Why does PMOS have to be 3x bigger than NMOS in a gate ?

[chihwt2003]

Transmission gate basic

Hi all,

Just want to ask why do we need to size Pmos aprrox 3 times bigger than NMOS for a transmission gate? WHat is the effect if both NMOS and PMOS are same size? Thanks.

CW

 

[zeeshanzia84]

Re: Transmission gate basic

The mobility of holes is less than that of electrons, thus PMOS devices are usually made larger so that charging of the output node can take place much quickly.

b/c current = charge/time

if you can't decrease time...why not increase the charge!

 

[Willt]

Re: Transmission gate basic

Hi friend,

For an inverter, the size of PMOS is 2-3 times larger than that of NMOS due to mobility difference between holes and electrons. But the sizing depends very much on the application. If we don't care about the output rising time, we could just simply make (W/L)p=(W/L)n

For transmission gate, the sizing of PMOS could be the same as NMOS because large PMOS increases its parasitic capacitance at the drain and the source. Thus, the speed is decreased in turn when (W/L)p=3(W/L)n.

You could just try three cases:
(1) (W/L)p=(W/L)n
(2) (W/L)p=2(W/L)n
(3) (W/L)p=3(W/L)n
Compare the propagation delay in these cases.

Hope it helps ~

Will

 

[zeeshanzia84]

Re: Transmission gate basic

thanks a lot WILLT. You're absolutely right....i just got it a bit mixed up.