OUTPUT CURVES

ID versus VDS

Fix the gate voltage, sweep the drain voltage, and record the drain current. Each curve is one gate voltage. Current first rises along a curve, then flattens once the channel pinches off near the drain.

Higher gate voltage, taller curve: that's the gate controlling the current.

THREE REGIONS

Cutoff, triode, saturation

Cutoff · VGS < Vth No inversion channel, only a tiny leakage current

Triode · VDS < VGS − Vth The channel spans source to drain, acting like a voltage-controlled resistor

Saturation · VDS ≥ VGS − Vth The channel pinches off near the drain and the current flattens

VGS − Vth is called the overdrive voltage.

TRIODE REGION

A tunable resistor

ID = k(VGS−Vth)VDS

For small VDS the current is proportional to VDS, with k = μ·C_ox·(W/L). The full expression is ID = k[(VGS − Vth)·VDS − VDS²/2]. More gate overdrive lowers the effective resistance, so the gate tunes the resistance.

In triode, the MOSFET is a resistor whose value the gate sets.

SATURATION REGION

A gate-controlled current source

ID = ½k(VGS−Vth)²

Once the channel pinches off, the current depends on the gate, not the drain. Example: with μC_ox = 100 µA/V², W/L = 10, and an overdrive of 0.5 V, the saturation current is ½ × 100 µA/V² × 10 × 0.25 V² = 125 µA.

Square law: doubling the overdrive quadruples the current.

UNIT 6 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered cutoff, triode, and saturation, with the square-law equations.