Vacuum electron devices

Abstract

From its invention in 1906 until about 1950 the vacuum tube has been the key element of electronics. Then it started to be replaced by semiconductor devices. From our everyday experience with consumer electronics we are inclined to conclude that the days of the vacuum tube are over. Nothing is less true, however. Many vacuum devices have been replaced by cheaper, smaller, more reliable and less power-consuming semiconductor devices. The low power consumption of semiconductors, which is essential for applications such as computers and electronic telephone exchanges, becomes their weak point in applications where high power is needed. Then tubes are difficult or downright impossible to replace by semiconductors. Also, at the very high frequencies, say above 300 GHz, semiconductor devices are scarce and tubes will continue to be used for quite some time to come. Figure 2.1 gives an impression of the power output that can be obtained from microwave tubes. In short, despite the enormous development of semiconductor technology, it is much too early to declare the vacuum tube a fossil of a past period. In fact, developments in the tube field have never stopped and still continue. Also, as is shown in the last section of this chapter, the tube field is able to generate new developments by borrowing from semiconductor technology.