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Introduction

2. A MOSFET in Saturation or as a Triode.

Charge carriers in MOSFET originate in Source, flow through Channel and into Drain. The total amount of charge that flows depends on how much charge is injected into channel from the Source. This is controlled by the Gate-Source voltage, Vgs.  The drain current Id may or may not depend on the overall voltage drop between Source and Drain. This applet shows a visual simulation of the MOS transistor characteristics as controlled by the Gate-Source bias, Vgs, and the Gate-Drain bias, Vgd.

First, this applet demonstrates the facts that the inversion channel at the Source-end is controlled by Vgs, and at the Drain-end by Vgd.  For an n-channel MOSFET, the inversion channel is present at the Source-end of channel if Vgs > Vt, and is present at the Drain-end of channel if Vgd > Vt. Use the arrows next to these voltages in the applet and observe that the blue n-channel is controlled at either end of the channel by these two voltages, respectively.

n-MOSFET: (1) Triode If Vgs > Vt and Vgd > Vt, then the n-channel is continuous all the way from S to D. The S and D are connected by a conductor (or a resistor) of a given resistance. The drain current increases if the voltage drop between S and D increases. The channel resistance depends on how much charge is injected at the S-end, which in turn is controlled by Vgs. The Drain current Id depends on both Vgs and Vgd (or Vds), and thus we call thgis region of operation a Triode.  (2) Saturation If Vgs > Vt and Vgd < Vt, then n-channel is presen (or induced) at the S-end, but the channel is depleted at the D-end. That is, the n-channel is pinched off at the Drain-end. What happens to all the electrons injected into channel from Source ? Think about it. [answer]. Once the drain-end of channel is pinched off, the current no longer depends on the voltage drop between S and D (actually there is a small dependence of Id on Vds. Why ? What is this phenomenon called ? answer).  (3) Cutoff  If Vgs < Vt (and of course, Vgd < Vt), then the no n-channel is present and no current flows.

p-MOSFET: The same discussion applies as in the NMOS case except that the inequality reverses between Vgs (and Vgd) and Vt.