*Question T6B05:
What type of transistor has a gate, drain, and source?*  

*Correct Answer is  B: Field-effect*  

### AI Response: (may contain errors)  

The correct answer to the question is **B: Field-effect**. 

### Explanation of the Correct Answer:
- A **Field-effect transistor (FET)** is a type of transistor that has three terminals: gate, drain, and source. 
- The gate terminal controls the flow of current between the drain and source terminals by creating an electric field, which modulates the conductivity of a channel.

### Why the Other Choices are Incorrect:
- **A: Varistor**
  - A varistor is a voltage-dependent resistor used to protect circuits from voltage spikes. It does not have a gate, drain, or source; instead, it has two terminals and operates based on the voltage applied across it.

- **C: Tesla-effect**
  - The term "Tesla-effect" is not a recognized type of transistor. It may refer to concepts related to Nikola Tesla's work, but it does not pertain to a specific transistor type with gate, drain, and source terminals.

- **D: Bipolar junction**
  - A **Bipolar Junction Transistor (BJT)** has three terminals as well, but they are called emitter, base, and collector. BJTs operate based on the flow of both electrons and holes, unlike FETs, which rely on electric fields.

### Summary:
- **Field-effect transistors (FETs)** are characterized by their gate, drain, and source configuration, making them suitable for various applications in electronics, including amplifiers and switches. 

Understanding the differences between these components is crucial for the Extra Class exam, as it helps in identifying the correct types of devices used in various circuits.