*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)  

A varactor diode is the correct answer because it is specifically designed to act as a voltage-controlled capacitor. Here's why:

1. **Why the correct answer is correct:**
   - **Varactor Diode (A):** A varactor diode, also known as a varicap or tuning diode, is a type of semiconductor device that changes its capacitance based on the voltage applied across its terminals. This property makes it useful in applications like voltage-controlled oscillators and frequency modulators, where tuning is required.

2. **Why the other choices are incorrect:**
   - **Tunnel Diode (B):** A tunnel diode is known for its ability to operate at very high speeds due to the quantum mechanical effect called tunneling. It is used in high-frequency oscillators and amplifiers, not as a voltage-controlled capacitor.
   - **Silicon-Controlled Rectifier (C):** A silicon-controlled rectifier (SCR) is a type of thyristor used for controlling power. It acts as a switch that can turn on or off the flow of current, but it does not function as a capacitor.
   - **Zener Diode (D):** A Zener diode is designed to allow current to flow in the reverse direction when a specific breakdown voltage is reached. It is used for voltage regulation, not for capacitance control.

3. **Useful Concept:**
   - The capacitance of a varactor diode is inversely proportional to the square root of the reverse bias voltage applied across it. This relationship can be expressed as:
     $$ C \propto \frac{1}{\sqrt{V}} $$
   - Where $C$ is the capacitance and $V$ is the reverse bias voltage.

Understanding these distinctions helps in identifying the correct application of each semiconductor device.