1
\$\begingroup\$

Can someone explain what does it mean when one say P:N sizing ratio of an inverter. FO=4 and equal rise and fall time.

\$\endgroup\$

1 Answer 1

2
\$\begingroup\$

The PMOS transistor has a lower driving strength then the NMOS transistor. For equal rise and fall times the PMOS transistor needs to have a larger width than the NMOS transistor.

Typically the PMOS transistor is about 3 times wider than the NMOS transistor.

FO is the fan-out, which is the number of gates that can be driven by that inverter.

\$\endgroup\$
1
  • \$\begingroup\$ Typically the PMOS transistor is about 3 times wider than the NMOS transistor And that is because the PMOS needs to be 3 times wider than an equivalent NMOS to get the same "drive strength". The reason for this is mostly mobility of carriers. NMOS rely on electrons and PMOS on holes. Hole mobility is roughly 3x less than electron mobility. \$\endgroup\$ Commented Jan 15, 2017 at 10:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.