Distinguishing Between Conductors, Insulators, and Semiconductors Based on Band Theory of Solids
Conductors
All metals are good electrical conductors, and their resistivities are on the order of . In conductors there is no forbidden energy gap between the balance and the conducting tape. Flounce and wire bands are partially filled at room temperature. Therefore, electrons can easily jump from the valence band to the conduction band. Because of this, current can easily flow through the conductor.
Insulators
The resistance of insulators is very high, on the order of . In insulators, the flounce bands are filled and the conduction bands are empty. The energy gap between valence band and conduction band is very large. This means that electrons cannot jump from the valence band to the conduction band. There are no free electrons in an insulator, so no current flows through it.
Semiconductor
Materials with intermediate resistivity values (on the order of) are called semiconductor materials. The energy gap between the valence band and the conduction band is very small. A semiconductor is a material that lies between a conductor and an insulator in its ability to conduct current. A semiconductor in its pure (intrinsic) state is neither a good conductor nor a good insulator. The most common single element semiconductors are silicon, germanium, and carbon. Compound semiconductors such as gallium arsenide are also commonly used.
Intrinsic Semiconductors
A pure semiconductor is known as an intrinsic semiconductor. The most common examples of intrinsic semiconducting materials are silicon. Each atom of silicon has four valance electrons. Moreover, each atom of silicon is surrounded by four atoms. A silicon (Si) atom with its four valence electrons shares an electron with each of its four neighbors to form a covalent bond. This effectively creates eight shared valence electrons for each atom and produces a state of chemical stability. The semiconducting materials have a negative temperature coefficient of resistivity. At low temperatures, the valence band is filled and the conduction band is empty. Thus the semiconducting materials behave like insulators at low temperatures. At comparatively higher temperatures, the electrons in the valance band acquire sufficient energy to jump into the conduction band.
As the temperature increases, the probability of the electrons jumping from valance to conduction band increases. Therefore, the conductivity of semiconductors increases with an increase in temperature. At absolute zero, the intrinsic semiconducting materials behave like insulators because they have no free electrons. But as the temperature of increases, the thermal agitation in the atoms breaks some covalent bonds which results in the formation of electron-hole pairs.
The electrons jump from the valance band to the conduction band by absorbing the thermal energy. As the result, the conductivity of semiconductors increases with an increase in temperature.
so the difference betw.een conductor and semiconductor and insulator is clear.
Extrinsic semiconductor
The semiconductors doped with some impurity are called extrinsic semiconductors. The conductivity of silicon and germanium can be drastically increased by the controlled addition of impurities to the intrinsic (pure) semiconductive material. This process, called doping, increases the number of current carriers (electrons or holes). The two categories of impurities are n-type and p-type.
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