*Question E6B04:
What type of semiconductor device is designed for use as a voltage-controlled capacitor?*  

*Correct Answer is  A: Varactor diode*  

### AI Response: (may contain errors)  

The correct answer to the question about the semiconductor device designed for use as a voltage-controlled capacitor is:

**A: Varactor diode**

### Explanation of the Correct Answer:
- **Varactor Diode**: This device is specifically designed to act as a variable capacitor. Its capacitance changes with the applied reverse voltage. The relationship between the capacitance ($C$) and the reverse voltage ($V_R$) can be approximated as:

$$C \propto \frac{1}{V_R^{n}}$$

where $n$ is a constant that depends on the specific diode. This property makes varactor diodes useful in applications like voltage-controlled oscillators (VCOs) and frequency modulation.

### Why the Other Choices are Incorrect:
- **B: Tunnel Diode**: This is a type of diode that exhibits negative resistance due to quantum tunneling. It is used primarily in high-frequency applications and oscillators, but it does not function as a voltage-controlled capacitor.

- **C: Silicon-Controlled Rectifier (SCR)**: An SCR is a four-layer semiconductor device that acts as a switch. It is used for controlling power in AC circuits and does not have the variable capacitance characteristic of a varactor diode.

- **D: Zener Diode**: A Zener diode is designed to allow current to flow in the reverse direction when a specific reverse voltage (Zener voltage) is reached. It is primarily used for voltage regulation, not as a variable capacitor.

### Summary:
- The varactor diode is the only device among the options that functions as a voltage-controlled capacitor, making it the correct answer. The other devices serve different purposes in electronic circuits.