*Question E6A02:
Which of the following semiconductor materials contains excess free electrons?*  

*Correct Answer is  A: N-type*  

### AI Response: (may contain errors)  

The correct answer is **A: N-type**. 

### Explanation:

1. **Why the correct answer is correct:**
   - N-type semiconductors are created by doping a pure semiconductor (like silicon) with elements that have more valence electrons than the semiconductor itself. For example, adding phosphorus (which has 5 valence electrons) to silicon (which has 4 valence electrons) introduces extra free electrons. These excess electrons are the majority charge carriers in N-type materials, allowing them to conduct electricity effectively.

2. **Why the other choices are incorrect:**
   - **B: P-type**: P-type semiconductors are doped with elements that have fewer valence electrons (like boron, which has 3). This creates "holes" (positive charge carriers) rather than excess free electrons.
   - **C: Bipolar**: This term refers to bipolar junction transistors (BJTs), which can be either N-type or P-type, but it does not specifically indicate the presence of excess free electrons.
   - **D: Insulated gate**: This term typically refers to insulated gate field-effect transistors (IGFETs), which use a gate to control the flow of current but do not inherently describe the type of semiconductor material or its charge carriers.

### Useful Concepts:
- **Doping**: The process of adding impurities to a semiconductor to change its electrical properties.
- **Majority and Minority Carriers**:
  - In N-type: Majority carriers are electrons; minority carriers are holes.
  - In P-type: Majority carriers are holes; minority carriers are electrons.

Understanding these concepts will help you differentiate between types of semiconductors and their charge carriers, which is crucial for the Extra Class exam.