Maharashtra State Board 11th Physics Solutions Chapter 14 Semiconductors
1. Choose the correct option.
2. Answer the following questions.
3. Answer in detail.
ii. As an outcome of the small distances between atoms, the resulting interaction amongst electrons and the Pauli’s exclusion principle, energy bands are formed in the solids.
iii. In metals, conduction band and valence band overlap. However, in a semiconductor or an insulator, there is gap between the bottom of the conduction band and the top of the valence band. This is called the energy gap or the band gap.
iv. For metals, the valence band and the conduction band overlap and there is no band gap as shown in figure (b). Therefore, electrons can easily gain electrical energy when an external electric field is applied and are easily available for conduction.
v. In case of semiconductors, the band gap is fairly small, of the order of 1 eV or less as shown in figure (c). Hence, with application of external electric field, electrons get excited and occupy energy levels in conduction band. These can take part in conduction easily.
vi. Insulators, on the contrary, have a wide gap between valence band and conduction band of the order of 5 eV (for diamond) as shown in figure (d). Therefore, electrons find it very difficult to gain sufficient energy to occupy energy levels in conduction band.
vii. Thus, an energy band gap plays an important role in classifying solids into conductors, insulators and semiconductors based on band theory of solids.
Intrinsic semiconductors | Extrinsic semiconductors |
1. A pure semiconductor is known as intrinsic semiconductors. | The semiconductor, resulting |
2. Their conductivity is low | Their conductivity is high even at room temperature. |
3. Its electrical conductivity is a function of temperature alone. | Its electrical conductivity depends upon the temperature as well as on the quantity of impurity atoms doped in the structure. |
4. The number density of holes (nh) is same as the number density of free electron (ne) (nh= ne). | The number density of free electrons and number density of holes are unequal. |
ii. During diffusion of charge carriers across the junction, electrons migrate from the n-side to the p-side of the junction. At the same time, holes are transported from p-side to n-side of the junction.
iii. As a result, in the p-type region near the junction there are negatively charged acceptor ions, and in the n-type region near the junction there are positively charged donor ions.
iv. The potential barrier thus developed, prevents continuous flow of charges across the junction. A state of electrostatic equilibrium is thus reached across the junction.
v. Free charge carriers cannot be present in a region where there is a potential barrier. This creates the depletion region.
vi. In absence of depletion region, all the majority charge carriers from n-region (i.e., electron) will get transferred to the p-region and will get combined with the holes present in that region. This will result in the decreased efficiency of p-n junction.
vii. Hence, formation of depletion layer across the junction is important to limit the number of majority carriers crossing the junction.
11th Physics Digest Chapter 14 Semiconductors Intext Questions and Answers
Internet my friend (Textbookpage no. 256)
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v.astr.gsu.edu/hbase/Solids/semcn.html
[Students are expected to visit above mentioned links and collect more information regarding semiconductors.]