Recall that placing a positive voltage at the gate of an n-type enhanced-mode FET will turn the switch on. Placing the same voltage at the gate of a p-type enhanced-mode FET will turn the switch off. Likewise, placing a negative voltage at the gate will turn the n-type off and the p-type on. These FETs always respond in opposite, or complementary, fashion to a given gate voltage. Thus, if the gates of an n-type and a p-type FET are connected any voltage applied to the common gate will operate the complementary pair, turning one on and leaving the other off. A semiconductor that pairs n- and p-type transistors this way is called a complementary metal-oxide semiconductor (CMOS). Because complementary transistor pairs can quickly switch between two logic states, CMOSs are very useful in logic circuits. In particular, because only one circuit is on at any time, CMOSs require less power and are often used for battery-powered devices, such as in digital cameras, and for the special memory that holds the date, time, and system parameters in personal computers.
