LESSON 1 . Electron- Energy bond in solids- Insulators-semiconductors and metals.
Atom
An atom is defined as the smallest particles into which an element can be divided and still retain
the chemical properties of that element. Atom basically consists of electrons, proton and
neutrons. The proton and neutron is altogether called as nucleus. The nucleus is the central
portion of an atom. It is having positive charge. Around the nucleus small negatively charged
particles called electrons revolve in different paths. The different paths are called either orbits or
shells.
The electron
An electron is a negatively charged particle. It is moving around the nucleus. It is having
negligible mass. Hence it is very mobile.
Properties
Charge of an electron = 1.6025 X 10-19 coulomb
Mass of an electron = 9.106 X 10-31 kg
The ratio of charge mass of an electron = 1.77 X 1011 coulombs / kg
Energy of electron
The electrons which are moving around the nucleus possess two types of energy.
1. Kinetic energy due to its motion
2. Potential energy due to the charge on the nucleus.
Hence, the total energy of the electrons are the sum of these two energies. The total energy
increases as its distance from the nucleus increases. Thus the electrons in the last orbit possess
very high energy as compared to the electrons in the inner orbits.
Nucleus
The central portion of an atom is called nucleus. It contains protons and neutrons. The proton is
a particle carrying a positive charge. The positive charge is numerically equal to the negative
charge of an electron. The neutron has no charge but it is having the same mass as the proton.
So, the nucleus of an atom is positively charged, the mass of an electron is (1/1845 that of a
proton) negligible. Hence the neutron and proton constitute the entire weight of an atom. This is
called atomic weight. In an atom, the number of electrons is equal to the number of
,protons. This number is called as atomic number. The negative charges of the electrons are
exactly balanced by the positive charges of the protons. Therefore net charge of an atom is zero.
The electrons are revolving around the nucleus in different orbits. Each orbit has different
energy level. The electrons moving in the outermost orbit have highest potential energy. They
can be easily disturbed by external influences. These electrons are known as free electrons or
valance electrons. The last orbit is known as valance orbit. The electrons moving in the inner
orbits which are closely situated to nucleus have very low potential energy. They are much
influenced by the central portion nucleus. Hence, they cannot be much disturbed by external
influences. They are known as bound electrons.
Energy band in solids
1. Valence band
2. Conduction band
3. Forbidden energy gap
(i)Valence band
Electrons in the outer most orbit of an atom are called valence electrons. The range of energies
possessed by valence electrons is known as valence band. This band may be completely or
partially filled. It is the highest occupied band.
(ii) Conduction band
The electrons which left the valence band are called free electrons. The band occupied by these
electrons is called the conduction band. This band is next in the valence band. It may either be
empty or partially filled with electrons. In the conduction band, electrons move freely and
conduct electric current through the solid.
(iii) Forbidden energy gap
The valence band and conduction band are separated by a gap on the energy band diagram is
known as forbidden energy gap. There is no allowed energy state in the forbidden energy
gap. If the width of the forbidden energy gap is greater means the valence electrons are tightly
bounded to the nucleus and vice versa.
, Insulator
Insulator is a substance through which the passage of current is not allowed. The electrons in the
valence band are bound very tightly to their parent atoms. Hence it requires very large electric
field to remove them from their nuclei, in terms of energy band, the valence band is full, the
conduction band is empty and the forbidden energy gap is very large between them. This is
shown in Fig. 1.1(a). Therefore a very high electric field is required to carry the electron from
the filled valence band into the empty conduction band. At higher temperature some electrons
may go to the conduction band and in turn the insulator resistance decreases i.e., an insulator has
negative temperature co-efficient of resistance.
Semiconductor
A semiconductor is a substance whose conductivity lies in between conductors and insulators.
At low temperature, the valence band is completely full and conduction band is completely
empty. Hence a semiconductor behaves as an insulator at low temperatures. As the temperature
is increased, more valence electrons cross over to the conduction band and the conductivity
increases. Hence electrical conductivity of the semiconductor increases, with rising temperature
i.e., a semiconductor has negative temperature co-efficient of resistance. In terms of energy band
the valence band is filled and the conduction band is empty. Moreover, the energy gap between
valence and conduction band is very small as shown in Fig. 1.1(b). Hence, it requires smaller
electrical field to push the electron from the valence band to the conduction band.
