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[SOLVED] Why vdsat is not equal to vgs-vth?

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pansoo

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hspice vdsat

I have simulated some circuits using Hspice.
But seeing the list file, vdsat is not equal to vgs-vth like below.


element 1:mn4 1:mn6
model 0:n.23 0:n.23
id 596.5613u 596.5613u
ibs -4.591e-18 -2.945e-20
ibd -1.1578u -4.532e-18
vgs 957.6501m 876.6040m
vds 2.5404 342.4499m
vbs -342.4499m 0.
vth 726.1315m 635.6738m
vdsat 191.1505m 195.5912m
beta 26.7565m 25.1268m
gam eff 559.7876m 576.1277m
gm 4.9592m 4.7878m
gds 20.6280u 42.1656u
gmb 1.2493m 1.4630m
cdtot 143.6392f 180.0130f
cgtot 407.4155f 412.1034f
cstot 443.0402f 471.4738f
cbtot 429.4820f 505.9530f
cgs 342.3662f 341.3931f
cgd 20.3584f 20.3584f


Is anyone knows that?
 

gte582w

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vg_s vth

because vdsat is not equal to vgs-vth
especially for short-channel devices
 

tarun_taurus

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vdsat vds(sat)

I guess generally,they are v.close....
the problem is you need to get a point where your switching of liner to sat. is happening and accurate W/L for that may not be applicable in spice/layout.
Also vdsat doesn't always remain vgs-vth due to lamda effect..., it starts increasing linearly!
 

pansoo

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vdsat always less than vgs-vth

I knows that vth is change, then vdsat is also changed.

But isn't it true that vdsat is definition of vgs-vth?

Thus I think vdsat equal to vgs-vth at the time of op point.
 

raduga_in

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vdsat hspice

Please check with the formula of the Vdsat in the manual
of the BSIM Model you are using..
 

pansoo

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formula for vdsat

ok. thanks. I knows that. :D

Oh! so complex vdsat --;;
 

fanrong

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vd sat formula

In hspice simulation result report file , u can see vdssat and vgs .vds, Vdssat =Vgs-vth , but vds is the real value . For mos working in saturate region , vds must
greater than vdssat
 

vivek_raj_verma

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vgs-vth

hi there,

see the equation of Vdsat in ur models. Also i wnat to know about above values.. at what point u got it. r they results of any simulaton
 

wszywps

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vdsat formula

Vdsat=Vgs-Vth is the begin of working in saturate region
 

Vamsi Mocherla

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Most important thing people are forgetting for vds,sat not equal to VGS-VTH is because of two things.

1. Velocity Saturation
2. Hortizontal field due to large VDS in short channel transistors.

Unfortunately, it is not because of channel length modulation.

Please refer to any standard circuits book or the BSIM manual.
 

allennlowaton

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i guess this is not a big problem..
as long as the Vds is larger than (Vgs-Vth)..which keeps the transistor
in saturation region...
 

rfsystem

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I could actual not refer to the equations but the vdsat is defined from the strong inversion to the subthreshold range. The first model equation known to me was the EKV model which use a single equation for both ranges. There was also the defintion of the vdsat.

In subthreshold the lower limit is Vdsat=m*Vt where m is the subthreshold slope.
 

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