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

Choices:  
A: An FET that exhibits a current flow between source and drain when no gate voltage is applied  
B: An FET that has no current flow between source and drain when no gate voltage is applied  
C: An FET that exhibits very high electron mobility due to a lack of holes in the N-type material  
D: An FET for which holes are the majority carriers    

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

To understand a depletion-mode FET, remember: depletion = default-on.

1) Why the correct answer is correct
- In a depletion-mode FET (JFET or depletion-mode MOSFET), a conductive channel already exists at $V_{GS}=0$, so current flows from drain to source with no gate bias.
- Applying a reverse gate voltage “depletes” carriers in the channel and reduces $I_D$. Sufficient reverse bias “pinches off” the channel and turns the device off.

2) Why the other choices are incorrect
- B: “No current at zero gate voltage” describes an enhancement-mode FET (default-off), not depletion-mode.
- C: Electron mobility is not what defines depletion-mode operation; this statement is unrelated to the depletion/enhancement distinction.
- D: “Holes are the majority carriers” simply describes a P-channel device; both N- and P-channel devices can be depletion-mode. Carrier type is not the defining feature.

3) Useful formulas and rules of thumb
- JFET (Shockley equation):
  $$I_D = I_{DSS}\left(1 - \frac{V_{GS}}{V_P}\right)^2$$
  where:
  - $I_{DSS}$ is the drain current at $V_{GS}=0$ (nonzero for depletion devices),
  - $V_P$ (pinch-off or cutoff voltage) is the gate voltage where $I_D \to 0$ (negative for N-channel JFETs).
  - At $V_{GS}=0$, $I_D = I_{DSS}$ → current flows with no gate bias.

- Depletion-mode NMOS:
  - Has $V_T < 0$, so at $V_{GS}=0$ the device conducts.
  - Apply negative $V_{GS}$ to reduce/stop conduction.

- Rule of thumb:
  - Depletion-mode = default-on (needs reverse bias to turn off).
  - Enhancement-mode = default-off (needs forward gate bias to turn on).