A depletion-type insulated-gate field-effect transistor (IGFET), usually called a depletion-mode MOSFET or D-MOSFET, has a conducting channel at zero gate-to-source voltage. For an N-channel device, making the gate more negative than the source reduces current; making it more positive increases current, within the device’s ratings. “Normally on” describes the channel’s zero-bias state, not a guaranteed current or a fully enhanced condition.
What “depletion-type IGFET” means
An IGFET controls current through an electric field from a gate that is insulated from the semiconductor. The gate behaves approximately like a capacitor: steady-state gate current is very small, but leakage and transient charging current remain, and the insulating layer can be damaged by excessive voltage or electrostatic discharge. A MOSFET is the best-known IGFET implementation. The name reflects a conventional metal–oxide–semiconductor structure; modern gate materials and dielectrics can differ from that simple description. All About Circuits explains the IGFET family and its relation to other field-effect transistors.
“Depletion” describes what one gate-voltage polarity does to an existing channel: it reduces the mobile carriers in that channel. It does not mean the transistor can only be depleted. A depletion MOSFET can generally be enhanced by applying the opposite polarity, subject to its particular limits.
Structure and symbol
Inside a basic N-channel device
A basic N-channel depletion IGFET has an N-type channel joining the source and drain, a P-type body or substrate around or beneath the channel, and an insulated gate over the channel. The channel is present when the gate and source are at the same potential. Many discrete devices internally connect the body to the source, but internal body connections and body-diode orientation must be confirmed in the manufacturer’s datasheet rather than assumed from a generic drawing.
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Reading the circuit symbol
In common textbook symbols, a solid channel line between source and drain indicates a pre-existing channel, while a broken or absent line is commonly used for an enhancement device. Symbol standards and manufacturer drawings vary, so treat the solid-line convention as a useful clue, not a universal rule. Check the part number and datasheet before designing around a symbol alone.
How gate voltage changes current
Use voltage relative to the source, not relative to ground:
VGS = VG − VS
For an N-channel depletion MOSFET, the gate’s electric field changes the channel’s carrier concentration. Negative VGS repels electrons from the channel region and reduces its conductivity. Positive VGS attracts additional electrons and can increase conductivity. The gate is not intended to conduct ordinary forward current into the channel. For a P-channel depletion device, reverse the voltage polarities: a positive gate-to-source voltage depletes the channel, and a negative one enhances it.
| Device | Channel at VGS = 0 | Gate-to-source change that generally increases N-channel current | Gate type |
|---|---|---|---|
| N-channel depletion MOSFET | Conducting | More positive | Insulated |
| N-channel enhancement MOSFET | No useful channel | More positive | Insulated |
| N-channel JFET | Conducting | Less negative or nearer zero, within limits | Reverse-biased PN junction |
For P-channel devices, reverse the voltage polarities. IGFETs and JFETs can both be normally on, but a JFET’s gate is a PN junction; forward-biasing it can cause substantial gate current. An insulated gate avoids that junction behavior, not the risk of gate damage. Toshiba’s JFET explanation describes the junction-gate distinction.
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Why the device is normally on—and what that does not promise
The channel is fabricated into the device, so an appropriate drain-to-source voltage can produce current when VGS is zero. For an idealized N-channel MOSFET, a negative threshold voltage is a useful sign of depletion-mode operation: at zero gate bias, VGS is above that threshold. The sign convention reverses for P-channel devices. This is an introductory model, not a substitute for the device’s specified operating curves.
At zero gate bias, current depends on drain voltage, device geometry, temperature, manufacturing variation, and the external circuit. It is not automatically the maximum rated drain current. Datasheets may specify zero-gate drain current (IDSS), a cutoff voltage (VGS(off)), or other operating points. “Normally on” therefore tells you the default channel condition, not the exact current a circuit will draw.
