output resistance of PMOS and NMOS. which has higher output resistance?

[surreyian]

Hello all,

the output resistance is given as
1) satuation region, R=1/(Lamda*Id).
2) linear region, R=1/[u.Cox.(w/l).(vgs-vth)]

In these cases, PMOS will have higher outout resistance as PMOS has lower mobility and Id. Is my understanding correct?

However in Razavi's design of analog CMOS integrated circuit, chapter 2. It states that for given dimensions and bias current, NMOS has higher output resistance. How does this statement come about?

 

[erikl]

Do not mix up static (Ro,s=Vds/Id) and dynamic (small signal, Ro,d=δVds/δId) output resistance!

 

[threekingtiger]

Hi. You can refer to Paul Gray's book about the channel length modulation of MOSFET. The output resistance is strongly related to the channel length and the doping level. Lamda=(1/L-(Xd))*d(Xd)/d(Vds). The doping level affects the width of depletion region near the SD diffustion. So if you really wanna compare the output resistance of the two devices of the same dimension, you need to know which one is much more doped than the other. However, knowing the result is usually not important for a designer because active load is frequently used and the Ro=rop//ron.