At elevated temperatures, as a result of an electric field, alkali-ion contamination from regions surrounding MOS transistors can increase in concentration at the edges of negatively biased gate electrodes, and can subsequently enter the channel regions of MOS transistors and adversely impact reliability. In contrast to alkali-ion contamination initially present in the gate oxide or gate electrode material, which can be detected by conventional high-temperature bias tests of MOS capacitors or transistors, alkali-ion migration from outside of a transistor into the channel region is more difficult to detect. Electrical characterization of n-channel MOS transistors with no significant contamination initially present in channel regions, but with alkali-ion contamination present in regions adjacent to transistors, indicated that lateral alkali-ion migration can increase n-channel transistor subthreshold leakage currents by over three orders of magnitude. Possible sources of lateral alkali-ion contamination are indicated, and test patterns and screening tests to detect susceptibility to lateral alkali ion migration effects are discussed.