Operating regions and useful equations
The familiar MOSFET regions are useful for understanding a depletion device, provided the threshold sign and operating conditions are applied consistently. In these equations, VTH is negative for the idealized N-channel depletion device, and μn, Cox, W and L represent channel parameters.
Cutoff
Making VGS sufficiently negative relative to the device’s cutoff condition reduces drain current to a level usable as “off” in a particular circuit. Real parts still have leakage; cutoff does not mean mathematically zero current. The necessary gate voltage varies by part and operating conditions.
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Linear or ohmic region
At relatively low VDS, the channel behaves approximately like a gate-controlled resistance. An idealized long-channel relation is:
ID ≈ μnCox(W/L)[(VGS − VTH)VDS − VDS2/2]
Saturation region
When the channel constricts near the drain, the long-channel square-law approximation is:
ID ≈ ½ μnCox(W/L)(VGS − VTH)2
In that idealized model, the approximate boundary is VDS ≥ VGS − VTH. These expressions explain trends; they are not universal design models. Channel-length modulation, mobility degradation, body effect, series resistance, temperature, and short-channel behavior alter real-device performance. Threshold voltage can also depend on geometry, drain voltage, and source-to-body bias. Use manufacturer curves and models for a real circuit. The MOSFET theory overview and this threshold-voltage study address device-theory effects.
Terminology needs care: in MOSFET descriptions, “pinch-off” often marks the onset of saturation near the drain. In JFET discussions, the term may describe cutoff-like behavior. Do not treat pinch-off, saturation, and cutoff as interchangeable. The threshold-voltage reference discusses the terminology.
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Biasing a depletion MOSFET
Source-resistor self-bias
A grounded gate and a source resistor can create negative VGS automatically. With VG approximately 0 V and VS = IDRS:
VGS = VG − VS ≈ −IDRS
If current rises, the source voltage rises; that makes VGS more negative and tends to push current back down. This is useful negative feedback, but it does not eliminate part-to-part or temperature variation.
Worked first estimate
Suppose the desired operating point is approximately 2 mA at VGS = −2 V. A first estimate is RS = 2 V ÷ 2 mA = 1 kΩ. This is only a starting point: confirm that the chosen part’s transfer curves support the operating point across temperature and production spread. Then check the approximate transistor dissipation, PD ≈ VDSID, and resistor dissipation, PR = ID2RS, against ratings and thermal conditions.
Fixed gate bias
An external negative supply or bias network can set an N-channel device’s VGS more directly. This is useful when the operating point needs deliberate control, but the bias must remain valid during startup, brownout, reset, power-down, and fault conditions. In floating or high-side circuits, calculate gate-to-source voltage rather than inferring it from the gate’s ground-referenced voltage.
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Current limiting and startup paths
A source resistor, sense resistor, or control loop can constrain a depletion device’s default conduction for current limiting or bias generation. A depletion MOSFET can also provide a startup path in a power converter, allowing a control circuit to begin operating before another switch takes over. In that use, the device may carry current while experiencing nearly the full input voltage. Verify its voltage rating, startup duration, safe operating area, thermal dissipation, and behavior if the downstream controller fails to start. A discrete device’s current spread may make it a poor precision regulator compared with a feedback-controlled source or dedicated regulator.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to check in the datasheet
Do not choose a part based on its threshold voltage alone. Threshold is typically specified at a small test current; it is not the gate voltage required for a low-resistance, fully enhanced state or a promise of a particular operating current.
| Parameter | Why it matters |
|---|---|
| VGS(th) | Threshold test point, usually at low drain current; not a full-on drive specification. |
| IDSS or zero-gate current | Shows current at a specified zero-gate operating condition, where provided. Read the stated VDS and limits. |
| VGS(off) | Gate-to-source voltage associated with cutoff, where specified; check its test conditions and spread. |
| VDS and ID ratings | Bound drain voltage and current, but do not by themselves establish safe operation at every combination. |
| Maximum VGS | Sets the gate-source stress limit for both polarities as specified by the manufacturer. |
| RDS(on), where specified | Check the test gate voltage and current; a value at one bias may not apply to the intended operating point. |
| gm, gate charge and capacitances | Help assess control sensitivity, switching behavior, and coupling of drain transients into the gate. |
| Leakage, thermal resistance, junction-temperature limit and safe operating area | Determine off-state behavior and whether steady or pulsed dissipation is safe in the intended conditions. |
Use the manufacturer’s minimum, typical, and maximum curves where available, and a manufacturer SPICE model for simulation when appropriate. A JFET’s Shockley equation is not a universal depletion-MOSFET model.
