Side-by-side comparison
| Parameter | MOSFET | JFET |
|---|---|---|
| Gate structure | Insulated gate (SiO₂ layer, no DC path) | PN junction gate (reverse-biased) |
| Input impedance | >10 MΩ (capacitive) | ~10⁹ Ω (reverse-biased junction) |
| Gate biasing | V_GS can be zero, positive, or negative | V_GS must be zero or negative (depletion only for N-channel) |
| Gate-drain leakage | Extremely low (<1 nA) | Slightly higher (reverse junction leakage) |
| Transconductance g_m | Higher, adjustable by W/L ratio | Lower, typically 1–10 mA/V |
| Noise (1/f) | Higher 1/f noise due to oxide interface traps | Lower 1/f noise; preferred for low-noise front-ends |
| Pinch-off voltage V_P | Not applicable (enhancement types have V_th) | Defined by doping; typically −1 to −6 V for N-channel |
| Fabrication | CMOS compatible; widely integrated | Discrete; harder to integrate in CMOS |
| Common devices | 2N7000, BS170, IRF540 | J112, J113, MPF102, 2N5459 |
| Application | Power switching, digital logic, RF power | Low-noise amplifiers, AGC circuits, analog switches |
Key differences
The JFET gate is a reverse-biased PN junction — N-channel JFETs like the MPF102 require V_GS ≤ 0 always; forward-biasing the gate by even 0.5 V damages it. MOSFETs have an insulated oxide gate and can accept positive, zero, or negative V_GS without damage. JFET has inherently lower 1/f noise because it lacks the Si-SiO₂ interface traps that plague MOSFETs — this is why J112 is preferred in the front end of high-end oscilloscopes and audio gear. MOSFETs offer higher transconductance and are CMOS-compatible, making them dominant in ICs; discrete JFETs like 2N5459 remain alive only in niche analog applications.
When to use MOSFET
Use a JFET when designing a low-noise preamplifier, a high-impedance buffer, or an analog switch where 1/f noise below 1 kHz matters critically. A J112 as the input FET in an electret microphone circuit gives a noise figure below 1 dB — no MOSFET of the same era matches that.
When to use JFET
Use a MOSFET when you need high transconductance, power switching capability, or CMOS integration. A 2N7000 MOSFET in a solid-state relay switches 200 mA at 60 V from a 5 V logic signal, something no JFET can do because a JFET cannot be enhanced beyond I_DSS.
Recommendation
For low-noise small-signal amplifiers below a few kilohertz, choose JFET (J112 or MPF102) — the lower 1/f noise floor is decisive. For everything else — power, integration, digital-compatible drive — choose MOSFET. If you're unsure, MOSFET is almost always the right default.
Exam tip: Examiners expect you to explain why a JFET is always operated in depletion mode while a MOSFET can operate in both enhancement and depletion — draw the I_D vs V_GS curve for each to show this clearly.
Interview tip: Interviewers in analog design roles ask about noise figure and why JFETs appear in oscilloscope input stages — explaining Si-SiO₂ interface trap noise in MOSFETs versus clean junction leakage in JFETs shows circuit-level thinking.