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Electron hole or electron vacancy is responsible for producing current in semiconducting materials. The hole is positively changed and is equal to the volume that of a negatively charged electron. The polarity of a hole is opposite to the charge of an electron. An electron-hole or electron vacancy is created when an electron leaves its original position to take up an empty space. In a semiconductor, electrons move out of the outermost shell of the atom forming an electron hole. The current in the semiconductors is mainly created by the electrons and the holes. P-type semiconductors are the ones where the holes are the most important charge carriers. The process in which the electrons and holes move in a semiconducting material under the effect of voltage is known as applying a forward or reverse bias.
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Keyterms: Electron Hole, Atom, Semiconducting material, Crystalline Solids, Semiconductors, n-type Semiconductors, p-type Semiconductors, Electron
Imperfections in Solids
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Crystalline solids are made of much smaller crystals that have defects in them which takes place while the crystallization process takes place at a fast pace. There are two types of defects: point defects and line defects. Point defect is the defect in the arrangement of an atom. It is classified into three types: stoichiometric defects, impurity defects, and non-stoichiometric defects.
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| Solid State Important Questions | Solid State MCQ | Solid State Ncert Solutions |
Difference between Conductors, Insulators, and Semiconductors
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| Conductors | Insulators | Semiconductors |
|---|---|---|
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Semiconductors
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Substances like silicon and germanium have a smaller gap between the valence and conductor band which makes their electrons jump to conduction. Such substances are known as intrinsic semiconductors. As the conductivity of these semiconductors is too low to be used in practical use, a significant amount of impurities are used to increase the conductivity. This process is known as doping. Doping is possible by Impurities that are rich in electrons or have a deficit of electrons as compared to intrinsic conductors producing electronic defects in them.
Electron Rich Impurities
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Negatively charged ions increase the conductivity of a substance which is why silicon doped with impurities rich in electrons is known as n-type semiconductors.
Electron Deficient Impurities
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An electron-hole or electron vacancy is created when the elements contain only 3 valence electrons, and the fourth valence electron is missing, creating an empty space where the neighboring electrons come to fill up the space. When group 13 elements such as boron, aluminum, etc are added to a pure semiconductor such as silicon or germanium, it is known as a p-type semiconductor. In a p-type conductor, current flows in one direction from hole to hole because they have more holes than electrons.
Use of n-type and p-type Semiconductors
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- Electronic components are made by n-type and p-type semiconductors.
- Rectifier uses diode which is a combination of n-type and p-type semiconductors
- Radio and audio signals are detected with the help of PNP and NPN translators.
- Photodiodes such as solar cells are used to convert light energy into electrical energy.
Things to Remember
- The stoichiometric defect is of two types- interstitial defect and vacancy defect
- Electrons are negatively charged whereas holes are positively charged.
- The absence of an electron in an atom is known as electron-hole or electron vacancy
- The production of additional electrons and holes increases the electrical conductivity
- Electric charge carriers are negatively charged electrons that move around freely and carry an electric charge
- A photovoltaic cell generates an electric current or voltage when it comes in contact with sunlight. The process is known as the photovoltaic effect.
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Sample Questions
Ques: Calculate the density of silver that forms ccp lattice and the length of the edge of its unit cell is 408.6pm. The atomic mass is given as 107.9 u. (3 marks)
Ans: we know that the lattice is ccp, the amount of silver atoms per unit cell = z = 4
Molar mass of silver = 107.9 g mol-1 = 107.9 x 10-3 kg mol-1
Edge length of unit cell = a = 408.6 pm = 408.6 x 10-12 m
Density,( d) = z.m/a3.NA
= 4 x (107.9 x 10-3 kg mol-1 )/(408.6 x 10-12 m)3 (6.022 x 1023 mol-1)
= 10.5 x 103 kg m-3
= 10.5 g cm-3
Ques: The cell edge of a body-centered cubic structure of an element is 288pm and 7.2g/cm3 is the density of the element. Calculate the number of atoms in 208g of the element. (3 marks)
Ans: volume of the unit cell = (288pm)3
= (288 x 10-12 m)3
= (288 x 10-10 cm)3
= 2.39 x 10-23 cm3
Volume of 208g of the element = mass/density
= 208g/7.2g cm-3
= 28.88cm3
Number of unit cells in the volume = 28.88cm3/ (2.39 x 10-23 cm3/unit cell)
= 12.08 x 1023 unit cells
Each bcc cubic unit cell contains 2 atoms
The total number of atoms in 208g = 2 (atom/unit cell) x 12.08 x 1023 unit cells
= 24.16 x 1023 atoms
Ques: What is the formula of a compound which has two elements X and Y where the atoms of element X acts as cations fill up all the octahedral voids and that of element y which acts as anions make ccp? (2 marks)
Ans: element Y gives rise to ccp lattice. The number of atoms in element Y is equal to the number of octahedral voids that are formed. The atoms of X occupy all the octahedral voids and their number is also similar to that of element Y. Therefore, the equal number of atoms present in elements X and Y can be expressed with a 1:1 ratio which makes the formula of the component as XY.
Ques: A compound has 2 elements, A and B. Atoms of element A hold 2/3rd of the tetrahedral voids and the atoms of element B make up the hcp lattice. What is the formula? (2 marks)
Ans: the number of tetrahedral voids formed is similar to the double of the number of atoms of element B. element A holds only 2/3rd of the atoms of element A.
Thus, the ratio of number of atoms of A and B = 2 x (2/3): 1 or 4:3
The formula of the compound = A4B3
Ques: Which among the following is true if the lattice constant of the semiconductor is reduced: conduction bandwidth, bandgap, valence bandwidth. (3 marks)
Ans: more energy is needed to remove one atom when the electrons are closely packed to the atom due to the fall in the lattice parameter of the semiconductor. This property enables the reduction in the width of the energy bands of both conduction band and valence. This results in the overall increase in the energy gap between the valence and conduction band.
Thus, conduction bandwidth and valence bandwidth decrease but the bandgap increases.
Ques: What is the nature of a semiconductor at 0? K? (2 marks)
Ans: it is neutral. The conductivity of a semiconductor increases and decreases with the change in temperature. Thus, the semiconductor at 0?K acts as an insulator as conductivity becomes 0.
Ques: Calculate the number of holes in an N-type semiconductor with 1015 cm-3 donors (2 marks)
Ans: an electron is produced with each ionized donor. Thus, n = 1015 cm-3
P = n2i/n = 1020 cm-3/1015 cm-3 = 105 cm-3
With the increase in temperature by 60oc. at 1015 cm-3, n is constant while p increases by about a factor of 2300.
Ques: In p-type silicon water, what is the value of n if p = 1017 cm-3? (2 marks)
Ans: n = n2i /p
=1020/1017 = 103 cm-3
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