Protect the gate and define the default state
An insulated gate has high input resistance, but its dielectric can be damaged by electrostatic discharge or excessive gate voltage. A nominally safe DC bias does not guarantee safety during fast drain-voltage transients: gate-drain capacitance can couple a transient onto the gate.
- Keep positive and negative VGS within the part’s absolute maximum ratings.
- Use a defined gate bias so the device cannot turn on unintentionally when a controller is disconnected or unpowered.
- Consider a gate resistor to limit transient current and a gate-source clamp or zener where exposure to transients warrants it; size protection for the actual circuit.
- Account for gate charge, Miller coupling, inductive transients, and startup behavior.
- Use ESD-safe handling. Gate-oxide overstress can cause permanent damage or degraded reliability.
A high-side gate voltage that appears positive relative to circuit ground can still be negative relative to the source. In a safety disconnect, a normally-on device can also leave a load energized if the control wire breaks or its controller loses power. Design the unpowered and fault states explicitly rather than assuming a control signal will always be present.
When a depletion IGFET is useful—and when another device fits better
| Choose or consider | It fits when | Main trade-off |
|---|---|---|
| Depletion MOSFET | The circuit needs a conducting default state, an insulated gate, or both depletion and enhancement control; examples include startup paths, simple current limiting, bias generation, active loads, and some analog or RF circuits. | Default-on current, gate protection, and device spread require deliberate control. It is not the ordinary default choice for low-loss power switching. |
| Enhancement MOSFET | The default should be off, or the job is ordinary logic-controlled or power switching where low conduction loss matters. | It needs suitable gate drive to establish its conducting state. |
| JFET | A normally-on junction-gate device suits an analog application, including some low-noise uses, and its gate remains within reverse-bias limits. | The gate is a PN junction; forward-bias restrictions and leakage differ from an insulated gate. |
| Dedicated current regulator or controller | Current accuracy, temperature stability, startup behavior, protection, or production repeatability matters more than a minimal discrete solution. | May add components or cost, but avoids relying on poorly controlled transistor spread. |
Depletion devices also appear in level shifting, gate-drive support, normally-on analog loads, simple pre-regulators, protection, and cascode arrangements. Their default conduction can be useful in a startup or bias function but is a liability when an unpowered system must keep a load off.
Common design mistakes
- Reading gate voltage against ground: use VGS = VG − VS, especially for high-side or floating-source circuits.
- Treating threshold as a turn-on or full-on voltage: VGS(th) is measured at a specified test current and does not establish low on-resistance or operating current.
- Assuming zero-gate current is controlled: a depletion device connected across a supply can draw excessive current at VGS = 0 unless an external element or feedback limits it.
- Assuming “off” means zero current: define the acceptable leakage or load current and check the specified cutoff behavior.
- Assuming an insulated gate tolerates any polarity: either polarity may be usable only within the part’s rated gate-source limits.
- Ignoring body-diode direction: package internals and diode ratings are part-specific; verify the datasheet before relying on a discrete MOSFET’s diode path.
- Confusing MOSFET saturation with JFET cutoff: “pinch-off” does not mean the same operating condition in every device family.
For the basic operating behavior, see All About Circuits’ depletion-type IGFET explanation and CircuitBread’s introductory treatment. For the common depletion/enhancement threshold-sign convention, see the depletion and enhancement modes overview; always use the selected manufacturer’s datasheet for design values.
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