Side-by-side comparison
| Parameter | Enhancement | Depletion MOSFET |
|---|---|---|
| Channel at V_GS = 0 | No channel; device is OFF | Channel exists; device is ON by default |
| Threshold voltage V_th | Positive for N-channel (+1 to +4 V) | Negative for N-channel (−1 to −6 V) |
| Operating mode | Enhancement only (channel induced by gate) | Can operate in both depletion and enhancement mode |
| Gate voltage to turn OFF | Reduce V_GS below V_th (toward 0 V) | Apply negative V_GS to pinch off channel |
| Biasing complexity | Simple; self-biasing not needed for switch | Requires negative bias supply or self-bias resistor |
| Circuit symbol | Dashed channel line (broken) | Solid channel line (continuous) |
| Common N-channel devices | BS170, 2N7000, IRF540N | 2N3797, BF960 (dual-gate depletion) |
| Typical application | Power switches, digital logic, inverters | Constant-current sources, VCOs, GaAs RF amps |
| Fail-safe behavior | Fails OFF — safe for power switching | Fails ON — risky for power loads |
| Prevalence in industry | Dominant in digital and power design | Niche; mostly RF and constant-current circuits |
Key differences
Enhancement MOSFETs need a gate voltage above V_th to create a channel — the BS170 starts conducting at V_GS ≈ +2 V and is fully off at 0 V, making it inherently safe to drive from a logic output. Depletion MOSFETs carry a doped channel at fabrication; the 2N3797 passes I_DSS at V_GS = 0 and needs a negative gate voltage to cut off. This fail-ON behavior makes depletion types unsuitable for most power switching, but ideal as a constant-current two-terminal device — connect gate to source (V_GS = 0) and the depletion MOSFET behaves like a current regulator diode. In GaAs RF IC design, depletion-mode HEMTs are standard because they offer higher electron mobility and lower noise at microwave frequencies.
When to use Enhancement
Use an enhancement MOSFET when the device must be OFF by default and turn ON only when driven by a logic or PWM signal. A 2N7000 enhancement MOSFET in a microcontroller-driven LED driver is safe at power-up — no accidental turn-on, no flyback current until intentionally switched.
When to use Depletion MOSFET
Use a depletion MOSFET when a constant-current source with minimal external components is needed. Connecting a depletion N-channel MOSFET (gate shorted to source) in series with a load creates a simple current limiter that regulates I_D to I_DSS without any feedback resistor.
Recommendation
In 9 out of 10 real circuit problems, choose enhancement MOSFET — it integrates cleanly with logic, has predictable biasing, and fails safe. Depletion type is worth knowing for constant-current applications and RF design, but it's the answer only when the question specifically demands depletion-mode behavior.
Exam tip: Exam questions frequently ask students to draw the transfer characteristic (I_D vs V_GS) for both types on the same axis — know that the depletion curve extends into negative V_GS with I_DSS at zero, while the enhancement curve starts at positive V_th.
Interview tip: Core company interviewers ask why enhancement MOSFETs dominate digital IC design over depletion types — the correct answer centers on fail-safe OFF state, simpler biasing, and CMOS compatibility, not just a memorized preference.