UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper II
Q1(c). Band structure of n-type and p-type semiconductors
Question
Draw the qualitative band structure of p-type and n-type semiconductors.
Answer in brief
n-type: filled donor level just below the conduction band. p-type: empty acceptor level just above the valence band.
Key points
- n-type: a group 15 atom has one extra electron in a donor level ED. Thermal energy lifts it into the conduction band.
- p-type: a group 13 atom gives an empty acceptor level EA. Valence-band electrons jump in and leave holes.
- Fermi level EF moves up towards the conduction band in n-type, down towards the valence band in p-type.
- Eg is unchanged. Only the carrier density n rises: σ = neμ.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coversband theoryn-type semiconductorp-type semiconductordonor and acceptor levelsFermi level
All parts of Question 1
- 1(a) Why Z > 1 for H₂ and He at all pressures5 marks
- 1(b) Corresponding state of H₂S for N₂ at 25 °C and 1 atm5 marks
- 1(c) Band structure of n-type and p-type semiconductors5 marks
- 1(d) Symmetry elements of orthorhombic Mg₂SiO₄5 marks
- 1(e) Fall of G with temperature at constant pressure5 marks
- 1(f) Z > 1 and f > P for the gas P(Vm − b) = RT5 marks
- 1(g) Overall equation and the role of each species5 marks
- 1(h) log k against pH for specific H⁺ catalysis5 marks
- 1(i) KCl salt bridge for a silver–lead cell5 marks
- 1(j) Equilibrium constant from standard potentials5 marks
- 1(k) Conditions for an acceptable wave function5 marks
- 1(l) No solution with E = 0 for the particle in a box5 marks
- 1(m) Discrete levels and the resonance condition5 marks
- 1(n) Rotational activity of CO and CO₂5 marks
- 1(o) Measuring light intensity with a chemical actinometer5 marks
- 1(p) Why watch hands glow in the dark5 marks