nMos inverter delay

                            Considering an nMos inverter with a Zp.u/Zp.d ratio as 4:1.Here the value of Rp.u is 10Kohms in 5micrometer technology. And so using the above ratio,we get Rp.u =4*Rp.d=40Kohms.In this case, the delay associated with the inverter depends on whether it's being turned ON or OFF. However,if we consider a cascaded inverter pair,then the delay over the input will be constant irrespective of the sense of logic level transition of the input to the first. Generally, we have an overall delay of 5t(t+4t). In general,the delay through a pair of nMos inverter is Td=(1+Zpu/Zpd)t.
                           Hence, for an nMos inverter it is 5t.A single 4:1 inverter exhibits unsymmetrical delays since the delay associated with the turning ON is, for example t,while the delay associated with turning OFF is 4t.