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heaton New Member Offline Posts: 2	meaning of vdsat Nov 29^th, 2004, 1:10pm guys, someone told me that vdsat is nothing but vgs-vth. however, my dc operating point report of a 0.18u nmos shows that vgs=515.5m, vth=492.9m, where vdsat=81m, a lot larger than vgs-vth. i'm really confused, any advice would be appreciated. thanks.
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extractor_man Guest	Re: meaning of vdsat Reply #1 - Nov 29^th, 2004, 10:57pm Vdsat is your maximum drain source voltage A value limited mainly by hot carrier injection in short channel MOS transistor.
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crhu

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Posts: 11

Re: meaning of vdsat
Reply #2 - Dec 21^st, 2004, 4:04pm

Hi, heaton,
What region does your transistor work? You need to
make sure that the transistor works at staturation/pinch
off region.

Vdsat, (aka veff (effective voltage) and delta v), is used to
check if the transistor works at saturation.

Vdsat is calculated with Ids and w/l, sqrt((2*Ids)/(kp*w/l)).
You can derive it from the saturaton current equation w/o channel length
modulation:
Ids = kp/2*(w/l)*(Vgs-vth)^2
Vdsat = vgs -vth @ saturation

If vgs > vth, and vds > vdsat, the transistor is in saturation.
In this case, vdsat=vgs-vth if channel length modulation
is omitted.
If vgs > vth, and vds < vdsat, in linear region.
In this case, vdsat != vgs-vth, because in linear
region there is no square law bt. Ids and (Vgs-vth).

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heaton New Member Offline Posts: 2	Re: meaning of vdsat Reply #3 - Dec 24^th, 2004, 11:08am crhu, My vds is around 400mv, larger than vdsat.
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crhu Junior Member Offline I come, I see, and I go Posts: 11	Re: meaning of vdsat Reply #4 - Dec 27^th, 2004, 6:04pm Hi, heaton, May I know which simulator you are using, Hspice or Spectre? If you can upload your netlist, I can have a look into it. Thanks. --Charlie
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Geoffrey_Coram

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Re: meaning of vdsat
Reply #5 - Jan 10^th, 2005, 2:46pm

I believe Vdsat = vgs - vth is an approximation from a time of much simpler MOS models. A glance at the BSIM3 source code shows

Vdsat = EsatL * Vgst2Vtm / (Abulk * EsatL + Vgst2Vtm)

where Vgst2Vtm = (vgs - vth) - 2 * (thermal voltage), EsatL is computed from the VSAT parameter and the effective length, and Abulk has some other dependencies.

Even in MOS level 3, we have vdsat = (vgsx-vth)/Fbody, so the body effect can change vdsat from the simple approximation.

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crhu

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Re: meaning of vdsat
Reply #6 - Jan 11^th, 2005, 1:26pm

Hi,Geoffrey and Keaton,
I went through the mosfet manuals (BSIM3v3 and BSIM4v3)
and you are right that vdsat is not equal to (vgs-vt)
since level3 as small geometry is considered.

I checked it too in a testcase and it shows that
vdsat != vgs -vth
//file: hspice.lis
...
element 0:m1 0:m2 0:m3 0:m4 0:m5
model 0:nch 0:nch 0:pch 0:pch 0:nch
region Saturati Saturati Saturati Saturati Saturati
id 1.2300u 1.2300u -1.2300u -1.2300u 2.4601u
ibs 0. 0. 0. 0. 0.
ibd 0. 0. 0. 0. 0.
vgs 632.2317m 632.2317m -792.4483m -792.4483m 600.0000m
vds 739.7834m 739.7834m -792.4483m -792.4483m 267.7683m
vbs 0. 0. 0. 0. 0.
vth 622.6295m 622.6295m -676.6599m -676.6599m 586.9457m
vdsat 46.5314m 46.5314m -87.5725m -87.5725m 47.4368m
...

Thanks.

--charlie

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yilmazerc New Member Offline Posts: 2	Re: meaning of vdsat Reply #7 - Jan 12^th, 2005, 6:14am hi all i think ican add something physical vdsat=vgs-vth is valid for long channel devices vdsat is smaller for short channells because of the velocity saturation due to the high electric field along the channel
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