Id and Ids difference

Status
Not open for further replies.
A

anhnha

Full Member level 6
Full Member level 6
Joined
Mar 8, 2012
Messages
322
Helped
4
Reputation
8
Reaction score
4
Trophy points
1,298
Activity points
3,684
I simulated NMOS transistors in cadence and see that there are two currents, Id and Ids.
Id : current entering the drain
Ids: current that enters the drain and leaves the source
Are they always equal?
 

Hi,

Ids is the drain to source current only (the one we are interested in for general analysis) but Id is the total drain current this may also include drain to bulk leakage.
 

anhnha said:
Ids: current that enters the drain and leaves the source
Click to expand...

No - I don´t think so (because there is no difference between drain and source current).
I think, the subscript stands for "short circuit" of the Vgs voltage (Id for Vgs=0).
 

LvW said:
No - I don´t think so (because there is no difference between drain and source current).
I think, the subscript stands for "short circuit" of the Vgs voltage (Id for Vgs=0).
Click to expand...

No LvW its not the case... even with a high VGS cadence will report both of them and Id is slightly greater than Ids
 

SIDDHARTHA HAZRA said:
No LvW its not the case... even with a high VGS cadence will report both of them and Id is slightly greater than Ids
Click to expand...

I didn´t say that Ids is the maximum current. In many books and data sheets it is the current for Vgs=0 (sometimes writtes as Idss)
 
Last edited:

I think SIDDHARTHA HAZRA is right
Have a look to this:

**broken link removed**
(see point 8 and point 48 of operating point parameters, towards the end of the document)
 

Hi, Anhnha;
Id is the darin current entering into MOS while there is some leakage in bulk that's why Ids is some how different from Id. you can verify it in CADENCE.
 

anhnha said:
Id : current entering the drain
Ids: current that enters the drain and leaves the source
Are they always equal?
Click to expand...

The question was if Id and a current called Ids are equal or not.

Without any doubt, Id is the actual drain current.
Because source current=drain current, we do not need a separate symbol Is.
However, with the aim to write a formula for the function Id=f(Vgs) we need the threshold voltage (Id=0) and another typical point for Id (which determines the slope of the curve).
And there was an agreement to use (for depletion types) the current Id for the condition Vgs=0.
In most cases, this current is written as Id,s or Id,ss.
 

LvW said:
The question was if Id and a current called Ids are equal or not.
Because source current=drain current, we do not need a separate symbol Is.
Click to expand...

Do to parasitics they aren't perfectly identical, some current (very very small) flows through the bulk.
 

albbg said:
Do to parasitics they aren't perfectly identical, some current (very very small) flows through the bulk.
Click to expand...

OK - we always have parasitic effects everywhere. Do you think this justifies a specific name? I don´t think so.
The main question is: What means anhnha (the questioner) when writing "Ids"?
 

Status
Not open for further replies.

Similar threads