Why does resistivity and temperature varies-
1.directly to metals
2.inversely to semiconductors
3.doesnot varies with temperature in alloys
||Explain all three conditions in details.||
1.directly to metals
2.inversely to semiconductors
3.doesnot varies with temperature in alloys
||Explain all three conditions in details.||
Variation of Resistivity in Conductors We know that current is the movement of free electrons from one atom to the other when there is a potential difference. In conductors no forbidden gap is present between the conduction band and valence band. In many cases both the bands overlap each other. The valence electrons are loosely bound to the nucleus in conductors. Usually metals or conductors have low ionization energy and so they tend to lose electrons very easily. When an electric current is applied the delocalized electrons are free to move within the structure. This is the case that happens in normal temperature.
When the temperature increases the vibrations of the metal ions in the lattice structure increases. The atoms starts to vibrate with higher amplitude. These vibrations in turn causes frequent collisions between the free electrons and the other electrons. Each collision drain out some energy of the free electrons and causing them unable to move. Thus it restricts the movement of the delocalized electrons. When the collision happens the drift velocity of the electrons decreases. This means that the resistivity of the metal increases and thus current flow in the metal is decreased. The resistivity increases means that the conductivity of the material decreases.
For metals or conductors, it is said that they have a positive temperature co – efficient. The value α is positive. For most of the metals, the resistivity increases linearly with increase in temperature for a range of 500K. Examples for positive temperature co – efficient include, silver, copper, gold etc.
Temperature dependence on resistivity for metals
Variation of Resistivity in Semiconductors Silicon is a semiconductor. 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.
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.
The curve is non - linear for a wide range of temperature.
Temperature dependence on resistivity for semiconductors
Variation of Resistivity in Insulators In insulators the forbidden energy gap between the conduction band and the valence band is high. The valence band is completely filled with the electrons. The forbidden gap between the two bands will be more than 3 e V. Thus it requires a high amount to energy for the valence electron to move to the conduction band. Diamond is an example for insulator. Here all the valence electrons are involved in the co valent bond formation and conduction does not happen. The electrons are tightly bound to the nucleus.
When the temperature is increased, the atoms of the material vibrate and it makes the valence electrons present in the valence band to shift to the conduction band. This in turn increases the conductivity of the material. When the conductivity of the material increases, it means that the resistivity decreases and so the current flow increases. Thus some insulators at room temperatures changes to conductors at high temperature. For insulators they have a negative temperature co – efficient. So the value of temperature co –efficient of resistivity, α is negative.
We know that current is the movement of free electrons from one atom to the other when there is a potential difference. In conductors no forbidden gap is present between the conduction band and valence band. In many cases both the bands overlap each other. The valence electrons are loosely bound to the nucleus in conductors. Usually metals or conductors have low ionization energy and so they tend to lose electrons very easily. When an electric current is applied the delocalized electrons are free to move within the structure. This is the case that happens in normal temperature.
When the temperature increases the vibrations of the metal ions in the lattice structure increases. The atoms starts to vibrate with higher amplitude. These vibrations in turn causes frequent collisions between the free electrons and the other electrons. Each collision drain out some energy of the free electrons and causing them unable to move. Thus it restricts the movement of the delocalized electrons. When the collision happens the drift velocity of the electrons decreases. This means that the resistivity of the metal increases and thus current flow in the metal is decreased. The resistivity increases means that the conductivity of the material decreases.
For metals or conductors, it is said that they have a positive temperature co – efficient. The value α is positive. For most of the metals, the resistivity increases linearly with increase in temperature for a range of 500K. Examples for positive temperature co – efficient include, silver, copper, gold etc.
Temperature dependence on resistivity for metals
Variation of Resistivity in Semiconductors
Silicon is a semiconductor. 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.
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.
The curve is non - linear for a wide range of temperature.
Temperature dependence on resistivity for semiconductors
Variation of Resistivity in Insulators
In insulators the forbidden energy gap between the conduction band and the valence band is high. The valence band is completely filled with the electrons. The forbidden gap between the two bands will be more than 3 e V. Thus it requires a high amount to energy for the valence electron to move to the conduction band. Diamond is an example for insulator. Here all the valence electrons are involved in the co valent bond formation and conduction does not happen. The electrons are tightly bound to the nucleus.
When the temperature is increased, the atoms of the material vibrate and it makes the valence electrons present in the valence band to shift to the conduction band. This in turn increases the conductivity of the material. When the conductivity of the material increases, it means that the resistivity decreases and so the current flow increases. Thus some insulators at room temperatures changes to conductors at high temperature. For insulators they have a negative temperature co – efficient. So the value of temperature co –efficient of resistivity, α is negative.
