Semiconductors
Matter
Matter is defined as everything that has mass and volume. Change in state of matter changes the structure and the arrangement of particles.
Forms of Matter :
- Solid
- Liquid
- Gas
Solids are classified in to two types depending on the structural arrangement of atoms.
- Crystalline solid
- Amorphous solid
- Monocrystalline
- Polycrystalline
Figure : Classification of Solid materials
Crystalline solid
A material in which an arranged in an orderly, repeating pattern of atoms within the material exists is called Crystalline Solid.
A crystal is one in which atoms or molecules are in three dimensional periodic arrangement to minimize the total energy. The periodicity may be same or different in different directions. The periodic positions of the atoms or molecules are called space lattice or crystal lattice. The geometrical representation of a crystal structure in terms of lattice points is called space lattice. A crystal consists of a three dimensional periodic array of atoms or molecules.They have regular structure and properties like physical, electrical, optical depends on the directions inside the crystal
Ex:
all metals, NaCl, diamond, KCl etc.
Atomic arrangement in crystalline materials is described by by a space lattice.
Space Lattice
A space lattice is a three dimensional array of points.
Space Lattice + Basis = Crytal Structure
Crystal structure is determined by x-ray diffraction analysis.
Amorphous solid
A material which don’t have regular arrangement of atoms and the atoms are irregularly arranged is known as Amorphous solid .
Hence they don’t have definite structure. The electrical and optical properties are same through out the material.ie. They are isotropic in nature.
Ex: Glasses, Polymers, Rubber.
Material
A substance that can be used for making
or doing something. Such substances are called Materials.
Materials can be divided on the basis of electrical properties in three categories :
Classification of Materials On the basis of Conductivity
Following is detailed classification of Materials.
Classification and sub classification of Materials
Solids are divided into three types on the basis of electrical conductivity (𝞼) or resistivity (𝜌) :
- Metals
- Semiconductors
- Insulators
Metals
The materials which possess very low resistivity ranging 𝜌 ~ 10–2 – 10–8 Ω m or high conductivity 𝞼 ~ 102 – 108 S m–1 is known as Metals.
Semiconductors
The materials which have resistivity or conductivity intermediate to metals and insulators and possess resistivity ranging 𝜌 ~ 10–5 – 106 Ω m and conductivity 𝞼 ~ 105 – 10–6 S m–1 is known as Semiconductors.
Insulators
The materials which have high resistivity ranging 𝜌 ~ 1011 – 1019 Ω m or low conductivity ranging 𝞼 ~ 10–11 – 10–19 S m–1is known as Insulators.
Semiconductors Fundamentals
A semiconductor is a material which has electrical conductivity between that of a conductor such as copper and that of an insulator such as glass. The conductivity of a semiconductor material increases with increasing temperature. All the elements of IV group of periodic table are semiconductors (C,Si,Ge) etc. The energy band gap between valence band and conduction band is less than 3 eV.
In a metallic conductor, current is carried by the flow of electrons. In semiconductors, current can be carried either by the flow of electrons or by the flow of positively charged holes in the electron structure of thematerial. Silicon is used to create most semiconductors commercially.
Semiconducting materials have some specific properties such that the addition of impurities and increasing of temperature increase their electrical conductivity, unlike metals. Further semiconductor devices have very small size and less weight and require a small power to operate.
Properties of a semiconductor.
1. The resistivity lies between 10-4 to 0.5 ohm metre.
2. At 0K, they behave as insulators.
3. The conductivity of a semiconductor increases both due to the temperature and impurities.
4. They have negative temperature coefficient of resistance.
5. In semiconductors both the electron and holes are charge carries and will take part in
condition.
Types of Semiconductors
There are two types of semiconductors: One is intrinsic and the other is extrinsic semiconductor.
Intrinsic Semiconductors
A pure semiconductor without any impurities is known as intrinsic semiconductor. In intrinsic semiconductor, the electron and hole concentrations are equal because these carriers within a pure material are created in pairs. For example, pure germanium (Ge) or Silicon (Si).
In an intrinsic semiconductor,
ne = nh = ni
where ne = the free electron density in conduction band, nh = the hole density in valence band, and ni = the intrinsic carrier concentration.
Extrinsic Semiconductors
A doped impure semiconductor is known as extrinsic semiconductor. Doping increases the conductivity of semiconductors. On the basis of doping extrinsic semiconductors are classified as n-type and p-type.
The majority carriers in n-type and p-type are electrons and holes, respectively; the minority carriers in n-type and p-type are holes and electrons, respectively. For example, when a small amount of pentavalent impurity like arsenic or phosphorus is added to a pure germanium (Ge) or Silicon (Si), it is known as n-type semiconductor; however, when a small amount of trivalent impurity like gallium or boron is added to a pure germanium (Ge) or Silicon (Si), it is known as p-type semiconductor.
Doping
The process of adding impurities to a semiconductor is called doping. The impurity is called dopant. The purpose of adding impurity is to increase either the number of free electrons or holes in the semiconductor crystal.
If a pentavalent impurity is added to the semiconductor, a large number of free electrons are produced in the semiconductor.
If a trivalent impurity is added to the semiconductor, large number of holes are produced in the semiconductor crystal.
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