A transistor computer is a computer which uses discrete transistors instead of vacuum tubes. The "first generation" of electronic computers used vacuum tubes, which generated large amounts of heat, were bulky, and were unreliable. A "second generation" of computers, through the late 1950s and 1960s featured boards filled with individual transistors and magnetic memory cores (see History of computing hardware). These machines remained the mainstream design into the late 1960s, when integrated circuits started appearing and led to the "third generation" machines.