge
carriers.
Therefore, in
semiconductors, the negatively charged electrons as well as the positively charged
holes act as the charge carriers that contribute to the conduction of electricity.
0 Comments