Introduction:
All materials can be classified into three groups, conductors, insulators, and semiconductors. In conductors the conduction and valance band are over laps and the electron will not find any obstruction to flow from one level to another level to cause conduction. In insulators the energy gap is more and ionic and covalent bonds tightly hold the atoms together. The probability of breaking this bond is very small. Hence there no free electron to conduct. Generally the materials having more than three electron volts of energy gap are known as Insulators. Roughly the conduction possibility in insulator is 1012 times less than in case of good conductors. There is a group of materials in which the energy gap (2-3eV) between the filled and unfilled zones is sufficiently small so that the electrons may be exited by thermal energy to move from the filled zone to empty zone. At absolute zero temperature the semiconductor will act as an insulator.
Semiconductor materials:
A semiconductor material is one whose electrical properties lie between those of insulators and good conductors. E.g. Germanium and Silicon.
Valence band:
The electrons occupying the outer most orbit or shell of an atom are called valence electrons. The band occupied by this electron is called valance band.
Conduction band:
The electrons, which have left the valance band, are called as conduction electrons. The band occupied by this electron is called conduction band.
Energy band:
The energy in an energy level of an isolated atom is called energy band. In terms of energy band, at room temp the semiconductor are classified as
- Partially filled conduction band.
- Partially filled valance band.
- A very narrow energy gap (1 ev) between conduction and valance band.
At absolute zero temperature there are no electrons in the conduction band of semiconductors and their valance band is completely filled. It means that at absolute zero temperature the semiconductor acts as an insulator. As the temp increases the semiconductor looses its insulating property and consequently gains conductivity. Semiconductors have negative temp coefficient of resistance.
Typical resistivities are of the following order.
Conductors =10-8ohm-m.
Semiconductors =103ohm-m
Insulators = 1012ohm-m.
Bonding in semiconductors
It has been observed that the semiconductors like Ge and Si have a crystalline structure. Both these materials are tetravalent i.e. they have 4 valance electrons. In order to acquire a stable electronic configuration, each atom shares its 4 electrons with 4 neighboring atoms and forms a covalent bond. The cross section of a Ge crystal lattice is shown in fig. The circles represent the atom cores. These cores consist of nuclei and the inner 28 electrons. The pair of line represents a covalent bond and a dot represents the valance electrons.
Properties of semiconductor:
Electronic property: The study of electrical and magnetic properties of the materials is called the electronic property.
Electrical conductivity: Electrical conductivity “σ” the reciprocal of resistivity of the material. This term describes the movement of electrical charges from one point to another point.
Current density
Electrical conductivity= ---------------------------------------------
Applied electric field intensity
i.e. σ =j /e =neμ
Where,
n= number of electrons
e= Charge of electrons
μ= Mobility (drift velocity acquired by the electron on application of unit electric field).
Conductivity of metals depends upon the presence of free electron in a crystal due to the metallic bond. In case of semiconductor the conductivity is due to charge carriers, which depends on many factors by purity of the semiconductor, temperature etc. The semiconductors can be classified as intrinsic and extrinsic (impurity or doped). The doped semiconductor can be further sub divided into two categories as electron or N-type and hole or P- type semiconductor.
Intrinsic semiconductor:
These are extremely pure. At temp close to absolute zero, the atoms of the crystal are covalent bonded and application of field does not cause any directional motion of the electrons. As the temp increases some of the electrons attain kinetic energy greater than the binding energy for the covalent bond. These electrons rupture and escape into the interstices of the lattice and become free. These electrons move freely giving rise to conduction.
Each electron, which moves into the interstitial space, becomes a conduction electron. Each electron, which moves out of the bond, leaves a vacancy or hole. An electron from the neighboring bond, thus creating the hole there occupies this vacancy. In this way a hole moves in the direction opposite to that of electron. Since each hole is an electron deficiency, it ascribes a ‘+’ charge. Thus in an intrinsic semiconductor the number of holes is equal to the number of electrons and the conduction is due to movement of both.
Extrinsic semiconductors:
These are also called as impurity or doped semiconductors. Conduction due to the electrons or holes of impurity atoms added to the semiconductor is known as extrinsic semiconductor. Impurity atoms have great influence on the electric conductivity of semiconductor materials.
There are two types of impurities which when added to the Germanium and silicon as a conductor matrix produces excess of electrons or holes. These are n type and p type semiconductors.
N-type crystals:
The four valance electrons of Ge and Si (belongs to iv group in periodic table) contribute to four co-valent bonds with the four nearest neighbors. When a pentavelant impurity such as an atom is added to tetravalent (Ge) matrix, only four of the five electrons of arsenic (As) participate in the co-valent bond formation with the four neighbors. The fifth electron does not participate in co-valent bond formation. Its bondage to the nucleus is also weak and because of high di-electric constant of Ge and energy of the bonding is some 250 times less.
At slightly elevated temperature the fifth electron gets detached. The semiconductor acquires the conductivity due to the free electrons. Since these electrons were not participating in the bond formation the positive ions do not behave as holes. These ions will not contribute for conduction and they will simply fix to the lattice (since the electron conductivity is the dominant factor for the conduction in a crystal with the pentavalent impurity. Such crystal is called N type crystal. Hence we can define the N-type semiconductor is one in which the electron conductivity is dominant factor for conduction in crystal with the pentavalent impurity (donor). For energy band diagram refer fig.
At elevated temperature the intrinsic electrons and holes of the matrix also contributes the conduction. The charge carriers whose concentration dominates the conduction process are called majority carriers and the charge carries of the opposite sign are called minority carriers.
