Physics Answers

Question 5

5.1​ The electron and hole mobilities in a Si sample are 0.135 and 0.48m2/V-s respectively. ​Determine the conductivities of intrinsic Si at 300 K if the intrinsic carrier concentration is 1.5 x 106 atom/m3. The sample is then doped with 1023 phosphorus atom/m3. Determine the equilibrium holeconcentration, conductivity, and position of the Fermi level relative to the intrinsic level.​(5)

5.2​ Indicate on an energy level diagram the conduction and valence bands, donor and acceptor states and the position of Fermi level for

(i) an intrinsic semiconductor.

(ii) a n-type semiconductor.

(iii) a p-type semiconductor.​​​​​​​​(5)

Question 3

If the velocity of sound in a solid is of the order 103 m/s, compare the frequency of the sound wave λ = 20 Å for (a) a monoatomic system and (b) acoustic waves and optical waves in a diatomic system containing two identical atoms (M=m) per unit cell of interatomic spacing 2.2 Å.​​​​​​(10) 

Question 4

4.1. ​The conductivity of silver is 6.5 x 107per Ohm per m and number of conduction electrons per m3 is 6 x 1028. Find the mobility of conduction electrons and the drift velocity in an electric field of 1 V/m. Given m = 9.1 x 10–31 kg and e = 1.602 x 10–19 C.​​​​​​​(5)

4.2​ What is Lorentz number? The thermal and electrical conductivities of Cu at 200C are 390 Wm-1K-1 and 5.87 x107W-1m-1 respectively. Calculate Lorentz number.​​       (5)​

Question 1​​​​​​​​​​​​

Compute the net potential energy of a simple Na+Cl- pair. The equilibrium distance between the ions is 0.28nm. The potential energy due to repulsion between electron sis given by Ur = β/r8​​​(10)

Question 2

2.1 α-Co has an hcp structure with lattice spacings of a= 2.51 A0 and c = 4.07 A0, β Co is fcc, with a cubic lattice spacing of 3.55 A0. What is the difference in density between the two forms?​​(5)

2.2 Sodium transform from bcc to hcp at about T = 23K. Assuming that the density remains fixed, and the c/ a ratio isideal, calculate the hcp lattice spacing a given that the cubiclattice spacing a' = 4.23 A0 in the cubic phase.      ​​​​​​​​(5)