Popcorn noise was first observed in early point contact diodes, then re-discovered during the commercialization of one of the first semiconductor op-amps; the 709.[3] No single source of popcorn noise is theorized to explain all occurrences, however the most commonly invoked cause is the random trapping and release of charge carriers at thin film interfaces or at defect sites in bulk semiconductor crystal. In cases where these charges have a significant impact on transistor performance (such as under an MOS gate or in a bipolar base region), the output signal can be substantial. These defects can be caused by manufacturing processes, such as heavy ion implantation, or by unintentional side-effects such as surface contamination.[4][5]