Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

[SOLVED] Effect of body effect on Drain current

Status
Not open for further replies.
iVenky

iVenky

Advanced Member level 2
Joined
Jul 11, 2011
Messages
584
Helped
37
Reputation
76
Reaction score
35
Trophy points
1,318
Location
College Station, Texas
Activity points
6,124
I have one simple doubt. If Vsb is more than 0 in NMOS then we have body effect which results in the increase of threshold voltage Vth. This means that the drain current should decrease (from the drain current equation) but I read in some book that the drain current will increase.

Please help me.

Thanks in advance.
 

Due to the larger Vsb, the depletion region between the channel and the substrate becomes wider therefore more immobile charge (left behind when the mobile carriers are repelled) becomes uncovered. This increases the current in the channel.
 
  • Like
Reactions: erikl

    erikl

    erikl

    Points: 2
    Helpful Answer Positive Rating
Yes thats correct, Due to body effect source impedance decreases and leads to increase in Drain current. As told by JoannesPaulus, immobile charge trapped in bulk depetion region add up to drain current.
 

When you change Vsb you change Vdb simultaneously and
you will elevate the drain field and leakage / impact ionization.
Neither of these pertain to the channel and its (effective) VT.
You are seeing non-gate-controlled leakage.
 

varunkant2k said:
... immobile charge trapped in bulk depletion region add up to drain current.
Click to expand...
... isn't just a sensible or lucky explanation :roll: SCNR!

But I was already able to understand JoannesPaulus' explanation :smile:
 

varunkant2k said:
"... Drain current .... increased due to immobile cahrge in the depletion region"
What is effect of Vth then? Is should reduce drain current?
I know that, back gate effect increase the trance conductance looked at source.
Expect if you can elaborate your point.
Click to expand...

First re-read JoannesPaulus' explanation above! And then:

"more immobile charge (left behind when the mobile carriers are repelled) becomes uncovered." This means the repelled mobile carriers (which have left the fixed=immobile dopant atoms) contribute to the drain current as additional body effect. And this, again, means that -- with the help of this body effect Vsb>0 -- for comparable drain current you need a lower Vgs than without this effect (i.e. Vsb=0), which seems to effectively lower Vth.

BUT KEEP IN MIND: This additional current doesn't emerge from the source, but will be delivered by the substrate=backgate.
And you trade the decrease of Vgs (for same drain current) against an increase of Vgb !

Can't explain it better, sorry!
 

Unfortunately, I need to rephrase my lost post.

We are talking a about Vsb > 0, or reverse biased source-bulk junction. A already stated, increasing the bias voltage increases Vth. Also Rdson is increased according to datasheets. But in which region of transistor operation do you expect an increase of drain current with increasing Vsb?

I can contribute an output characteristic for varying Vsb that contradicts this assumption. It's for a depletion mode transistor, but there shouldn't be a principal difference.

 

I would expect it for a transistor operating near its
Vds(max) in the "off" condition - where the drain current
is not entirely defined by the channel subthreshold
conduction.

The chart looks like it's for an older thick oxide long
channel FET. If you see a visible "curl" in the Id-Vd
curves (as is common at short channel) you can bet
that more D-B reverse bias isn't going to be helpful.
 

I would expect it for a transistor operating near its
Vds(max) in the "off" condition - where the drain current
is not entirely defined by the channel subthreshold
conduction.
Click to expand...

I rather assume it's the normal behaviour of a basic MOSFET. In a visual explanation, Vbs is working in the same direction as Vgs. Making the bulk or gate more negative relative to source reduces Id. This behaviour is also suggested by the Level 1 MOS equations, and confirmed by own MOSFETs measurements with separate bulk terminal.

So as a simple question, under which conditions do you see an opposite behaviour?

P.S.: I possibly misunderstood your quoted post. Did you mean the "regular" (in my view) behaviour according to the shown curve or the opposite discussed in previous posts?
 
Last edited:

...If you see a visible "curl" in the Id-Vd curves (as is common at short channel) you can bet that more D-B reverse bias isn't going to be helpful.
Click to expand...
With Vsb>0, We always see a reduction in Drain current, where Id = gm*Vgs+gmb*Vbs. This response can be verified by increased Vth.
I have attached another image ( FVM already attached one image) shows clearly this response.
ID vs Vbs.JPG
 

JoannesPaulus said:
Due to the larger Vsb, the depletion region between the channel and the substrate becomes wider therefore more immobile charge (left behind when the mobile carriers are repelled) becomes uncovered. This increases the current in the channel.
Click to expand...

I could not understand how immobile charge uncovered increases current in the channel. Can someone explain this how current is assisted by immobile ions? Does it help in increasing the minority carriers which are mobile because ultimately the current is established by minority carriers?
 

soumendu89 said:
I could not understand how immobile charge uncovered increases current in the channel. Can someone explain this how current is assisted by immobile ions? Does it help in increasing the minority carriers which are mobile because ultimately the current is established by minority carriers?
Click to expand...

I think this only concerns the leakage current from bulk to the channel: For VSB>0 the space charge region extends ~√VSB , and so increases the channel leakage current. This has nothing to do with the bulk to channel control ("back gate control"), which reduces the drain current with increasing VSB.
 
Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top