JFET operation: difference between "pinch off by Vgs" and "pinch off by Vds"?

JFET operation: difference between "pinch off by Vgs" and "pinch off by Vds"?

Hi all,

I have a question about JFET operation.
(1) In JFET, Vgs<Vp means channel is pinched off, so JFET is off and no current flows from source to drain.
(2) When Vgs>Vp and Vds>Vgs-Vp, channel is pinched at the drain end, so current is not further increased with increased Vds.
The channel is both pinched off in (1) and (2), but (1) and (2) denote totally different JFET operating condition.
Does anyone know how to explain this?

Thanks~

Re: JFET operation: difference between "pinch off by Vgs" and "pinch off by Vds"?

Terminology

• The first pinch-off that you refer to(Vgs < Vp)a voltage between the gate and source is applied to reverse bias the gate-source pn-junction, thereby widening the depletion layer of this junction,restricting its cross-sectional area of conducting channel.
• The depletion layer is so-called because it is depleted of mobile carriers and so is electrically non-conducting for practical purposes.

• In the second pinch-off that you refer to when,Vds > Vgs - Vp,is when saturation is achieved,and drain to source current can no longer be increased at that Vgs.In this case there is current flowing from drain to source,unlike the previous case.

Re: JFET operation: difference between "pinch off by Vgs" and "pinch off by Vds"?

Hi rahdirs,

Thanks for your answer.
I just don't understand why JFET channel is both pinched off in off condition and saturation condition, but different Vds-Id characteristic.
The only difference seems to be the pinched off location (near gate and near drain).

Re: JFET operation: difference between "pinch off by Vgs" and "pinch off by Vds"?

It is pinched-off location that makes difference in operation.

When Vgs < Vp,no electrons flow from source to drain & there is only a depletion region between source and drain.

When Vgs > Vp & Vds > Vgs - Vp:-

• When Vds = Vgs - Vp, the n-channel pinches off at the drain. With further increase in Vds, (Vds > Vgs - Vp),the pinch-off point moves farther from the drain, shrinking the channel, merging the depletion regions of the drain and channel together.
• But the pinch-off point has a voltage of Vgs - Vthres and still attracts electrons from the source. When the electrons reach the pinch-off point, they are injected into the depletion region between the drain.
• Because the pinch-off voltage remains more or less constant and Vds is subject to increase, the field between them becomes stronger for increasing Vds. The electrons that are injected are accelerated through the depletion region to the drain. Thus, (conventional+) current actually flows from drain to source.

Still,not clear ?????? Then can,you rephrase your question ???

Re: JFET operation: difference between "pinch off by Vgs" and "pinch off by Vds"?

Thank you rahdirs.
Do you mean that the difference is that there is channel in saturation region, but no channel in off region?

Here is my question.
In saturation region (depletion region between drain and channel), electrons (in NJFET) are swept from the pinch off point (at channel) to drain.
In off region (depletion region between drain and source), why electrons are not swept from the pinch off point (near source, there is no channel) to drain?

Re: JFET operation: difference between "pinch off by Vgs" and "pinch off by Vds"?

Yes,the presence & absence of channel.

Are you asking that if electrons can be swept off from pinch-off point to drain in saturation,why can't they be swept off from source to drain in cut-off(Vgs < Vp).

Yes,they can be swept off even with Vgs < Vp,but you would need very high Vds in that case at which the MOSFET breaks down.

Electrons are swept through at a particular value of electric field.(E=V/L).
In saturation,that L is small(from pinch-off point to drain terminal), & hence they can be swept through,but in Vgs < Vp,they have to be swept through the entire channel length L,hence the need for very high Vds.

See in the attached image,you can see current flow from drain to source,for the case Vgs < Vp after the VBR point.

Re: JFET operation: difference between "pinch off by Vgs" and "pinch off by Vds"?

Thank you very much, rahdirs.
You answer exactly what I ask!!

Here comes another question, please.
Punch through occurs when two depletion regions touch each other, and current increases significantly when voltage increases.
When JFET is in off and saturation regions, drain's depletion region touches source's depletion region. Why punch through does not happen?
Thanks~~