Maximum Stacked (series) Transistors?

Hi all,

I am a bit confused by how to determine the maximum number of permissible serial transistors in a logic gate in 0.18 (or technology really). Is there a limit? My thought would be that if for example, the number of series PMOS is too great, Vout of the digital gate would not rise high enough to excite the NMOS of the next gate in the logic path.

For example: is it possible to create a 5-input NAND with 0.18 technology? How about a NOR? (series PMOS instead of NMOS). How about a 6- or 7-input gate? Where is the limit?

As an experiment, I have tried testing this in Cadence, for example creating a simple chain of 3 NMOS transistors, with a small capacitor with an initial vale of 1.8V on the top of the stack. Simulation shows that the capacitor is fully discharged through the NMOS chain, as opposed to only partially discharging to 1.8V - 3 * Vth.

Thanks for any help/clarifications you can provide!

torontograd said:

I am a bit confused by how to determine the maximum number of permissible serial transistors in a logic gate in 0.18 (or technology really). Is there a limit? My thought would be that if for example, the number of series PMOS is too great, Vout of the digital gate would not rise high enough to excite the NMOS of the next gate in the logic path.

Click to expand...

No. In static CMOS, the transistors swing all the way to cut-off, so there is no Vt drop. You're thinking about Vt drops in analogue circuits where the stacked transistors are all kept in saturation.

For example: is it possible to create a 5-input NAND with 0.18 technology? How about a NOR? (series PMOS instead of NMOS). How about a 6- or 7-input gate? Where is the limit?

Click to expand...

Yes, Yes, Yes, the limit is your target speed.

Are you sure you're really a University of Toronto grad student? ;p

What if he is a control systems (etc.) grad student. There is no reason to be caustic - being a grad student does not imply solid digital circuit skills.

Haha! Thanks for the comeback, and also thanks for the help roadbuster, much appreciated.