Hi~ Actually Im fine with calculating the delay but I havent got the concept clearly enough. Both nmos and pmos have their equivalent RC models. I understand how the RC model of nmos works when its on or off but I dont get how the pmos turns out this way. When its off, the upper part wont work because its open circuit but when its on, isnt the resistor supposed to be included? Why isnt it there in the inverter's equivalent RC model when the pmos is on?