MOSFET operation: enhancement vs depletion
MOSFETs operate in two main modes: enhancement and depletion. In enhancement-mode MOSFETs, the device is normally off; applying a gate voltage creates a conductive channel, allowing current flow. In depletion-mode MOSFETs, the device is normally on; applying a gate voltage of opposite polarity depletes the channel, reducing or stopping current flow. Thus, enhancement MOSFETs require a gate voltage to conduct, while depletion MOSFETs conduct without gate voltage and require it to turn off.
MOSFET operation: enhancement vs depletion
MOSFETs operate in two main modes: enhancement and depletion. In enhancement-mode MOSFETs, the device is normally off; applying a gate voltage creates a conductive channel, allowing current flow. In depletion-mode MOSFETs, the device is normally on; applying a gate voltage of opposite polarity depletes the channel, reducing or stopping current flow. Thus, enhancement MOSFETs require a gate voltage to conduct, while depletion MOSFETs conduct without gate voltage and require it to turn off.
💡 Key Takeaways
- Understand how a MOSFET acts as a voltage-controlled conductor in a circuit.
- Differentiate enhancement-mode MOSFETs (normally off, require gate bias to conduct) from depletion-mode MOSFETs (normally on, conduction can be reduced by gate bias).
- Learn how threshold voltage governs turning on an enhancement-mode MOSFET and why depletion-mode devices conduct with zero gate bias.
- Compare the typical current–voltage (Id–Vgs) characteristics for both modes and how Vgs controls Ids.
- Identify common use cases for each mode, such as enhancement-mode devices for switching and depletion-mode devices for biasing or special analog applications.
❓ Frequently Asked Questions
What is a MOSFET and how does the gate voltage control current between drain and source?
A MOSFET is a metal-oxide-semiconductor field-effect transistor that uses a voltage at its insulated gate to modulate the conductivity of a channel between drain and source. The gate voltage forms or modulates this channel, allowing current to flow, while ideally no gate current flows.
What is an enhancement-mode MOSFET and how does it turn on?
An enhancement-mode MOSFET is normally off at zero gate voltage. For an n-channel device you apply a positive Vgs above its threshold to create an inversion channel and conduct current; for a p-channel device you apply a negative Vgs below its threshold. The current increases with |Vgs| beyond the threshold.
What is a depletion-mode MOSFET and how does it turn off?
A depletion-mode MOSFET is normally on at zero gate voltage because a conductive channel already exists. Applying gate bias depletes the channel and reduces current: an n-channel device turns off with negative Vgs (toward zero current) whereas a p-channel device turns off with positive Vgs.
How do enhancement and depletion MOSFETs differ in zero-bias conduction and threshold behavior?
Enhancement devices are off at Vgs = 0 and require a threshold voltage to form a channel; depletion devices are on at Vgs = 0 and have a threshold (often negative for n-channel) that requires opposite bias to turn off. This affects how they are biased in circuits.