Also called popcorn noise or burst noise. This is one of those noise sources for which no single source has been theorized to fully explain it. The best understanding is that it is caused by random trapping and release of charge carriers at thin film interfaces or at defect sites in bulk semiconductor crystal. What happen are random step-like transitions between discrete voltage levels and these steps are at random intervals ranging from milliseconds to seconds. They are usually related to silicon contamination with heavy metals or lattice structure imperfections.