*Question E6A09:
What is a depletion-mode field-effect transistor (FET)?*  

*Correct Answer is  A: An FET that exhibits a current flow between source and drain when no gate voltage is applied*  

### AI Response: (may contain errors)  

A depletion-mode field-effect transistor (FET) is a type of FET that is normally "on" when no gate voltage is applied. This means that there is a current flow between the source and drain without any external gate voltage. Let's break down the choices:

1. **Correct Answer (A):**  
   - A depletion-mode FET is designed to conduct current between the source and drain when the gate voltage is zero. This is because the channel is naturally open, allowing current to flow. Applying a negative gate voltage (for an N-channel FET) reduces the current flow by depleting the channel of charge carriers.

2. **Incorrect Choices:**
   - **Choice B:** This describes an enhancement-mode FET, which is normally "off" when no gate voltage is applied. In enhancement-mode FETs, a positive gate voltage (for N-channel) is required to create a conductive channel.
   - **Choice C:** While electron mobility is a factor in FET performance, this choice does not accurately describe the operation of a depletion-mode FET. The presence or absence of holes in the N-type material is not the defining characteristic of depletion-mode operation.
   - **Choice D:** This describes a P-channel FET, where holes are the majority carriers. However, it does not specifically relate to the depletion-mode operation, which is about the default state of the channel being conductive.

**Key Concept:**
- **Depletion-mode FET:** Normally "on" (conducting) with no gate voltage.  
- **Enhancement-mode FET:** Normally "off" (non-conducting) with no gate voltage.

Understanding the difference between depletion-mode and enhancement-mode FETs is crucial for identifying their behavior in circuits.