PS&PD of a mosfet 8lambda+W (8lambda+2W)

[leohart]

PS,PD of a mosfet

what is the PS and PD of the following mosfet?
It is said 8lambda+W in this slide,but in Allen's book,he said it is 8lambda+2W?
Which one is right?Give your opinion pls guys~

 

[zhaozenghui]

PS,PD of a mosfet

thanks!!!!

 

[leohart]

PS,PD of a mosfet

anyone have any comments?

 

[ezt]

Re: PS,PD of a mosfet

Dear leohart,

I guess the relation in the attached picture is correct, since one side of the Source/Drain junction is faced to the active channel and hence there exists no field-implant on that edge to result in the expected parasitic cap (or the parasitic cap would be very smaller!). That's why if in the same picture there were two fingers of gate poly-Si, the middle junction would have "Perimeter = 4 λ" and NOT "4λ+W" for computing its parasitic capacitance, since the middle junction would face active channels on its both sides.
This relation is used also in "Analog Integrated Circuit Design" by Johns & Martin [P. 99, Footnote]

Regards,
EZT

 

[leohart]

PS,PD of a mosfet

thanks, but I dont understand why "or the parasitic cap would be very smaller!" and it should be 8lambda for the middle junction I suppose...

 

[ezt]

Re: PS,PD of a mosfet

Dear leohart,

"Since there is no field-implant along that edge, the sidewall capacitance is therefore smaller there". These are almost the exact words in Johns&Martin book. Well, I think there may still exist some small parasitic cap. due to fringing or some other phenomena but definitely it is small enough that could be ignored!

About "4λ" or "8λ" it really depends on the set of Design Rules (DRs) of your processing technology. I think that sentence in my last reply could be replaced with "the middle junction would have "Perimeter = 8λ" and NOT "8λ+W" for computing its parasitic capacitance". However, it can be right, since the middle junction may have no contact (in many structures) and hence in many lambda-based DRs the distance between two adjacent gate fingers may be at least "2λ", hence the perimeter of the middle junction would reduce to "4λ".

Regards,
EZT