why is contamination delay lower bound?

Question

I am taking a course from edX called computation structures: 1.Digital Circuits. When the course explained about CMOS timing, there was propagation delay(tpd) and contamination delay(tcd). I understood that tpd has to be upper bound but I couldn't understand why tcd should be lower bound. Shouldn't tcd be upper bound too?

Answer 1

Contamination delay is the time from an input change to when the output starts to change, propagation delay is the time from an input change to when the output has reached a stable, valid level. During the time between min contamination delay and max propagation delay the output may not be valid.