CONDUCTORS , INSULATORS AND SEMI CONDUCTORS
All substances are classified into three types known as conductors, insulators and semi conductors based on their ability to allow the electrical current to pass through them.
Conductors : Those substances which allow electrical current to pass through them completely are known as conductors.
Examples are metals, alloys and fused electrovalent compounds.
Insulators : Those substances which do not allow electrical current to pass through them are known as insulators. Examples of insulators are wood, silk, cotton, glass, rubber, organic compounds like benzene and carbon tetra chloride.
Semiconductors : Those substances that allow the electrical current to pass through them partially (or) possess very low electrical conductivity are known as semi conductors. Examples of semi conductors are Silicon, Germanium, oxides of Manganese, Cobalt oxide, Titanium dioxide etc.
Conductors of electricity are further classified into two types depending on their mode of transport of electrical charges inside them. These are electronic conductors and electrolytic conductors.
Metallic conductors : Metals and alloys conduct electricitiy due to the movement (mobility) of delocalised outershell electrons present inside them and are known as electronic conductors (or) metallic conductor. Electrical conduction through metals do not bring about any chemical transformations.
Electrolytic conductor : Electrovalent (or) ionic compounds conduct electrical current in their dissolved state (in solutions) or in their fused state only. These compounds consists of ions of opposite charges and in aqueous solution they exist as ions. In the presence of applied electrical field, these ions migrate to respective electrodes exhibiting electrical conductivity. This mobility of ions is responsible for conduction of electricity through electrolytes and is referred as electrolytic conduction. Passage of current through electrolytes is accompanied by chemical changes also. The differences in the properties of electronic and electrolytic conductors are given below :
Pure metals and their solid solutions such as alloys are called as metallic conductors.
Free and mobile electrons of the metallic atoms or alloys are responsible for electrical conductance.
Positive holes in the metals move in the opposite direction to electrons.
There is no chemical change in the material when electricity is passed.
There is only flow of electrical energy but there is no transfer of matter.
Conductivity of metal decreases with increase in temperature due to the enhanced thermal vibration of metal atoms disrupting the movement of electrons passing through them.
Electrovalent (or) ionic compounds conduct electricity through their ions present in fused state or in dissolved state.
Ions with positive and negative charges conduct electricity and move towards cathode and anode respectively.
Electrolysis occurs when electrical current is passed through electrolytic solutions.
Chemical change occurs.
There is actual transfer of matter since ions move towards respective electrodes.
The conductivity of electrolytes increases with increase in temperature. This is due to increase with ionic mobility.
Semi conductors : Certain type of solids like pure silicon and germanium which are poor conductors of electricity at normal temperature become good conductors either at high temperatures or in the presence of impurities like Arsenic or Boron. There are two types of semi conductors known as intrinsic and extrinsic semiconductors.
Intrinsic semi conductors : In the intrinsic type, these solids have very low conductivity at room temperature but at high temperatures one of the interatomic covalent bonds between Silicon (or) Germanium atoms are broken heterolytically such that free electrons and corresponding positive holes are created. When electrical field is applied these electrons migrate along the direction of the applied electricfield causing electrical conductivity in them. The positive holes move in opposite direction to that of the movement of electrons.
Extrinsic semi conductor : In the extrinsic type of semi conductors addition of impurities like Arsenic or Boron causes appreciable increase in the electrical conductivity. This effect can be obtained as follows :
N-type semi conductor : In silicon and germanium crystals, each atom is covalently bonded to four neighbours so that all its four valence electrons are tied down. Thus in the pure state these elements are poor conductors. Suppose an atom of arsenic is introduced in place of silicon or germanium in the crystal lattice. Arsenic has five valence electrons, four of which will be utilised in the formation of covalent bonds and the remaining electron is free to move through the lattice. This leads to enhanced conductivity.
p-type semi conductor : Now let a Boron atom be introduced in place of Silicon atom in the crystal lattice. A Boron atom has only three valence electrons. It can form only three of the four bonds required for a perfect lattice. Thus it is surrounded by seven electrons (one of Si) rather than eight. Thus electron vacancy or a 'positive hole' in the lattice is produced. Another electron from the bond of the adjacent Si atom moves into this hole, completing the four bonds on the B atom. This electron also leaves a hole at its original site. In this way electrons move from atom to atom through the crystal structure and the holes move in the opposite direction. Therefore the conductivity of the material improves.
Semi conductors which exhibit conductivitiy due to the flow of excess negative electrons, are called n-type semiconductors (n for negative).
Semiconductors which exhibit conductivity due to the positive holes, are called p-type semiconductors (p for positive).