Does the depletion regions touch in a JFET at pinch-off?

[matbob]

Hi everyone,

(all references to an N channel JFET)

The book: 'Solid State Electronics Devices' by Ben G. Streetman says that the depletion regions on both sides in a JFET MEET near the drain as the drain voltage is increased which essentially pinch off the channel(with gate voltage = 0).After this the drain current cannot increase and the electrons from the channel enter the electric field of the depletion region and flow to the drain contact.

Now, the famous book: 'Integrated Electronics: Analog and Digital Circuits and Systems' by Millman and Halkias says that it is not possible to completely pinch-off the channel (ie, the depletion regions cannot touch), since the drain current cannot go to zero which would result in the removal of the ohmic drop along the channel which is required for pinch-off. So a dynamic equilibrium is present.

Which one is right? Please help!

Also please tell me whether the depletion regions can touch when Vds=0 (drain to source voltage = 0) and gate voltage is increased to pinch-off voltage Vp.

 

[flatulent]

They are both correct in their own way. The stream of charge carriers through the depletion region is a conducting effect itself. One way to look at this is there is a voltage gradient across the part of the channel that is not pinched off. This accelerates the charge carriers to a mechanical velocity. They continue with this velocity through the depletion region and arrive at the drain. This same acceleration occurs at the base emitter region of a BJT.

 

[rituparna dhar]

@ flatulant..but if the depletion regions touch each other then how the electrons can flow?

 

[erikl]

how the electrons can flow?

A FET source can inject charge carriers either via a (perhaps gate-generated) channel, or even directly into a depletion region.

A depletion region (in static mode) is depleted of charge carriers, because there is an electric field which draws (and accelerates) charge carriers out of this region -- either thermally or radiation (light) generated ones in the depletion region, or somehow introduced into it from outside of the depletion region (s. below). Such stream of charge carriers caused by an electric field is called drift current.

If charge carriers are introduced into the depletion zone -- either thermally generated charge carriers outside of the depletion region, which get into it by diffusion: this is called diffusion current -- or by injection (via emitter, source or channel) -- this is called injection current. Once in the depletion region, all these charge carriers also get accelerated and then contribute to the drift current.