Gate, source and drain

Switch a load with an electric field

MOSFET means Metal Oxide Semiconductor Field Effect Transistor: an insulated gate uses an electric field to control a conducting path. In this n-channel example, electrons are the negative charge carriers; gate is the control, source is at the 0 V reference and drain connects to the load. A milliwatt (mW) is one thousandth of a watt.

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EXPERIMENT BENCH

Change the circuit settings, predict the readings, then check a challenge. These are learning models, with the assumptions explained in each lesson.

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Why it works

Illustrative switch: gate voltage at least 3 V selects 2 Ω on resistance; otherwise the path is open. Real turn-on curves, gate capacitance and the body diode are omitted; a real threshold is not a guarantee of full conduction. G, D and S label gate, drain and source. VGS is gate-to-source voltage (Vg here); VDS is drain-to-source voltage, equal to drain voltage here because source is 0 V.

Vg is gate voltage relative to source; Vs is supply and RL load resistance. When on, I = Vs/(RL + 2 Ω), drain voltage = I × 2 Ω and switch loss = I² × 2 Ω. The gate supplies control, not the load energy.

Difficulty

Build · Build it
Power on at 5 V, gate 0 V and load 100 Ω. Keep the load off.
Need a hint?
Vs = 5 V; Vg = 0 V; RL = 100 Ω → 0 mA
Explore · Change one thing
Keep supply 5 V and load 100 Ω; set gate to 5 V. Read about 49.02 mA.
Need a hint?
Vs = 5 V; Vg = 5 V; RL = 100 Ω → 49.02 mA
Solve it · Solve the challenge
At supply 12 V and gate 5 V, obtain 20–30 mA with switch loss no greater than 2 mW.
Need a hint?
Vs = 12 V; Vg = 5 V; RL = 400 Ω → 29.85 mA; 1.782 mW

Extra reading and circuit recipes