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What a 2N7000 MOSFET does
MOSFET 2N7000 — A transistor switched by voltage, not current. Its gate draws nothing, so any signal can drive it.
What it is
A MOSFET is a transistor controlled by voltage instead of current. The 2N2222 you know opens when you push current into its base. The 2N7000 opens when you raise the voltage on its gate, and the gate is insulated from everything else by a glass-thin layer of oxide: no current flows into it at all. The name spells that out: Metal-Oxide-Semiconductor Field-Effect Transistor. An electric field through the oxide does the switching.
Same black half-cylinder as the 2N2222. Flat face toward you, legs down, left to right: Source, Gate, Drain. The source goes to −, the load hangs between + and the drain, and the gate is the control. If you have wired an NPN, this is the same shape: source where the emitter was, drain where the collector was, gate where the base was.
Why does it matter that the gate takes no current? Because then anything can drive it. A finger on a touch pad, a 100 kΩ divider, a logic chip output that can barely manage a milliamp: all of them can swing a gate between 0 V and 4.5 V, and the MOSFET will switch 200 mA for them.
How it works
The threshold. Below a gate-to-source voltage of about 2 V (the datasheet allows 0.8 to 3 V part to part; the site's model uses 2.0 V) the channel is closed and nothing flows drain to source. Above it the channel opens, and the more gate voltage, the wider it opens.
How much it can pass. In the model, with the drain well above the source, I_drain = ½ × 0.1 × (V_gate − 2)² amps. At 4.08 V on the gate that is 0.05 × 2.08² ≈ 216 mA available. An LED lane only wants 11 mA, so the MOSFET is nowhere near its limit and simply behaves like a small resistor: R_on ≈ 1 ÷ (0.1 × (4.08 − 2)) ≈ 4.8 Ω, and 11 mA across 4.8 Ω drops only 0.05 V. The drain sits a twentieth of a volt above ground: a closed switch.
Setting the gate voltage. A 10 kΩ from the button and a 100 kΩ pull-down form a divider: 4.5 × 100 ÷ 110 = 4.09 V. The 41 µA that flows through the pair goes into the 100 kΩ, none into the gate. Change the 10 kΩ to 100 kΩ and the gate drops to 2.25 V, just above threshold: I_drain = 0.05 × 0.25² ≈ 3 mA, a dim LED. A 2N2222 with its base resistor made ten times bigger would get a tenth of the base current and go dark; the MOSFET only cares what voltage arrives.
The pull-down is not optional. The gate is a tiny capacitor (tens of picofarads) with nothing to drain it. Left floating it holds whatever charge it last picked up and the MOSFET sits half on, flickering. The 100 kΩ bleeds it to 0 V in microseconds whenever the button is released.
Limits. 200 mA continuous drain current, 60 V drain-to-source, and the gate must never go beyond ±20 V from the source. At 4.5 V you are nowhere near any of them, but the oxide is thin: a static spark from a carpet can exceed 20 V. Touch the metal of your workbench before handling it.
The numbers
| Pinout (flat face toward you) | S – G – D — source to −, load on the drain |
|---|---|
| Gate threshold | 0.8–3 V — sims use 2.0 V |
| Gate current | 0 — a few picoamps of leakage |
| On-resistance at 4 V gate | ≈ 5 Ω — 2N7000 datasheet: 5.3 Ω max at 4.5 V, 75 mA |
| Max drain current | 200 mA — 500 mA pulsed |
| Max drain-source voltage | 60 V |
| Gate voltage limit | ±20 V — static can exceed this — touch earth first |
See it working
The The Switch That Uses No Current circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Gate left floating. Random on, off and half-on. Fix: 100 kΩ (10 kΩ to 1 MΩ all work) from gate to source.
- Source and drain swapped. The 2N7000 has a built-in diode from source to drain, so wired backwards it conducts all the time through that diode. Fix: flat face toward you, legs S-G-D, source to −.
- Gate driven from a 3 V battery. Some parts need up to 3 V just to start opening; at 3 V you may get a dim, warm MOSFET. Fix: 4.5 V or more on the gate, or a logic-level MOSFET rated for 2.5 V drive.
- Expecting it to drive a motor. 200 mA is the limit and a stalled hobby motor wants more. Fix: a bigger MOSFET (IRLZ44N) or the 2N2222 for small motors, always with a flyback diode.
- Handling it on a dry day. The oxide is only nanometres thick. Fix: touch something earthed before you pick it up; keep it in its antistatic bag.
Questions people ask
- What is the difference between a MOSFET and a normal transistor?
- A bipolar transistor like the 2N2222 is switched by current flowing into its base, so the thing driving it has to supply that current. A MOSFET is switched by voltage on its gate and draws no current, so a weak signal can switch a strong load.
- Does a MOSFET need a gate resistor?
- Not to limit current, since the gate takes none. It needs a pull-down (or pull-up) so the gate is never left floating, and in fast circuits a small series resistor tames the current that charges the gate capacitance each time it switches.
- What is the pinout of a 2N7000?
- TO-92 package, flat face toward you, legs down: source, gate, drain from left to right. The same positions an NPN uses for emitter, base, collector.
Projects that use this part
- 🪶 The Switch That Uses No Current — A MOSFET turns the LED fully on, yet its gate draws nothing. Hold the button, then tap the 10 kΩ to make it 100 kΩ and see the gate still win. L5 · 30 min
- 🌙 The Night Gate — A night light switched by a MOSFET. The gate takes no current, so the moment it turns on is plain voltage-divider math you can work out exactly. L5 · 30 min
verified against: onsemi — 2N7000 / 2N7002 / NDS7002A datasheet · Kuphaldt — Lessons in Electric Circuits, Insulated-gate field-effect transistors (MOSFET) · SparkFun — Transistors