Strange problem in ASSURA RCX

[sureshnaidu]

the extracted transistor parameters are not same as in the schematic view.i have divided the transistor into four fingers.when i check its dc parameters the Cgs value of one finger is differing from the other finger.The first finger Cgs and the Third finger Cgs values are same and second finger and fourth finger Cgs are same.Interestingly Cgs and Cgd of first finger are equal to Cgd and Cgs of second finger respectively.i have many transistors in my design and this problem occurring for all the transistors. my post layout simulations are worst compared to prelayout simulation results.Iam thinking that the problem is due to above mentioned reason.what is the reason for this.can any one explain.thanks is advance

 

[JoannesPaulus]

It is not a problem, ASSURA is just extracting your layout: when you have 4 fingers, you have 3 drains and 2 sources (or 2 drains and 3 sources), alternating. The capacitance is going to be different for every finger.
I am surprised that fingers 1-3 and 2-4 match, I would have expected 1-4 and 2-3 but of course it depends on your layout.

 

[sureshnaidu]

It is not a problem, ASSURA is just extracting your layout: when you have 4 fingers, you have 3 drains and 2 sources (or 2 drains and 3 sources), alternating. The capacitance is going to be different for every finger.
I am surprised that fingers 1-3 and 2-4 match, I would have expected 1-4 and 2-3 but of course it depends on your layout.

what happened if i have 30 fingers.Cgs of all the fingers is need to be same right.if i add Cgs of all the fingers it is noway near to the value from schematic.

 

[JoannesPaulus]

what happened if i have 30 fingers.Cgs of all the fingers is need to be same right.if i add Cgs of all the fingers it is noway near to the value from schematic.

That's the effect of your layout... Especially for deep sub-micron designs, this is one of the major issues.

 

[sureshnaidu]

JoannesPaulus, please see the image that i have attached.The Cgs and Cgd of successive fingers interchanged

 

[JoannesPaulus]

RCX does not really know which one is the drain and which one is the source. The transistor is symmetric. This behavior is absolutely normal.