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.
What is difference between conductors and insulators?
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.
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.