A semiconductor is doped with a donor impurity

Options

A. The hole conecntration increases.
B. The hole concentration decreases.
C. The electron concentration increases.
D. the electron concentration decreases.

Answer

To solve the question regarding the effects of doping a semiconductor with a donor impurity, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Semiconductor**:
- Start with the concept of an intrinsic semiconductor, which is pure and has equal numbers of electrons and holes at thermal equilibrium.

2. **Doping with Donor Impurity**:
- Doping refers to the addition of impurities to the semiconductor to change its electrical properties. In this case, we are doping with a donor impurity.

3. **Types of Doping**:
- There are two types of doping: donor doping (which adds extra electrons) and acceptor doping (which creates holes). Since we are focusing on donor impurities, we will consider their effects.

4. **Characteristics of Donor Impurities**:
- Donor impurities are typically pentavalent, meaning they have five valence electrons. When added to a semiconductor like germanium or silicon (which are tetravalent and have four valence electrons), one of the donor's electrons does not participate in bonding.

5. **Formation of Free Electrons**:
- When a donor atom is incorporated into the semiconductor lattice, four of its five electrons will form covalent bonds with the neighboring semiconductor atoms. The fifth electron becomes a free electron, which can move through the crystal lattice.

6. **Effect on Electron and Hole Concentration**:
- The introduction of free electrons increases the electron concentration in the semiconductor. Since the number of holes (which are the absence of electrons) is related to the number of electrons, the increase in electron concentration results in a decrease in hole concentration.

7. **Conclusion**:
- Therefore, after doping the semiconductor with a donor impurity, we conclude that:
- The electron concentration increases.
- The hole concentration decreases.

### Final Answer:
- The correct options are:
- **Electron concentration increases** (Option 3 is correct).
- **Hole concentration decreases** (Option 2 is correct).