N-channel MOSFET · Channel & Curves — Voltage Regulation

Along the way

1
Three views, one bias. Symbol, silicon, and the \(I_D\)–\(V_{\mathrm{DS}}\) family all show the same voltages. Move a slider — all three update.
2
Threshold opens the channel. Source is 0 V. When \(V_{\mathrm{GS}}\) exceeds \(V_{\mathrm{th}}\), electrons (drawn as −) collect under the oxide — inversion charge \(Q_n\). Below threshold: cutoff, almost no channel current.
3
Triode (linear). Small \(V_{\mathrm{DS}}\): the channel runs source to drain but thins toward the drain. \(I_D\) rises with \(V_{\mathrm{DS}}\). The gray hatch under the channel is depletion — mobile holes are gone. Current rides in the thin teal sheet above it, not throughout the hatch.
4
Saturation is pinch-off. Large \(V_{\mathrm{DS}}\): \(Q_n\) goes to ~0 at the drain end. Electrons that arrived in the inversion sheet are swept across the drain-end depletion to n⁺ D; \(I_D\) barely grows. Some books call this “active” — not BJT saturation, and not a switch fully on.
5
The boundary. Pinch-off starts near \(V_{\mathrm{DS,sat}} \approx V_{\mathrm{GS}} - V_{\mathrm{th}}\). Left of that line on the family: triode. Right: saturation.
6
Body diode is built in. The p-type body and n⁺ drain are a PN junction you did not add as a part. On a 3-terminal FET the body is tied to the source, so the diode’s anode is at S and cathode is at D. Gate off and drain below source → that diode conducts.

Source stays at 0 V. Saturation is drawn perfectly flat. The silicon cut is a cartoon, not a process cross-section.

Lab · Bias the N-FET · watch the channel

Source at 0 V · gold dot is the operating point

Cutoff —

\(V_{\mathrm{GS}}\) (V) \(V_{\mathrm{DS}}\) (V) \(V_{\mathrm{th}}\) (V) \(k\) (A/V²)

Symbol · enhancement NMOS + body diode

\(I_D\) vs \(V_{\mathrm{DS}}\) · family · gold = you

\(I_D\) vs \(V_{\mathrm{GS}}\) · at your \(V_{\mathrm{DS}}\)

Silicon · inversion, depletion, body diode (not to scale)

Regime—

\(V_{\mathrm{ov}}\)—

\(V_{\mathrm{DS,sat}}\)—

\(I_D\)—

\(V_{\mathrm{DS}}/I_D\)—

\[ V_{\mathrm{ov}} = V_{\mathrm{GS}} - V_{\mathrm{th}},\quad V_{\mathrm{DS,sat}} = \max(V_{\mathrm{ov}},0) \]

→ —

\[ I_{D,\mathrm{lin}} = k\bigl[V_{\mathrm{ov}} V_{\mathrm{DS}} - V_{\mathrm{DS}}^2/2\bigr] \]

→ —

\[ I_{D,\mathrm{sat}} = \tfrac12 k\, V_{\mathrm{ov}}^2 \quad (V_{\mathrm{DS}} \ge V_{\mathrm{DS,sat}}) \]

→ —

\(k\) is lumped strength (\(\mu C_{\mathrm{ox}} W/L\)). \(V_{\mathrm{DS}}/I_D\) is just that ratio — it is not \(R_{\mathrm{DS(on)}}\) except in deep triode. Saturation is drawn perfectly flat; real parts slope a little.

Try it: (1) Cutoff — \(V_{\mathrm{GS}}=1\,\mathrm{V}\) below threshold; no channel. (2) Triode — \(V_{\mathrm{GS}}=5\,\mathrm{V}\), \(V_{\mathrm{DS}}=0.5\,\mathrm{V}\); continuous channel, \(I_D\) climbs with \(V_{\mathrm{DS}}\). (3) Saturation — keep \(V_{\mathrm{GS}}=5\,\mathrm{V}\), raise \(V_{\mathrm{DS}}\) past \(V_{\mathrm{ov}}\); watch pinch-off on the drain side and the flat part of the curve. (4) Sweep \(V_{\mathrm{GS}}\) on the right plot — threshold is where current takes off.

Analog Intuition

N-channel MOSFET · channel & curves

Lab · Bias the N-FET · watch the channel