N-channel MOSFET

Part 5 · Voltage Regulation

Key point: past threshold an inversion channel forms. Raise \(V_{\mathrm{DS}}\) to \(V_{\mathrm{GS}}-V_{\mathrm{th}}\) and it pinches off. That flat \(I_D\) is MOSFET saturation. A switch that is on sits at small \(V_{\mathrm{DS}}\), in triode.

Source is 0 V. \(k = 1~\mathrm{A/V^2}\), \(V_{\mathrm{th}} = 1.50~\mathrm{V}\). Saturation is drawn flat.

1
Below threshold, no channel.
\(V_{\mathrm{GS}} = 1.00~\mathrm{V}\). \(V_{\mathrm{ov}} = -0.50~\mathrm{V}\). \(I_D = 0.00~\mathrm{mA}\).
2
Small \(V_{\mathrm{DS}}\) is triode.
\(V_{\mathrm{GS}} = 5.00~\mathrm{V}\), \(V_{\mathrm{DS}} = 0.60~\mathrm{V}\). \(V_{\mathrm{ov}} = V_{\mathrm{DS,sat}} = 3.50~\mathrm{V}\). \(I_D = 1.92~\mathrm{A}\). The channel still reaches the drain.
3
Pinch-off starts at \(V_{\mathrm{DS,sat}}\).
Same gate. \(V_{\mathrm{DS}} = 3.50~\mathrm{V}\), right on \(V_{\mathrm{ov}}\). \(I_D = 6.13~\mathrm{A}\). Left of this line on the family is triode. Right is saturation.
4
Further \(V_{\mathrm{DS}}\) does not raise \(I_D\).
\(V_{\mathrm{DS}} = 8.00~\mathrm{V}\). \(I_D\) is still 6.13 A. \(V_{\mathrm{DS}}/I_D = 1.31~\Omega\). That ratio is not \(R_{\mathrm{DS(on)}}\). \(R_{\mathrm{DS(on)}}\) is the deep-triode slope.
5
The body diode is already in the silicon.
Gate at 0 V, channel off, \(I_D = 0.00~\mathrm{mA}\). The p-body and n+ drain are a diode: anode at the source, cathode at the drain. This lab does not drive \(V_{\mathrm{DS}}\) negative, so that diode stays off.

Level-1 square law. The silicon cut is a cartoon. Saturation here is pinch-off, not a BJT “active” region and not a switch that is fully on.

Bias the N-FET

Source at 0 V · gold dot is this bias

Cutoff —

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

Symbol · body diode anode at S

\(I_D\) vs \(V_{\mathrm{DS}}\) · gold dot is this bias

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

Silicon · inversion sheet, depletion, body diode · not to scale

Regime—

\(V_{\mathrm{ov}}\)—

\(V_{\mathrm{DS,sat}}\)—

\(I_D\)—

\(V_{\mathrm{DS}}/I_D\)—

\(V_{\mathrm{ov}} = V_{\mathrm{GS}}-V_{\mathrm{th}}\)

→ —

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

→ —

\(I_{D,\mathrm{sat}} = \tfrac12 k V_{\mathrm{ov}}^2\)

→ —