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Design >> Analog Design >> body effect
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Message started by abhilash_172 on May 12^th, 2017, 5:03am

Title: body effect
Post by abhilash_172 on May 12^th, 2017, 5:03am

Hi all,

i have a doubt related to body effect in nmos, i am using 0.18um models , i have given Vds = 1.8v and Vgs = 0.7v and i varied body voltage Vb from -10 to 1v .
As Vb is increased vth decreases ,id increases and as far as vb<0 body source junction is reverse biased,so is = id.
what happens as vb > 0 ? body source junction is forward biased and how id will be changed?

Below is the graph of id vs vbs i got

Title: Re: body effect
Post by Maks on May 13^th, 2017, 11:48pm

For Vbs until ~+0.7 V, everything goes as expected - Vt goes down, Id goes up.

After ~0.7V, you are strongly forward biasing source/body diode, and device goes into a crazy non-linear very high current regime. I am not surprised seeing Id going down - it's quite likely that the potential barrier for carriers to go form source to body/substrate becomes lower than the barrier to go into channel, so carriers are redirected from drain to bulk, hence reducing Id. There is no much meaning into biasing the device into this regime, even SPICE models may not be accurate here.

Max

Title: Re: body effect
Post by abhilash_172 on May 14^th, 2017, 11:25pm

okay it makes sense,since source body junction is forward biased,only some electrons reach the drain end compared to earlier case,hence current is less.
we can also neglect hole current since source is heavily doped compared to body

Title: Re: body effect
Post by AnilReddy on May 17^th, 2017, 5:47am

Hi Abhilash,

One question, the graph you provided is a drain current graph vs the body voltage..right?

As you go more negative on the body voltage, i think the current should increase because Vt drops, right?

Please clarify...