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: Understanding the Concept:
This question relates to the fundamentals of semiconductor physics. The conductivity of a material depends on the number of free charge carriers available to conduct electricity. A pure semiconductor (intrinsic semiconductor) has a relatively low number of charge carriers (electrons and holes, created by thermal excitation). Doping is the process of intentionally adding impurities to a pure semiconductor to significantly increase the number of charge carriers.

Step 2: Detailed Explanation:
The conductivity (\(\sigma\)) of a semiconductor is given by the formula: \[ \sigma = e(n_e\mu_e + n_h\mu_h) \] where: - \(e\) is the elementary charge.
- \(n_e\) is the concentration of free electrons.
- \(n_h\) is the concentration of holes.
- \(\mu_e\) and \(\mu_h\) are the mobilities of electrons and holes, respectively.
In a pure (intrinsic) semiconductor at room temperature, the concentration of electrons is equal to the concentration of holes (\(n_e = n_h = n_i\)), and this intrinsic carrier concentration \(n_i\) is very small.
Doping involves adding impurity atoms:
1. N-type Doping: If a pentavalent impurity (e.g., Phosphorus, Arsenic) is added to a tetravalent semiconductor (e.g., Silicon), each impurity atom donates one free electron. This drastically increases the electron concentration (\(n_e \gg n_i\)). Electrons become the majority charge carriers, and conductivity increases significantly.
2. P-type Doping: If a trivalent impurity (e.g., Boron, Aluminum) is added, each impurity atom creates a 'hole' (an absence of an electron). This drastically increases the hole concentration (\(n_h \gg n_i\)). Holes become the majority charge carriers, and conductivity increases significantly.
In both cases, doping substantially increases the concentration of majority charge carriers (\(n_e\) or \(n_h\)), which directly leads to a large increase in the overall conductivity (\(\sigma\)) of the semiconductor.

Step 3: Final Answer:
When an impurity is doped in a pure semiconductor, the number of charge carriers increases, and therefore, the conductivity of the semiconductor increases.