P-type crystals:
This type of semiconductor is formed by adding a trivalent impurity like gallium, indium (having three valence electron). When indium is added to Ge, the indium atom lacks one electron to form four co-valent bonds with four Ge neighbor.
The resultant vacancy in the four bonds represents a hole. The trivalent impurities make available positive carrier or holes that can accept electron. These impurities are called acceptors. In the P-type semiconductor the predominant conduction is due to holes. As the temp increases an electron from the neighboring Ge-Ge co-valent bond goes and fills this vacancy. Thus making a negative indium ion bonded to the system with a hole in the Ge matrix. For energy band diagram refer fig.
17.5 Factors affecting the electrical conductivity
Temperature:
Structure disorders increases with increase of temp causing scattering of electrons. This result in low conductivity. Except at room temp, the resistively varies linearly with temp and the relationship is given as below
ρt=ρ20 [1+α (t-20)]
Where ρt - is the resistivity at t0 c
ρ20 - is resistivity at 20°C
α - is temp coefficient of resistivity per degree centigrade.
Impurities:
They cause drastic changes in resistivity. Local field around the solute atom is different from that present in the remaining portion of a material. This local fields cause scattering of electrons and hence reduces conductivity. The dependence of the resistivity on single impurity is given by “Nordium rule” ρt=Ax (1-x) where x is concentration. A is constant which depends on the base metal and the impurity.
Plastic deformation:
Imperfection in crystals like vacancies, dislocation, grain boundary, etc, give raise to scattering of electrons. Both resistivity and strain hardness depends on the number of dislocations present. That is way the resistivity (ρs) is high in cold work metal and annealing can reduce this, which removes imperfections.
Total resistivity =ρ=ρt +ρx+ρs=ρt+ρ0
(ρ0=ρx+ρs)
ρo=residual resistivity
Magnetic proprieties:
Some materials are magnetic in nature while others are not. Magnetic materials are involved in partially all-electrical apparatus starting form electromagnets, transformers, etc. It is essential to understand the differences between the various types of magnetic materials in terms of the magnetic properties of atoms and interaction between them.
Magnetism:
It is due to the motion of charges. The magnetic properties of the substances depend upon the presence of dipole moments. A charged particle having an angular momentum contributes to the dipole moment. There are three contributions to the angular momentum of an atom.
- Orbital angular momentum of an electron.
- Electrons spin angular momentum.
- Nucleus spin angular momentum
The total magnetic dipole moment of an atom is sum of the dipole moment due to each of the above forms of angular momentum.
The orbital angular momentum gives rise to induced dipole moment, which has a direction opposite to the applied magnetic field. Major observable magnetic behavior of an atom is due to the electrons spin. Orbital angular momentum of an electron and electron spin angular momentum gives rise to para magnetism and the nucleus spin angular momentum gives rise to diamagnetism. Atoms with filled electrons shells has zero spin.
Magnetization (M):
It is defined as the magnetic moment per unit volume. The magnetic susceptibility per unit volume is defined as
X=μ0M/B
Where μ0= Permeability of free space, B= macroscopic magnetic intensity, X= dimensionless. Diamagnetic substances have negative susceptibility and Para magnetic have positive susceptibility.
SL.
NO
|
ELEMENTS
|
CHEMICALS SYMBOLS
|
FORBIDEN ENERGY GAP
IN ev
|
APPLICATION
|
1
|
Indium antimonide
|
InSb
|
0.18
|
Infrared detectors
|
2
|
Lead telluride
|
Pb Te
|
0.33
|
-do-
|
3
|
Lead sulphide
|
PbS
|
0.37
|
-do-
|
4
|
Germanium
|
Ge
|
0.72
|
P-N junction devices
|
5
|
Silicon
|
Si
|
1.1
|
-do-
|
6
|
Gallium arsenide
|
GaAs
|
1.34
|
Tunnel diodes
|
7
|
Cadmium telluride
|
CdTe
|
1.45
|
Photocells
|
8
|
Cadmium sulphide
|
CdS
|
2.45
|
-do-
|
Super conducting materials:
Certain metals and large number of inter metallic compounds exhibits zero resistivity and undetectable magnetic permeability when they are cooled below a critical transition temp which nearer to absolute zero temp. This property refers to as super conductivity and electrical materials are then known as super conductor.
Thermal magnetic properties:
The characteristics temp at which a substance becomes super conductive depends on the strength of the magnetic field, whether the field is applied externally or is the result of current and the position of the super conductor in the periodic table. The critical values of temp and the magnetic field (Ho) at (0K) for several super conductors are shown in the table below.
Sl.no
| Super conductor material |
Magnetic filed (H0) in A/M at 0k (multiply by 79.6)
|
Transition temperature Tc in zero field k
|
1
|
Al
|
106
|
1.2
|
2
|
Hg
|
413
|
4.2
|
3
|
Nb
|
2000
|
9.2
|
4
|
Sn
|
305
|
3.7
|
5
|
Ti
|
20
|
0.4
|
6
|
V
|
1310
|
5.0
|
7
|
Nb3Sn
|
5000
|
18.1
|
8
|
V3Si
|
17.1
| |
9
|
NbN
|
16.0
| |
10
|
MoC
|
8.0
| |
11
| CuS |
1.6
|
It has been found at metals having low conductivity’s at room temp and metals with 3.5 or 7 valance electrons show super conductivity. These generalizations have lead to the formulation of many intermetallic compounds. So far 24 of the metallic elements are known to be super conductors. Possible application of super conductor may include frictionless electric motors, improved magnetic lenses for electron microscopes and noiseless amplifiers.
COMMENTS