Complementary Metal Oxide Semiconductor. A type of semiconductor that has been, until the EOS D30, widely unavailable for digital cameras. CMOS semiconductors use two circuits, negative and positive polarity circuits. Because only one of the circuits can be on at once, CMOS chips are less energy consuming than other chips that utilize simply one type of transistor. This is a clear advantage of the CMOS sensor over the standard CCDs in use today.