The single chart that tells you everything

Engineers describe a transistor with one chart: the I-V curve. Sweep the drain voltage with the gate held at a fixed value, plot the resulting current, and you have the device's behavior in your hand. Repeat for several gate voltages and you get the full family of curves that lives on the front page of every transistor's datasheet.

Key Concept

I-V Curve

A plot of current (I) versus voltage (V). For a transistor, almost always drain current Id versus drain-source voltage Vds, parametrized by gate-source voltage Vgs.

Key Concept

Cutoff Region

Vgs is below threshold. The channel is closed and Id is essentially zero (just leakage). The transistor is OFF.

Key Concept

Linear (Triode) Region

Vgs > Vth and Vds is small. The channel acts like a voltage-controlled resistor — Id rises roughly linearly with Vds. Used when transistors stand in for analog resistors or small-signal switches.

Key Concept

Saturation Region

Vgs > Vth and Vds is large enough (Vds ≥ Vgs − Vth). The channel 'pinches off' near the drain and Id stops rising — it saturates at a value set mostly by Vgs. This is the region used for digital switching and analog amplification.

Key Concept

Drain Current (Id)

The current flowing from drain to source through the channel. The number you ultimately care about — it tells you how strongly the transistor 'pushes' the next stage.

Diagram · Interactive I-V explorer

interactive

Vgs: 1.00 V

Vgs 0 V (cutoff)Vth = 0.5 V1.5 V

Drag the Vgs slider to redraw the curve. The shaded zones show linear vs saturation regions. Hover anywhere to read exact (Vds, Id) values.

Checkpoint · +5 XP

In the saturation region, what happens to Id as Vds increases?

Why the I-V curve is the fingerprint. From a single family of curves you can extract: threshold voltage, saturation current (drive strength), output resistance (analog gain), transconductance (gm), and even subthreshold swing if you plot it on a log scale. Two transistors with the same node and same Vdd can have wildly different I-V curves — that's why benchmarking matters.

Try It Yourself

Run a real I-V sweep

Open the Test Lab, run an I-V Characteristic Sweep on a Silicon planar FET, then on a GaN Crystal-EM hybrid. Compare the curve shapes — is the GaN saturation current higher? Sharper transition?

Open Test Lab

Lesson Summary

An I-V curve plots drain current (Id) against drain voltage (Vds) for a fixed gate voltage (Vgs).
Three regions: cutoff (no current), linear (Id rises with Vds), saturation (Id flattens — channel pinched off).
Run the sweep at several Vgs values and you get a family of curves — a transistor's fingerprint.
From an I-V curve you can read transconductance, output resistance, and the saturation current — everything that matters.

Test Your Knowledge · +50 XP

What does an I-V curve plot?

In the cutoff region:

The boundary between linear and saturation occurs roughly at:

Why is saturation the region used for digital switching?

Increasing Vgs (above Vth) does what to the saturation current?