Indian Institute of Technology RoorkeeDepartment of Metallurgical and Materials Engineering

MT-202 Electrical and Electronic Materials

MTEMax. Marks: 25

Time: 1.5 hour1 i Two atoms from a distance, r = ∞ when brought closer, what are the forces that can act

on them at an equilibrium distance, ro,In what way?

Ans: equal and opposite two forces e.g attractive force due to the Columb attraction between the electrons of one atom to the protons of the other ( opposite charges). Repulsive force due the electron coming near to another ( same kind of charge)

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ii Inside a cubic lattice, show the line of intersection between planes (1Ī0) and (111) on the sectional plane (1Ī 0).

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iii What is the Miller Index of the above mentioned line?

MI Direction AO is :

½,1/2,0 ---- 0,0,1

0,0,0 ------ -1/2,-1/2, 1

000 --------- -1,-1, 2

MI AO: [TT2]

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iv Is this line perpendicular to the cube diagonal [111], justify? 1

v Draw a Diamond Cubic unit cell. Indicate (110) in the unit cell in a separate diagram and mark the atoms on this plane. In spite of having more number of atoms per unit cell, the packing factor is less than FCC, why?

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vi What is the need for producing single crystals for electronic applications?

Single crystals do not have any grain boundaries , which are also the scattering centers of the electrons. Semiconductors already have lesser electrical conductivity and if the grain boundaries are present the conductivity may go near to an insulator. Hence single crystals are used for electronic applications.

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vii What is the basic criteria needed to grow a compound semiconductor on a given substrate epitaxially? State an application of the grown crystal.

Lattice matching- essential condition for the epitaxial growth Matching of lattice structures between two different semiconductor

materials, allows a region of band gap change to be formed in a material without introducing a change in crystal structure.

It allows construction of advanced light-emitting diodes and diode lasers.For example, gallium arsenide, aluminium gallium arsenide, and aluminium arsenide have almost equal lattice constants, making it possible to grow almost arbitrarily thick layers of one on the other one.

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2 i In the presence of an applied field, when an electron is moving in a particular direction, the trajectory between collision is parabola. Why?

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Ans: There is a force eE acts on an electron when it is moving in an applied electric field , E.

ii Na is a monovalent metal (BCC) with a density of 0.9712 g cm-3. Its atomic mass is 22.99 g mol-1, N=6.022x1023 mol-1. The drift mobility of electrons in Na is 53cm2V-1s-1. Consider the collection of conduction electrons in the solid. If each Na atom donate one electron to the electron sea, estimate the mean separation between the electrons?

Calculate the electrical conductivity of Na and compare this with the experimental value of 2.1x 107 Ω-1m-1 and comment on the difference. Given e = 1.602x10 -19 C.

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iii A 70% Cu -30% Zn brass electrical component has been made of powdered metal and contains15vol. % porosity. Assume that the pores are dispersed randomly. Estimate the effective electrical resistivity of the brass component at room temperature. Given: ρo for cu = 17 nΩ m and C = 300 nΩ m.

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