Question

1. Transfer curves for three W’s of PMOS. Explain how PMOS W affects transfer curve.
2. Compare the simulated switching threshold with calculated one.
3. List the reasons for keeping PMOS wider than NMOS
4. Will the inverter response be same if instead of width you change length of PMOS instead of width

Answer 1

1) Transfer curve for three W's of PMOS

For Symmetric inverter

is transconductance ratio

is transconductance of NMOS

is transconductance of PMOS

And

So, if Width of P is increases the value of k_R will be less than than one and inverter threshold shifts to higher values.

More W is incresed more will be inverter threshold voltage.

Transfer curve of three W's of PMOS

As W's of PMOS increases W/L ratio increases also. Hence the curce threshold shifts towards right side.

2)

i) For 1st when (W/L)_P is 1/2 the simulated threshold is less than 2.5.

ii)  For 2nd when (W/L)_P is 2 the simulated threshold is little bit than 2.5.

iii) For 3rd when (W/L)_P is 4 the simulated threshold is more than 2.5.

3)

3. The reasons for keeping PMOS wider than NMOS:

The mobility of holes is less than mobility of mobility of electrons. So, to have k_R =1 for symmetric inverter PMOS should be kept wider than NMOS. So, that threshold voltage should be half of Vin ( Vin/2).

4) No, if the Length of PMOS is changed then it will be directly proportional to k_R and as L is incresed k_Rwill increases and threshold voltage shifts to lower value(i.e, less than 2.5)