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Nikihi posted an Question
January 20, 2021 • 02:55 am 30 points
  • IIT JAM
  • Physics (PH)

Explain me first point in detail

Explain me first point in detail.... why registivity decreases... generally it increases with temperature

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    Dhairya sharma

    In semiconductors the forbidden gap between the conduction band and the valence band is small. At 0K, the valence band is completely filled and the conduction band may be empty. But when a small amount of energy is applied, the electrons easily moves to the conduction band. Silicon is an example for semiconductor. Under normal circumstances silicon act as a weak conductor. Each silicon atom is bonded to 4 other silicon atoms. The bonds between these atoms are co valent bonds where the electrons are in fixed positons. So at 0K, the electrons does not move within the lattice structure. Hence When the temperature in increased the forbidden gap between the two bands becomes very less and the electrons move from the valence band to the conduction band. Thus some electrons from the co valent bonds between the Si atoms are free to move within the structure. This increases the conductivity of the material. The conductivity increases means the resistivity decreases. "Thus when the temperature is increased in a semiconductor, the density of the charge carriers also increases and the resistivity decreases." For semiconductors it is said that they have a negative temperature co – efficient. So the value of temperature co –efficient of resistivity, α is negative.

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    can you please provide me exact formula for the temperature coefficient and what will be the modified form when it is negative please

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    ρ = m/n e2ԏ, where e is the charge on an electron, ԏ is the average time taken between each collision or the relaxation time of electrons and m is the mass of the electron, n is the charge density. It is clear that when the temperature is increased the average speed of the electrons increases and thus more collision occurs. Thus the relaxation time between each collisions decreases.For semiconductors, when the temperature is increased the conductivity of the material is increased. It means that the resistivity of the material is decreased and so the current flow is increased. For semiconductors they have a negative temperature co – efficient. So the value of temperature co –efficient of resistivity, α is negative.

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    Dhairya sharma best-answer

    In semiconductors the forbidden gap between the conduction band and the valence band is small. At 0K, the valence band is completely filled and the conduction band may be empty. But when a small amount of energy is applied, the electrons easily moves to the conduction band. Silicon is an example for semiconductor. Under normal circumstances silicon act as a poor conductor. Each silicon atom is bonded to 4 other silicon atoms. The bonds between these atoms are co valent bonds where the electrons are in fixed positons. So at 0K, the electrons does not move within the lattice structure. Hence When the temperature in increased the forbidden gap between the two bands becomes very less and the electrons move from the valence band to the conduction band. Thus some electrons from the co valent bonds between the Si atoms are free to move within the structure. This increases the conductivity of the material. The conductivity increases means the resistivity decreases. "Thus when the temperature is increased in a semiconductor, the density of the charge carriers also increases and the resistivity decreases." For semiconductors it is said that they have a negative temperature co – efficient. So the value of temperature co –efficient of resistivity, α is negative.

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    i hope it'll help you!

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