Continue to Site

# the differences between subthreshold and saturation area???

Status
Not open for further replies.

#### sunjiao3

##### Member level 5
subthreshold constant current source

Hi, friends. In saturation area of CMOS, Vds≥Vgs-Vth. while in subthreshold

area, usually, Vth-100mv≤Vgs≤Vth+100mv. Thus, someone say that saturatin

doesn't go against subthreshold. Is it right? What are the relationships and

differences between these 2 areas? Do they go against each other? Or do they

overlap some part of each other?

Thank you very much !

Re: the differences between subthreshold and saturation are

In the first part of your question you are talking about Vds after that you are mentioned about Vgs.
Then you try to compare these two different regions and ask about "saturation goes (???) or doesn't go against subthreshold". What does that mean?

Re: the differences between subthreshold and saturation are

In saturation you say that VDS > VGS-VT
In subthreshould, generally
VGS-VT <0

Transistor will be in saturation depending on first eqation. You should look for VDSAT which is no longer VGS-VT in subthreshold.

in addition to the above mentioned mathematical formulas, the conceptual difference is that, in saturation, the MOS acts as a constant current source, where as a in subthreshold, it has exponential characteristics like a BJT

Re: the differences between subthreshold and saturation are

Thank you all very much. Well, all I want to know in this problem is whether the

saturation and subthreshold are 2 mutually exclusive area. That is, a mos

transistor can not work in both subthreshold and saturation areas.

Re: the differences between subthreshold and saturation are

The Mos transistor can not, per definition, be at both saturation and subthreshold on the same time. The physics is different in the 2 cases.
On saturation region, the current flowing between the source and the drain is, mostly, a "drift current" due to the electic field between the source and the drain.
On subtreshold region the current is dominated by the diffusion phenomena (the electric field is weak).
If the Vgs<Vt and Vds is high, you will have a large depletion area in the drain-bulk diode, but you will not form a channel!

### sunjiao3

Points: 2
subthreshold
2(Vgs-Vth)>>Vds
saturation
Vgs-Vth<Vds

Re: the differences between subthreshold and saturation are

Hey guys, most of you should be sent to the starting block!!!!

Saturation means that the drain current saturates to a certain value, independent of Vds. This can be reached in weak (subthreshold), moderate and strong inversion!!!!

Why don't you re-read Tsividis book!!!!

The approximation saturation ~ Vds > Vgs-Vth is only valid for stron inversion. As the transistor approches weak inversion, Vds_sat has a theoretical limit to 2-4 times Vt (Vt=kT/q).

As somebody else mentioned it, in weak inversion (subthreshold; get used to weak inversion) drift phenomena dominates the drain current, but the transistor can still be saturated!!!! Actually, in very low power designs, you must place nearly all transistors in WI, and get a reasonable gain from your transistors because they are "saturated"!!!

A very good approximation, valid in all operations regions (WI, MI and SI) of the saturation Vds is

Vds_sat = 2/[n*(gm/Id)]

Give you anyway some margin.

Guys, in this topic you deceived me;-)

Points: 2