Atom
An atom is defined as the smallest particles into which an element can be divided and still retain
the chemical properties of that element. Atom basically consists of electrons, proton and
neutrons. The proton and neutron is altogether called as nucleus. The nucleus is the central
portion of an atom. It is having positive charge. Around the nucleus small negatively charged
particles called electrons revolve in different paths. The different paths are called either orbits or
shells.
The electron
An electron is a negatively charged particle. It is moving around the nucleus. It is having
negligible mass. Hence it is very mobile.
Properties
Charge of an electron = 1.6025 X 10-19 coulomb
Mass of an electron = 9.106 X 10-31 kg
The ratio of charge mass of an electron = 1.77 X 1011 coulombs / kg
Energy of electron
The electrons which are moving around the nucleus possess two types of energy.
1. Kinetic energy due to its motion
2. Potential energy due to the charge on the nucleus.
Hence, the total energy of the electrons are the sum of these two energies. The total energy
increases as its distance from the nucleus increases. Thus the electrons in the last orbit possess
very high energy as compared to the electrons in the inner orbits.
Nucleus
The central portion of an atom is called nucleus. It contains protons and neutrons. The proton is
a particle carrying a positive charge. The positive charge is numerically equal to the negative
charge of an electron. The neutron has no charge but it is having the same mass as the proton.
So, the nucleus of an atom is positively charged, the mass of an electron is (1/1845 that of a
proton) negligible. Hence the neutron and proton constitute the entire weight of an atom. This is
called atomic weight. In an atom, the number of electrons is equal to the number of
,protons. This number is called as atomic number. The negative charges of the electrons are
exactly balanced by the positive charges of the protons. Therefore net charge of an atom is zero.
The electrons are revolving around the nucleus in different orbits. Each orbit has different
energy level. The electrons moving in the outermost orbit have highest potential energy. They
can be easily disturbed by external influences. These electrons are known as free electrons or
valance electrons. The last orbit is known as valance orbit. The electrons moving in the inner
orbits which are closely situated to nucleus have very low potential energy. They are much
influenced by the central portion nucleus. Hence, they cannot be much disturbed by external
influences. They are known as bound electrons.
Energy band in solids
1. Valence band
2. Conduction band
3. Forbidden energy gap
(i)Valence band
Electrons in the outer most orbit of an atom are called valence electrons. The range of energies
possessed by valence electrons is known as valence band. This band may be completely or
partially filled. It is the highest occupied band.
(ii) Conduction band
The electrons which left the valence band are called free electrons. The band occupied by these
electrons is called the conduction band. This band is next in the valence band. It may either be
empty or partially filled with electrons. In the conduction band, electrons move freely and
conduct electric current through the solid.
(iii) Forbidden energy gap
The valence band and conduction band are separated by a gap on the energy band diagram is
known as forbidden energy gap. There is no allowed energy state in the forbidden energy
gap. If the width of the forbidden energy gap is greater means the valence electrons are tightly
bounded to the nucleus and vice versa.
, Insulator
Insulator is a substance through which the passage of current is not allowed. The electrons in the
valence band are bound very tightly to their parent atoms. Hence it requires very large electric
field to remove them from their nuclei, in terms of energy band, the valence band is full, the
conduction band is empty and the forbidden energy gap is very large between them. This is
shown in Fig. 1.1(a). Therefore a very high electric field is required to carry the electron from
the filled valence band into the empty conduction band. At higher temperature some electrons
may go to the conduction band and in turn the insulator resistance decreases i.e., an insulator has
negative temperature co-efficient of resistance.
Semiconductor
A semiconductor is a substance whose conductivity lies in between conductors and insulators.
At low temperature, the valence band is completely full and conduction band is completely
empty. Hence a semiconductor behaves as an insulator at low temperatures. As the temperature
is increased, more valence electrons cross over to the conduction band and the conductivity
increases. Hence electrical conductivity of the semiconductor increases, with rising temperature
i.e., a semiconductor has negative temperature co-efficient of resistance. In terms of energy band
the valence band is filled and the conduction band is empty. Moreover, the energy gap between
valence and conduction band is very small as shown in Fig. 1.1(b). Hence, it requires smaller
electrical field to push the electron from the valence band to the conduction band.
