This is a snap from Chapter number 5 CMOS inverter, Digital integrated circuit by Jan M Rabaey .
I just wanted to know from where this \$I_{peak}\$ is measured. The direct path current exist till the output goes from \$V_{tn}\$ to \$V_{DD}-V_{tp}\$. But this current is peak value of which current? Current through the PMOS drain /source or the NMOS drain/source? While simulating which of the four currents should I take?
The current author talks about is the short-circuit current \$I_{sc}\$ relevant to transient power dissipation in a CMOS inverter.
\$I_{sc}\$ flows from \$V_{DD}\$ to \$V_{SS}\$ when both PMOS and NMOS are conducting for a brief period of time while switching is happening at the input of the inverter. It is modelled/approximated as a spike within this time interval.
From the same book:
You can measure this current as the drain-source current through either PMOS or NMOS within the time interval where 'shorting' is happening. \$I_{peak}\$ is the peak value of \$I_{sc}\$.
A crude circuit to simulate the behavior of this current.
The peak current is the current need to charge (discharge) the gate of the MOSFET and it's mostly limited by the gate resistance and other parasitics (wiring inductance and various capacitances). In fact the gate current will also charge the parasitic gate-drain and gate-source capacitance (very important fact for power work, but it seems your book is about logic).
The basic idea about CMOS is that at rest nothing flows and bias current is almost null but during switch the gates need to be charged/discharged and that creates the dynamic dissipation which is the bulk of heat generation in CMOS logic (there's a little bit of cross conduction during switch, too).
The current spikes are not actually triangles but exponentials IIRC (the gates are more or less capacitors).
EDIT OOPS I read the question bad. Your peak current is actually the cross conduction between the CMOS pair. While one FET opens the other one is still closing so there is some VDD-VSS shoot through. It depends on the overlap of the switching curves of the P- and the N- channels
