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What's the physical meaning of Vdsat in Spectre?

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hebu

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We can see the parameter Vdsat in Cadence Spectre after we perform the
DC simulation. But I can't figure out what's the physical meaning of it?
I guess the vdsat is the overdrive voltage at first, but it's not exact the same
as Vgs-Vth. The Vdsat is smaller than Vgs-Vth. So anyone has the idea what is
Vdsat?
 

spectre vdsat

hebu said:
We can see the parameter Vdsat in Cadence Spectre after we perform the
DC simulation. But I can't figure out what's the physical meaning of it?
I guess the vdsat is the overdrive voltage at first, but it's not exact the same
as Vgs-Vth. The Vdsat is smaller than Vgs-Vth. So anyone has the idea what is
Vdsat?
Click to expand...
Comparting Vds with Vdsat you can easily determine is your DC biasing OK.
If Vds>Vdsat mos works in saturation
If Vds<Vdsat mos works in triode region
 
vdsat nmos

pixel said:
hebu said:
We can see the parameter Vdsat in Cadence Spectre after we perform the
DC simulation. But I can't figure out what's the physical meaning of it?
I guess the vdsat is the overdrive voltage at first, but it's not exact the same
as Vgs-Vth. The Vdsat is smaller than Vgs-Vth. So anyone has the idea what is
Vdsat?
Click to expand...
Comparting Vds with Vdsat you can easily determine is your DC biasing OK.
If Vds>Vdsat mos works in saturation
If Vds<Vdsat mos works in triode region
Click to expand...

Why not vds>vgs-vth ? How to decide the vdsat?
 

vdsat spectre

hebu said:
Why not vds>vgs-vth ? How to decide the vdsat?
Click to expand...
I dont know, but when I want to see does circuit work well I look at Vdsat. Until now I did not have any problems. I suspect that equation Vgs-Vth is maybe first order approximation?
 

vds vdsat

I think Vdsat is the voltage at which the carriers travel with saturated velocty not VGT...
 

vdsat linearity

hebu said:
We can see the parameter Vdsat in Cadence Spectre after we perform the
DC simulation. But I can't figure out what's the physical meaning of it?
I guess the vdsat is the overdrive voltage at first, but it's not exact the same
as Vgs-Vth. The Vdsat is smaller than Vgs-Vth. So anyone has the idea what is
Vdsat?
Click to expand...


Hi, did you get the Vdsat using Spectre command line?
In hspice ,Vdsat≠Vgs-Vth,too.There is equations for Vdsat in Spice model files.
 

region 3 but vds < vdsat

for short channel devices,you should make some modification of Vdsat.

Vdsat=(Vgs-Vt)//(L*Esat)

Esat is the saturate Electircal field of device,you can get it from model parameter Vsat=u*Esat/2
 

    V

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vds versus vdsat

Hi,
Vdsat is the minimum voltage that is required to keep the transistor in saturation.

In analog design it is a important parameter. Because if the Vds is less then Vdssat then it can cause the transistor to go into triode region, and the linearity and other imp parameters changes.

thanks
sarfraz
 

nmos vdsat derivation

I think Vdsat>Vgs-Vth,in long length MOS,Vds>Vgs-vth is in saturation area,
But the practical and even short length MOS, must Vds>Vgs-vth+Δ ,so Vdsat=
Vgs-vth+Δ,than you can get saturation area.
 

overdrive voltage vdsat

Why Vdssat not equal to Vgs - Vth ?
 
relation between vdsat and vds

Vds = vgs -vth and the conditions what Pixel stated are right to analyse the operating state of Transistor.
 

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