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Enhancement vs Depletion MOSFET: Key Differences

An enhancement-mode MOSFET has no channel when the gate voltage is 0 V; current flows only after a gate voltage creates an inversion layer (normally off). A depletion-mode MOSFET has a channel built in, so current flows even with the gate at 0 V, and it stops when a gate voltage of the opposite polarity removes the channel (normally on). In an n-channel device, the threshold voltage is positive for enhancement mode and negative for depletion mode. Enhancement-mode devices are used in almost all applications, including CMOS logic.

AspectEnhancement modeDepletion mode
With the gate at 0 VNo channel; off (normally off)Channel present; on (normally on)
Threshold voltage sign (n-channel)PositiveNegative
Threshold voltage sign (p-channel)NegativePositive
How the channel formsThe gate voltage induces an inversion layer at the substrate surfaceA thin channel layer is formed at the surface in advance, for example by ion implantation
How to stop current (n-channel)Lower the gate voltage below the threshold (0 V is enough)Apply a negative gate voltage to deplete the channel
Origin of the nameThe gate voltage enhances the channelThe gate voltage depletes the channel
Circuit symbolThe channel line is broken into three dashesThe channel line is a single solid line
If the drive circuit stopsTurns off, so it is safeMay turn on, so care is needed to avoid short circuits
Typical usesAlmost all applications: CMOS logic, power MOSFETs, analog circuits, etc.Load devices in early NMOS logic, constant-current sources, power supply start-up circuits, etc.

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Enhancement mode
Almost always. Setting the gate to 0 V reliably turns it off, so it is used for switches in logic circuits and for power switches that must turn off safely.
Depletion mode
When you want current to flow without a gate voltage: load devices in place of resistors, simple constant-current sources with the gate tied to the source, and start-up circuits that run before the power supply has come up.
Using normally-on devices
Devices that are inherently normally on, such as GaN HEMTs, are used as normally-off devices either by modifying the gate structure or with a cascode configuration in series with a low-voltage silicon MOSFET.

FAQ

Why do power applications require normally-off devices?

If the gate drive circuit loses power or fails, a normally-on device turns on, which can short the supply and cause a large current to flow. An enhancement-mode device is off with its gate at 0 V, so it is safe.

In what circuits have depletion-mode devices been used?

In NMOS logic before CMOS became mainstream, depletion-mode devices with the gate tied to the source were used as load devices in place of resistors. Today they are still used in simple constant-current sources and power supply start-up circuits.

How do you tell them apart in a circuit symbol?

By the line representing the channel. An enhancement-mode device is drawn with the line broken into three dashes, showing that there is no channel without a gate voltage. A depletion-mode device has a channel from the start, so it is drawn with a single solid line.

Can you tell them apart by the threshold voltage?

Yes. In an n-channel device, a positive threshold voltage means enhancement mode and a negative one means depletion mode. In a p-channel device the signs are reversed: negative means enhancement mode and positive means depletion mode.

Check your understanding

Which is a characteristic of a depletion-mode MOSFET?

  1. AIt has no gate
  2. BA channel exists even at a gate voltage of 0 V, so current flows
  3. CNo current flows even when the gate voltage is above the threshold voltage
Show answer and explanation

Answer: B. A channel exists even at a gate voltage of 0 V, so current flows

A depletion-mode MOSFET has a built-in channel and conducts even at 0 V gate voltage (normally on). For an n-channel device, a negative gate voltage removes the channel and stops the current. Early NMOS logic used them as load devices. Because they conduct when power is lost, they require safety considerations; GaN HEMTs are also inherently normally on, so special structures are used to make them normally off.

What is the condition for an enhancement-mode n-channel MOSFET to turn on (conduct)? (VGS: gate-source voltage, Vth: threshold voltage)

  1. AVGS is 0 V
  2. BVGS is greater than Vth
  3. CVGS is negative
Show answer and explanation

Answer: B. VGS is greater than Vth

In an enhancement-mode nMOSFET, a positive gate voltage above Vth draws electrons to the surface of the p-type substrate, forming an inversion layer (channel) so current flows between source and drain. At VGS = 0 V there is no channel and the device is off (normally off), which suits logic and makes power circuits fail safe. Vth is typically 0.2–0.4 V in advanced logic and about 1–4 V in power devices.

What is the sign of the threshold voltage (relative to the source) of an enhancement-mode p-channel MOSFET?

  1. APositive
  2. BNegative
  3. CAlways 0 V
Show answer and explanation

Answer: B. Negative

In a pMOS, applying a gate voltage below the source forms a hole inversion layer at the surface of the n-type body (n-well), turning it on. An enhancement-mode pMOS therefore has a negative threshold voltage (e.g., −0.4 V). In circuits, with the source tied to VDD, it turns on when the gate is more than |Vth| below VDD. Design documents often compare magnitudes |Vth| to avoid sign confusion.

  1. Why are power devices required to be 'normally off', with no current flowing at 0 V gate voltage?
  2. Which configuration combines a normally-on GaN HEMT in series with a low-voltage silicon MOSFET so it can be used safely with normally-off behavior?

Practice MOSFET questions

Key terms explained

Threshold voltage
The gate voltage at which a channel (inversion layer) forms in a MOSFET and current starts to flow. Written as Vth.
Normally-off
The property of passing no current when the gate voltage is 0 V. It is required of power devices for safety, since current stops even if the drive circuit fails.
Inversion layer
A layer formed at the semiconductor surface by the gate voltage, where carriers of the type opposite to the substrate gather. It becomes the MOSFET's channel.

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