The thyristor, the GTO and the IGBT
In the 1960s a new type of valve became available: the thyristor. The thyristor displaced the mercury-arc technology by semiconductor technology. The advantages of the semiconductor technology are:
· Greater power density
· Higher switching speed
· Lower weight per volume
· Less losses
· Avoiding the toxic aspects of handling mercury
In the periodic table there are metals and nonmetals. In their pure form metals conduct electricity, whereas nonmetals mostly do not. In steel, for instance, many of the atoms lost their outermost electrons entirely, and those electrons float aimlessly inside the steel. Inside an insulator, glass, for instance, the outermost electrons are much more tightly bound to their atoms and have no interest to move under the influence of an electric field. There is also an interesting group of nonmetals that display intermediate levels of conductivity: germanium, silicon, gallium arsenide, and silicon carbide. It was Michael Faraday who, in 1833, recognized semi-conduction in silver sulfide, but the phenomenon was not understood until in the 1930s the band theory of conduction was formulated.
The conductivity of semiconducting materials can be influenced by selectively inserting impurities into the crystal. These inserted atoms occupy positions in the crystal lattice that otherwise would be occupied by an atom of the substrate material. If the inserted atoms have more electrons in their outer band than the host material, we get n-type material. The additional electrons are free to move through the crystal and increase the conductivity. If the inserted atoms have fewer electrons in their outer band, we get p-type material, and this creates the so-called holes. Because the electrons from neighboring atoms are able to occupy these positions, the holes themselves have mobility and behave like positive charge carriers and also increase conductivity.
A diode is a simple example of a semiconductor device. A p-zone adjoins an n-zone on the same crystal, and if a voltage is applied, current can flow from the p-zone to the n-zone. In the p-zone, holes will flow toward the p–n junction, in the n-zone electrons will flow toward the p–n junction, and the holes and electrons recombine at the junction. If the polarity of the voltage reverses, charge carriers are depleted from the p–n junction and the conduction stops. To make a diode a switchable valve, the conductivity must be triggered externally. In 1947 the transistor was invented at Bell Laboratories by John Bardeen, William Shockley, and Walter Brattain. It uses an electric field to control the availability of charge carriers in a germanium crystal.
In 1950 William Shockley described the principle of the thyristor. A thyristor is an p–n diode with additional layers between the outer p- and n-zones. These layers prevent conduction, but when, at a third contact, the gate, a current is injected, this area floods with charge carriers and, when a forward voltage is present between the anode and the cathode, a current can flow. Conduction does not cease until the current crosses zero. In the 1980s the gate turnoff thyristor (GTO) came in the market. A GTO is a thyristor that can be turned off by applying a current to the gate in the reverse direction to that required to turn it on. A further development in semiconductor technology was the introduction of the insulated gate bipolar transistor (IGBT). An IGBT is a power semiconductor controlled by voltage (instead of by current as the GTO) and capable of faster operation, permitting higher switching frequencies.