The figure of merit of digital IC is defined as: 

Figure of merit = Propagation Delay × Power Dissipation

For the best IC operation, FOM should be as small as possible.

Units: ns × mW

= pJ (pico Joule)

Propagation delay (tpd­):

tPHL = delay time in going form High to low logic

tPLH = Delay time in going from low to High logic

Power dissipation (PD):

PD(avg) = Icc × Vcc

VCC = power supply

ICC = avg collector current calculated as the average of the High and low current, i.e.