MOSFETS and pn junctions in substrate / source and drain

[mofet]

Hi


I attached a sheet that shows my doubts about MOSFETs.
It is used to say that to body doesn't matter that much in MOSFET analysis. I do not know why is this.
Later, in some books they refer the effect body (I didn't read yet about it, but must be for very accurate models. It seems (?) that body effects must be minor.. only being important in critical applications).

But my doubts are:
1 - If we apply a positive voltage in DRAIN (Vds) that has n+ (plenty of electrons ) is this turns out the n+ region in one p-region ?

2 - Why connecting a substrate(=body) terminal to the source terminal put the pn junctions reverse-biased!?! (see the sheet attached, please). And in this way they told that the body doesn't matter much in operation device??

3 - If really Vgs is turning out more positive and positive... it will pull down all holes to the base of the substrate and it has no anymore carriers - (no free electrons neither holes) causing a depletion layer.... the positive charges (from Vgs positive) will attract the negative charges to the other plater under gate... these negative charges comes from n+ drain and n+ source regions... forming a n-channel. But if this is so... how electrons then can travel in an one negative channel if they would be repealed again to the source (electrons will come from source to the drain in the current - provided that Vds is positive)? Please help me in these questions. If anything is not clear, ask me , I'll try to expose better my doubts.


ps English is not my native language, so sorry for any unclear sentence, or typos.

 

[john blue]

It also depends on whether it is NMOS or PMOS

---------- Post added at 01:14 ---------- Previous post was at 01:12 ----------

as well as it is a normal type or depleted type

 

[mofet]

it is a nMOS and normal type (not depleted).

 

[john blue]

What you mean by is m+ turns out now a p drain? If i am not mistaken, you are asking if i apply +ve there , will it change to p drain? If that's the case, it won't. The n+ is due to the doping concentration as well as types of dopant.

---------- Post added at 14:09 ---------- Previous post was at 13:59 ----------

Why connecting a substrate(=body) terminal to the source terminal put the pn junctions reverse-biased!?! Yup. Based on your drawing, you can see that the +ve part of your diode is connected to ground and your negative part of the drain is connected to supply. Thus, the diode is reverse biased. For the other side, it is not biased.

---------- Post added at 14:11 ---------- Previous post was at 14:09 ----------

And in this way they told that the body doesn't matter much in operation device??

Yes. Now, it doesn't matter much. Your source and the body of the mosfet is having the same potential now. If you dont connect your substate body, it means you are leaving the body at a floating potential which is undetermined.

---------- Post added at 14:15 ---------- Previous post was at 14:11 ----------

Drain current is enhanced when body is not grounded

---------- Post added at 14:20 ---------- Previous post was at 14:15 ----------

Because of the difference of potential at the Drain and Source, electrons will flow

 

[mofet]

What you mean by is m+ turns out now a p drain? If i am not mistaken, you are asking if i apply +ve there , will it change to p drain? If that's the case, it won't. The n+ is due to the doping concentration as well as types of dopant.

It is n+ not m+. And yes, I was telling that if we apply a positive voltage (+ve) in the drain it would ch
ange the n-drain to a p-drain... And you told that it is nor true... I thought that to bias a diode it would be enough to apply a voltage as it happens with Vds in the drain... Being positive it would turn out a p-region now...
So we could have now a diode with forward bias in which the other extremity is pointing to the n-induced channel... And finally could enable current from drain to source (not talking about the real movement of electrons).

Yes. Now, it doesn't matter much. Your source and the body of the mosfet is having the same potential now. If you dont connect your substate body, it means you are leaving the body at a floating potential which is undetermined.

Got it, thanks.

[/COLOR]Drain current is enhanced when body is not grounded

Didn't know about this. So in reality the body is usually not grounded, correct?

Because of the difference of potential at the Drain and Source, electrons will flow

I know. But with a n-channel induced, I thought it could repeal the electrons coming from source (same charges...). (talking now about the direction of the real movement of electrons and not the conventional current = from drain to source). Maybe the n-channel itself is the "running" corridor of electrons.

 

[john blue]

Didn't know about this. So in reality the body is usually not grounded, correct? Depends, nowadays it is grounded using metal contact during fabrication

---------- Post added at 14:48 ---------- Previous post was at 14:47 ----------

Maybe the n-channel itself is the "running" corridor of electrons. Yup it is