Advertisements
Advertisements
Questions
Draw the necessary energy band diagrams to distinguish between conductors, semiconductors and insulators.
How does the change in temperature affect the behaviour of these materials ? Explain briefly.
Write any two distinguishing features between conductors, semiconductors and insulators on the basis of energy band diagrams.
Solution 1
Conductors: (i) For conductors, the valence band is completely filled and the conduction band can have two possibilities—either it is partially filled with an extremely small energy gap between the valence and conduction bands or it is empty, with the two bands overlapping each other, as shown.
(ii) On applying even an small electric field, conductors can conduct electricity.
Insulators: (i) For insulators, the energy gap between the conduction and valence bands is very large. Also, the conduction band is practically empty, as shown.
(ii) When an electric field is applied across such a solid, the electrons find it difficult to acquire such a large amount of energy to reach the conduction band. Thus, the conduction band continues to be empty. That is why no current flows through insulators.
Semiconductors: (i) The energy band structure of semiconductors is similar to that of insulators, but in their case, the size of forbidden energy gap is much smaller than that of the insulators, as shown.
(ii) When an electric field is applied to a semiconductor, the electrons in the valence band find it comparatively easier to shift to the conduction band. So, the conductivity of semiconductors lies between the conductivity of conductors and insulators.
Solution 2
Energy Band Diagram for a Conductor:
In conductors, the conduction and the valence band overlap each other.
As the temperature increase, the conductivity of the conductors decreases due to increase in the thermal motion of the free electrons.
Energy Band Diagram for an Insulator :
Here, the valence band is completely filled and the conduction band is empty. The energy band gap of the insulator is quite large. So, on increasing the temperature, the electrons of the valence band are not able to reach the conduction band. Therefore, electrical conduction in these materials is impossible.
Energy Band Diagram for a Semiconductor
In semiconductors, the valence band is totally filled and the conduction band is empty but the energy gap between conduction band and valence band is quite small. At 0 K, electrons are not able to cross even this small energy gap and, hence, the conduction band remains totally empty. Therefore, the semiconductor at 0 K behaves as an insulator. At room temperature, some electrons in the valence band acquire thermal energy greater than energy band gap, which is less than 3 eV and jump over to the conduction band where they are free to move under the influence of even a small change in the temperature.
APPEARS IN
RELATED QUESTIONS
Distinguish between a metal and an insulator on the basis of energy band diagrams ?
The conduction band of a solid is partially filled at 0 K. Will it be a conductor, a semiconductor or an insulator?
When a semiconducting material is doped with an impurity, new acceptor levels are created. In a particular thermal collision, a valence electron receives an energy equal to 2kT and just reaches one of the acceptor levels. Assuming that the energy of the electron was at the top edge of the valence band and that the temperature T is equal to 300 K, find the energy of the acceptor levels above the valence band.
The product of the hole concentration and the conduction electron concentration turns out to be independent of the amount of any impurity doped. The concentration of conduction electrons in germanium is 6 × 1019 per cubic metref conduction electrons increases to 2 × 1023 per cubic metre. Find the concentration of the holes in the doped germanium.. When some phosphorus impurity is doped into a germanium sample, the concentration o
The conductivity of an intrinsic semiconductor depends on temperature as σ = σ0e−ΔE/2kT, where σ0 is a constant. Find the temperature at which the conductivity of an intrinsic germanium semiconductor will be double of its value at T = 300 K. Assume that the gap for germanium is 0.650 eV and remains constant as the temperature is increased.
(Use Planck constant h = 4.14 × 10-15 eV-s, Boltzmann constant k = 8·62 × 10-5 eV/K.)
A semiconducting material has a band gap of 1 eV. Acceptor impurities are doped into it which create acceptor levels 1 meV above the valence band. Assume that the transition from one energy level to the other is almost forbidden if kT is less than 1/50 of the energy gap. Also if kT is more than twice the gap, the upper levels have maximum population. The temperature of the semiconductor is increased from 0 K. The concentration of the holes increases with temperature and after a certain temperature it becomes approximately constant. As the temperature is further increased, the hole concentration again starts increasing at a certain temperature. Find the order of the temperature range in which the hole concentration remains approximately constant.
(Use Planck constant h = 4.14 × 10-15 eV-s, Boltzmann constant k = 8·62 × 10-5 eV/K.)
In a semiconductor, the forbidden energy gap between the valence, band and the conduction band is of the order of
The valance of an impurity added to germanium crystal in order to convert it into p-type semiconductor is
- Assertion (A): In insulators, the forbidden gap is very large.
- Reason (R): The valence electrons in an atom of an insulator are very tightly bound to the nucleus.
The energy required by an electron to jump the forbidden band in silicon at room temperature is about ______.
