[UPSC CSE 2005] Band theory of solids (Q82, 10+5+5M)

(i) Discuss briefly the concept of effective mass in semiconductors and explain the significance of negative effective mass. (ii) In isolated atoms, the electrons have discrete and definite energies but in solids they have bands of energies. Explain why. (iii) A proton, deuteron and \alpha-particle accelerated through the same potential difference, on entering a region of uniform magnetic field applied normal to their plane of motion, move in circular orbits. Compare the radii of the orbits described by them.