In terms of their electrical properties,materials can be classified into three groups:
When atoms combine to form a solid,crystalline materials,they arrange themselves in symmetrical pattern.The atoms within crystal structure are held together by covalent bonds,which are created by the interaction of the valence electrons of the atoms. Silicon is a crystalline material.All materials are made up of atoms.These atoms contribute to the electrical properties of material,including its ability to conduct electrical current.
An insulator is a material that does not conduct electrical current under normal conditions.Most good insulators are compounds rather than single element materials and have very high resistivities. Valence electrons are tightly bound to the atoms:therefore ,there are very few free electrons in an insulator.Examples of insulators are rubber,plastics,glass,mica,and quartz.
In terms of energy bands,it means that insulators have:
- Full valence band.
- An empty conduction band.
- A large energy gap (of several e V) between them.
For conduction to take place,electrons must be given sufficient energy to jump from the valence band to the conduction band.Increase in temperature enables some electrons to go to the conduction band which fact accounts for the negative temperature coefficient of resistance of insulators.
A conductor is a material that easily conducts electrical current.Most metals are good conductors.The best conductors are single element materials,such as copper(Cu),silver(Ag),gold(Au),which are characterized by atoms with only one valence electron very loosely bound to the atom.These loosely bound valence electrons become free electrons.Therefore,in a conductive material the free electrons are valence electrons.
In terms of energy bands ,it means that electrical conductors are those which have overlapping valence and conduction bands.In fact ,there is no physical distinction between the two bands.Hence the availability of a large number of conduction electrons.
Another point worth noting is that in the absence of forbidden energy gap in good conductors,there is no structure to establish holes.The total current in such conductors is simply a flow of electrons.It is exactly for this reason that the existence of holes was not discovered until semiconductors were studied thoroughly.
A semiconductor material is one whose electrical properties lie in between those of insulators and good conductors.Example are:Germanium and silicon.
In terms of energy bands,semiconductors can be defined as those materials which at room temperature have:
- Partially filled conduction band.
- Partially filled valence band.
- A very narrow energy gap (of the order of ( 1 eV) between them.
At 0ºk ,there are no electrons in the conduction band of semiconductors and their valence band is completely filled.It means that at absolute zero temperature ,a piece of Ge or Si acts like a perfect insulator.However,with increase in temperature,width of the forbidden energy band is decreased so that some of the electrons are liberated into the conduction band.In other words,conductivity of semiconductors increases with temperature.It means that they have negative temperature coefficient of resistance.
Types of semiconductors:
Semiconductor may be classified as under:
- Intrinsic or pure semiconductors
- Extrinsic or impure semiconductors
An intrinsic semiconductors is one which is made of the semiconductor material in its extremely pure form.
Common examples of such semiconductors are:pure germanium and silicon which have forbidden energy gaps of 0.72 eV and 1.1 eV respectively.The energy gap is so small that even at ordinary room temperature,there are many electrons which possess sufficient energy to jump across the small energy gap from the valence to the conduction band.However,it is worth noting that for each electron liberated into conduction band,a positively charged hole is created in the valence band.When an electric field is applied to an intrinsic semiconductor at a temperature greater than 0ºK,conduction electrons move to the anode and the holes in the valence band move to the cathode.Hence,semiconductor current consists of movement of electrons and holes in opposite direction in the conduction and valence band respectively.
Alternatively,an intrinsic semiconductor may be defined as one in which the number of conduction electrons is equal to the number of holes.
Those intrinsic semiconductors to which some suitable impurity or doping agent has been added in extremely small amount are called extrinsic or impurity semiconductors.
Usually,the doping agents are pentavalent atoms having five valence electrons (antimony,arsenic,phosphorus,bismuth) or trivalent atoms having three valence (gallium,indium,aluminium,boron) .Pentavalent doping atom is known as donor atom because it donates or contributes one electron to the conduction band of pure germanium.The trivalent atom,on the other hand ,is called acceptor atom because it accepts one electron from germanium atom.
Why doping material are called impurities?
The reason why doping materials are called impurities is that they alter the structure of pure semiconductor crystals.
Depending on the type of doping material used ,extrinsic semiconductors can be further subdivided into two classes:
- N- type semiconductors
- P-type semiconductors
N-type extrinsic semiconductors:
This type of semiconductor is obtained when a pentavalent material like antimony (Sb) is added to pure germanium crystal.fig
Each antimony atom forms covalent bonds with the surrounding four germanium atoms with the help of four germanium atoms with the help of four of its five electrons.The fifth with electron is superfluous and is loosely bound to the antimony atom.Hence,it can be easily exited from the valence band to the conduction band by the application of electric field or increase in its thermal energy.Thus,practically every antimony atom introduced into the germanium lattice contributes one conduction electron without creating a positive hole.Antimony is called donor impurity and makes the pure germanium an N-type (N for the negative ) extrinsic semiconductor and with the N in negative charge carrier.
In addition to the electrons and holes intrinsically available in germanium,the addition of antimony greatly increases the number of conduction electrons.Hence,concentration of electrons in the conduction band is increased and exceeds the concentration of holes in the valence band.Because of this ,Fermi level shifts upward towards the bottom of the conduction band.
In terms of energy levels,the fifth antimony electron has an energy level (called donor level) just below the conduction band.Usually,donor level is 0.054 eV for silicon.It is seen from the above description that in an N-type semiconductor ,electrons are the majority carries while holes constitute the minority carriers.
P-type Extrinsic Semiconductor:
This type of semiconductor is obtained when traces of a trivalent impurity like boron (B) are added to a pure germanium crystal.
In this case,the three valence electrons of boron atom from covalent bonds with four surrounding germanium atoms,but one bond is left incomplete and gives rise to a hole.
Thus, boron which is called an acceptor impurity ,causes as many positive holes in a germanium crystal as there are boron atoms thereby producing a P-type (P for the positive ) extrinsic semiconductor.
As an aid to memory ,the student should associate the letter P in acceptor with the P in “p-type ” extrinsic semiconductor and with the P in positive charge carrier.
In this type of semiconductor ,conduction is by means of holes in the valence band.Accordingly,holes form the majority carriers whereas electrons constitute minority carriers.Since concentration of holes in the valence band is more than the concentration of electrons in the conduction band,Fermi level shifts nearer to the valence band.The acceptor level lies immediately above the Fermi level.Conduction is by means of movement at the top of valence band,the acceptor level readily accepting electrons from the valence band.