CONDUCTORS, INSULATORS AND CONDUCTIVITY
Conduction of electricity in electric circuits takes place due to the presence of excess electrons in materials called conductors. Electrons move in the direction in which the potential has been applied. The ability of a conductor to conduct electricity is directly proportional to the material’s area of cross section and inversely proportional to its length.
Where, G is conductance
σ is conductivity
A is area of cross section
is length of conductor
Insulators are materials which have very low conductivity as a result they are unable to conduct electricity.
The conductivity of conductors (usually metals) is very high and the conductivity of insulators is very low.
A semiconductor has electrical conductivity intermediate to that of a conductor and an insulator. Semiconductors differ from metals in their characteristic property of decreasing electrical resistivity with increasing temperature. The comprehensive theory of semiconductors relies on the principles of motion of electrons through a lattice of atoms. Current conduction in a semiconductor occurs via mobile or free electrons and holes, collectively known as charge carriers. Certain pure elements found in Group IV of the periodic table are semiconductors. The most commercially important of these elements are silicon and germanium. Semiconductor materials are useful because their behaviour can be manipulated by the addition of impurities, known as doping .Doping a semiconductor with a small amount of impurity atoms greatly increases the number of charge carriers within it. When a doped semiconductor contains excess holes it is called "p-type", and when it contains excess free electrons it is known as "n-type". Germanium, gallium arsenide, and silicon carbide are common dopants. A pure semiconductor is often called an “intrinsic” semiconductor. The electronic properties and the conductivity of a semiconductor can be changed in a controlled manner by adding very small quantities of other elements to the intrinsic material. This is typically achieved in crystalline silicon by adding impurities of boron or phosphorus to the melt and then allowing it to solidify into the crystal and the semiconductor is termed "extrinsic".
Photoconductivity is an optical and electrical phenomenon in which a material becomes more electrically conductive due to the absorption of electromagnetic radiation such as visible light, ultraviolet light,infrared light or gamma radiations. Band gap refers to the energy difference between the top of the valence band and the bottom of the conduction band; electrons are able to jump from one band to another. To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the band gap, or to excite the impurities within the band gap. When light is absorbed by a material such as a semiconductor, the number of free electrons and electron holes changes and raises its electrical conductivity.
LIGHT DEPENDENT RESISTOR
A photo resistor or light dependent resistor (LDR) is a resistor whose resistance decreases with increasing incident light intensity; in other words, it exhibits photoconductivity A photo resistor is made of a high resistance semiconductor. If light falling on the device is of high enough frequency, photons absorbed by the semiconductor give bound electrons enough energy to jump into the conduction band. The resulting free electron (and its hole partner) conduct electricity, thereby lowering resistance.
A diode is a two-terminal electronic component with an asymmetric transfer characteristic, with low (ideally zero) resistance to current flow in one direction, and high (ideally infinite) resistance in the other. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p-n junction connected to two electrical...