Question

When a semiconductor is doped with a donor impurity

• A. The hole concentration increases.
• B. The hole concentration decreases.
• C. The electron concentration increases.
• D. The electron concentration decreases.

Answer 1

The correct option is C The electron concentration increases.

When a semiconductor is doped with a donor type such as arsenic or phosphorous, which has five valence electrons, the donor atom replaces the Si or Ge atom.

As a result, four out of the five electrons of the donor atom form a covalent bond by sharing an electron with four atoms of silicon.

However, the fifth electron is free to move. Therefore, every donor atom replacing Si or Ge, one free electron is contributed by the donor atom and hence the electron concentration increases.

Also, due to the breaking up of covalent bonds at room temperature, equal number of electrons and holes are produced.

Thus, the total number of holes in the n-type semiconductor is less compared to the number of free electrons, but the doping with donor atom does not decrease the hole concentration, it only increases the electron concentration.

Hence, option $\left(C\right)$ is correct.