Electronic circuitry has undergone tremendous changes since the advancement of a triode, a type of electron tube that consists of three elements namely filament, grid and plate. In a triode, the amount of current from the cathode to the plate (i.e. controlled current is a function of both the grid to cathode voltage (the controlling signal) and the plate to cathode voltage (the electromotive force available to push electrons through the vacuum). A circuit of a triode is illustrated below:
With the development of the transistor (a semiconductor device), the electronic circuits became considerable reduced in size. This was mainly due the fact that a transistor was inexpensive, more reliable, consumed less power and much smaller in size than an electron tube.
Whilst semiconductor technology has almost rendered the electron tube technology obsolete, there are some applications that still employ electron tubes. In fact, in particular applications electron tubes perform better than semiconductor devices. In the following section we will explore the application capabilities of electron tubes in comparison to the semiconductor technology.
Specialized electron tubes like hydrogen thyratrons and krytrons are used in the field of high-power and high-speed circuit switching; they are able to switch far greater amounts of current at a faster rate than semiconductor devices. The thermal and temporal limits of semiconductor physics limits the switching abilities of semiconductor devices, on the other hand, electron tubes which don’t operate on the same principles are not affected by the same problem.
The superior thermal tolerance of electron tubes ensures their dominance over semiconductor devices in high-power microwave transmitter applications. Electron conduction through semiconducting materials is greatly affected by temperature, on the other hand electron conduction through electron/vacuum tubes is not impacted by temperature. As a result, the thermal limits of semiconductor devices are quite low as compared to that of electron tubes. Being able to operate electron tubes at far larger temperatures than equivalent semiconductor devices allows electron tubes to dissipate more thermal energy for a given amount of dissipation area, which makes them much smaller and lighter in continuous high-power applications.
One more benefit of electron tubes over semiconductor devices in high-power applications is their ability to be rebuilt. For example, when a large electron tube fails, it may be disassembled and repaired at far less cost than if you were to purchase a new electron tube. On the other hand, if a semiconductor device fails, whether large or small, there is generally no means of repair.
Vacuum/electron tubes being less complex in their fabrication than semiconductor devices, are potentially inexpensive to manufacture, nonetheless, the huge volume of semiconductor devices production in the world largely offsets this theoretical advantage. Semiconductor production is rather complex involving perilous chemical substances and requiring enhanced clean assembly environs.
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Lastly, electron tubes possess distinct advantage of low drift over a wide range of operating conditions. Unlike semiconductor devices whose barrier voltages, β ratios, bulk resistances and junction capacitances may change substantially with changes in device temperature and/or other operating conditions, the primary characteristics of a vacuum/electron tube remain nearly constant over a wide range in operating conditions, as those characteristics are determined fundamentally by the physical dimensions of the electron tube’s structural elements i.e. the cathode, grid(s) and plate rather than the interactions of subatomic particles in a crystalline lattice. However, it is important to note that, electron tube performance tends to drift more than semiconductor devices when measured over long periods of time, and here we are talking about years. This is mainly due to the tube aging caused by vacuum leaks i.e. when air enters the inside of a vacuum tube; its electrical characteristics become irreversibly altered. This is one of the main reasons why vacuum tubes do not last as long as their respective solid-state counterparts. This problem can be mitigated by maintaining vacuum at high level, making it possible to have excellence performance and long lifespan.
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