Confusion over DIBL and Body Effect

[diarmuid]

Hello,

Put simply:

- Body effect extends the depletion region around the source.
- DIBL effect extends the depletion region around the drain.

!!! BUT !!!

- Body effect increase Vth whilst DIBL reduces it.

How does this happen since both DIBL and Body effects seem to do the same thing i.e. increase depletion regions???

Thanks,

Diarmuid

 

[Dominik Przyborowski]

DIBL is related to longitudinal electric fields, while body effect to source-bulk voltage. It's two different physical phenomena.

 

[diarmuid]

Thanks Dominik.

My understanding of DIBL is the depletion region around the drain extends with increasing Vd. For small channel devices this significantly reduces the area needed to be
depleted by Vg hence lowering Vth.

My understanding of body effect is the depletion region around the source increases with increasing Vs. Would this also not reduce the threshold voltage?

Thanks,

 

[dick_freebird]

You seem to have the sign reversed. Negative body voltage
(on NMOS) relative to source increases reverse-bias and
increases VT(eff).

 

[diarmuid]

Ok so here is my understanding (wrt NMOS):

Body effect - Increased Vsg increases the depletion region formed between the channel and the bulk. This increases the fixed -ve charge in the channel which
in turn increases the Vth.

DIBL - Increased Vds extends the drain bulk depletion further into the channel. For small channel devices this significantly reduces the channel needed to be depleted
hence reducing Vth.

Or put it another way:

Both effects extend depletion regions. But ...

Body effect extends the depletion region vertically where as DIBL extends it more laterally.

Does this sound correct?

 

[srikanth1986]

I've been wondering about the same "paradox", Diarmuid. And I have not come across any satisfactory explanation that explains it in terms of depletion region physics. I think it might take an explanation in terms of energy/band diagrams to clarify this seeming contradiction.

One difference between the phenomena of Body Effect and DIBL I can think of, however, is this (consider an NMOS):

Body Effect, like DIBL, widens the depletion region under the gate. But it also has the effect of impeding the passage of electrons from the source to the inversion channel under the gate because of the increased depletion region. This would translate to higher gate bias to induce the same channel charge. This problem, on the other hand, is absent in the case of DIBL since the electrons need not be pulled into the inversion layer from the drain for an NMOS since the required charge flow is always from source-to-drain.

This is just a guess. Corrections are welcome.

Ok so here is my understanding (wrt NMOS):

Body effect - Increased Vsg increases the depletion region formed between the channel and the bulk. This increases the fixed -ve charge in the channel which
in turn increases the Vth.

DIBL - Increased Vds extends the drain bulk depletion further into the channel. For small channel devices this significantly reduces the channel needed to be depleted
hence reducing Vth.

Or put it another way:

Both effects extend depletion regions. But ...

Body effect extends the depletion region vertically where as DIBL extends it more laterally.

Does this sound correct?

 

[timof]

In fact, there is no paradox.

dick_freebird stated it right.

If, for n-type MOSFET, you apply positive voltage to the drain, Vt is decreased.
The physical mechanism is the lowering of the potential barrier for electrons going from the source to the channel.

Also, the same way, if you apply positive voltage to the body/bulk - Vt is decreased.

If you apply negative (reverse voltage for source/body p-n junction) voltage to the bulk - Vt goes up.

It agrees with the intuitive physics - applying positive voltage to other terminals (with respect to the source of the electrons - transistor source), attracts more electrons from the source, reduced the potential barrier, increases source/drain current, and reduced Vt.