A transistor computer, now often called a second generation 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 unreliable. A second generation of computers, through the late 1950s and 1960s featured circuit boards filled with individual transistors and magnetic core memory. These machines remained the mainstream design into the late 1960s, when integrated circuits started appearing and led to the third generation computer.