Question

What are doped semiconductors? Explain the effect of (i) doping and (ii) temperature on the conductivity of semiconductors.

Hint:

Remember: Semiconductor conductivity increases with temperature, opposite to metals.

Answer 1

Step 1: Definition.
Pure semiconductors (Si, Ge) have limited conductivity. When a small quantity of impurity is added, they become doped semiconductors (extrinsic type).
Step 2: Effect of doping.
- Doping with pentavalent impurities (P, As) gives N-type semiconductor (extra electrons as majority carriers).
- Doping with trivalent impurities (B, Al) gives P-type semiconductor (holes as majority carriers).
- Conductivity increases drastically because of increased number of charge carriers.
Step 3: Effect of temperature.
- As temperature rises, more covalent bonds break, releasing additional charge carriers.
- Hence conductivity increases with temperature (unlike metals where resistance increases).
Step 4: Conclusion.
Thus, doping and increase in temperature both increase the conductivity of semiconductors.