Why PMOS always have bigger size than NMOS?

Device Physics doubts related to diode, transistor, BJT, MOSFET, JFET.
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Arvind
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Joined: Wed Apr 09, 2014 3:11 pm
Location: Noida, India

Why PMOS always have bigger size than NMOS?

Post by Arvind » Wed Apr 09, 2014 4:01 pm

Why PMOS always have bigger size than NMOS in an inverter?
Last edited by Arvind on Wed Apr 09, 2014 4:10 pm, edited 2 times in total.

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Narveer
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Location: Bangalore, INDIA

Re: Why PMOS always have bigger size than NMOS?

Post by Narveer » Wed Apr 09, 2014 5:29 pm

NMOS has electrons as majority charge carriers and PMOS has hole as majority charge carriers. Electrons has mobility ~2.7 times higher the holes. The main reason behind making PMOS larger is that rise time and fall time of gate should be equal and for this the resistance of the NMOS and PMOS should be the same. This can be achieved only by sizing the PMOS ~ 3 times to the NMOS sizing.

nikhilp
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Joined: Tue Apr 15, 2014 10:52 am

Re: Why PMOS always have bigger size than NMOS?

Post by nikhilp » Tue Apr 15, 2014 12:35 pm

Yes as explained above, the mobility difference does cause this width difference, but this is true for higher nodes like > 45nm. As we go on lower nodes esp. at 28 and 16nm as I am working on these nodes, these is very less difference in mobility for holes and electrons and we size PMOS and NMOS equal unless otherwise required.
Now I dont know exactly in physical terms what is happening on lower nodes, It maybe due to shorter channels the field is too high and mobilities have saturated,not sure.

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