In 1957, General Electric introduced the silicon controlled rectifier (SCR), a three-terminal p-n-p-n device. It replaced the gas-filled tubes used previously, which were difficult to operate and unreliable, revolutionizing efficient control of electric energy and electrical machines. The symmetrical alternating current switch (TRIAC), the gate turn-off thyristor (GTO), and the large integrated gate-commutated thyristor (IGCT) evolved from the SCR, becoming the foundation of modern power electronics.