Question

When an impurity is doped in a pure semiconductor, the conductivity of the semiconductor

• A. increases
• B. decreases
• C. remains the same
• D. becomes zero

Hint:

Remember that the entire purpose of doping a semiconductor is to control and increase its conductivity. This process is fundamental to creating all modern semiconductor devices like diodes, transistors, and integrated circuits. Doping turns a poor conductor (intrinsic semiconductor) into a useful one (extrinsic semiconductor).

Answer 1

The Correct Option is A

Step 1: Compare carrier concentrations before and after doping. A pure (intrinsic) semiconductor has only thermally generated electron-hole pairs, so very few carriers and high resistivity.
Step 2: Introduce the dopant. Doping deliberately adds impurity atoms that release additional charge carriers (electrons in n-type, holes in p-type) at room temperature.
Step 3: Link carriers to conductivity. More free carriers mean more current for the same applied voltage, which by \(\sigma = ne\mu\) directly raises the conductivity.
Step 4: Therefore doping an intrinsic semiconductor makes it a much better conductor; the conductivity increases, confirming option (i). This is precisely why devices use doped, not pure, semiconductors.
\[\boxed{\text{Doping} \Rightarrow \text{conductivity increases}}\]