Substrate noise injection is evaluated for a 0.25-/spl mu/m CMOS technology, to determine the mechanisms that contribute to substrate crosstalk. At the transistor level, we find that impact ionization current and capacitive coupling from the junctions are the most significant contributors to substrate current injection. An investigation of substrate fluctuations at a circuit level included switching transients, capacitive damping, and separate substrate biasing. This investigation revealed that voltage transients on power-supply lines can be the dominant source of substrate fluctuations. Finally, a statistical analysis of signal cancellation in an integrated circuit was performed. The results indicate that more cancellation will take place for the high-frequency noise components than for the average and low-frequency components. As a consequence, the dc and low-frequency components of the transient that result from an individual switching event can not be neglected even if they are a relatively small fraction of the single transient.