*Question E6A03:
Why does a PN-junction diode not conduct current when reverse biased?*  

*Correct Answer is  C: Holes in P-type material and electrons in the N-type material are separated by the applied voltage, widening the depletion region*  

### AI Response: (may contain errors)  

To understand why a PN-junction diode does not conduct current when reverse biased, let's break down the correct answer and the other choices.

### Correct Answer: 
**C: Holes in P-type material and electrons in the N-type material are separated by the applied voltage, widening the depletion region.**

- **Explanation**: When a diode is reverse biased, a voltage is applied in the opposite direction to the forward bias. This causes the holes in the P-type material and the electrons in the N-type material to be pulled away from the junction. As a result, the depletion region (the area around the junction where no charge carriers are present) widens. This increased width of the depletion region prevents current from flowing through the diode, as there are not enough charge carriers available to conduct electricity.

### Why the Other Choices are Incorrect:

- **A: Only P-type semiconductor material can conduct current**  
  - **Explanation**: This statement is incorrect because both P-type and N-type materials can conduct current, but they do so in different ways. P-type materials conduct via holes, while N-type materials conduct via electrons. A diode requires both types to function properly.

- **B: Only N-type semiconductor material can conduct current**  
  - **Explanation**: Similar to choice A, this statement is misleading. N-type material can conduct current, but it cannot do so alone in a diode. The interaction between P-type and N-type materials is essential for diode operation.

- **D: Excess holes in P-type material combine with the electrons in N-type material, converting the entire diode into an insulator**  
  - **Explanation**: This statement is incorrect because, while holes and electrons do recombine at the junction, this process occurs primarily in the forward bias condition. In reverse bias, the depletion region widens, and the diode does not become an insulator; it simply prevents current flow.

### Useful Concepts:
- **Depletion Region**: The area around the PN junction where mobile charge carriers are depleted due to the electric field created by the junction.
- **Reverse Bias**: A condition where the positive terminal of the voltage source is connected to the N-type material and the negative terminal to the P-type material, leading to a widening of the depletion region.

Understanding these concepts will help you grasp the behavior of diodes in different biasing conditions, which is crucial for the Extra Class exam.