When the gate voltage (-vgs) is made more negative, the channel width decreases until no more current flows between channels and the source is said to be pinched (similar to cut-offregion for bjt). the voltage at which the drain closes is called the pinch-off voltage, (vp).
Pinch-off is not closed. during the pinch-off operation mode, the discharge current does not decrease to zero. otherwise, the id current becomes constant, remains relatively independent of the voltage vds. The pinch-off mode of the fet transistor is similar to the linear operating region of the bjt transistor.
With the usual common-source hook of the n-channel jfet, when the gate voltage of the vgs becomes more and more negative, the channel becomes narrower as the depletion zone moves into the channel from the side. eventually this depletion zone meets. but the channel is not closed! instead, the channel becomes part of the length of the constant width.
in this channel, the landslide damage occurs when a small part of the channel tries to close. but every time this happens, a larger voltage appears in the closed part, which pushes back the dzs, and opens the channel again.
As long as the channel’s pinch-off mode behaves very strangely: it’s no longer a resistance. instead, when the source-voltage voltage of the duct increases, the conductive channel grows physically longer! This is a magical resistor, a resistor that tries to maintain a constant current even with the changed voltage placed on it.
Simply put, a fet in pinch-off mode is a voltage-controlled constant current source, while a fet outside the pinch-off is a voltage-controlled resistor. for example, we can use fets as variable resistor, to act as audio volume control or analog potentiometer. we do this by keeping the vgs value low, so the channel remains wide open and does not go into pinch-off mode.
If we examine the series of operating curves for a fet, the id vds graph, the pinch-off mode is in the region where the id curves have flat tops, or they slope gently upward with a large increase in voltage vds.
So, if this is pinch-off, then what is pinched closed? it’s at the bottom of the family curve, on the big values of vgs, where the channel is actually closed, and id is zero at all vds value.
Finally, where does the misconception about pinch-closed versus pinchoff mode come from? maybe it’s from the fact that, when the pinch-off starts, the ac resistance of the drain-source channel will skyrocket. it becomes very large, ideally unlimited (which causes the curve v / i to be flat on it.) but this is not dc resistance.
there may be amperes flowing in your mosfet power-drain circuit, even when the channel resistance is very large. that’s the main feature of any constant current source. has a significant current, while for ac and dynamic changes, it behaves as an open circuit. but, ohms and unlimited open circuits does not mean zero current where the current sources are related. and so, when the vgs becomes large and the channel becomes the current source, it does not pinch closed. otherwise it’s all weird! entering pinch-off operation mode.
