The term semiconductor is applied to solid materials which, due to their structure – their lattice structure –and depending on the temperature, have a larger or smaller number of electrons (holes) which are free to move. Due to these mobile charge carriers, the material has a conductivity of a greater or smaller magnitude or, if we consider the reciprocal of this, the specific electric resistance of semiconductors at room temperature is in a range between 10 -2 and 106 Ωcm. Materials which have no mobile charge carriers, and thus have even higher specific resistances, are referred to as ”insulators” (glass, mica, amber). Materials which retain their conductivity even at the lowest temperatures, and which in normal circumstances have significantly lower specific resistances, are called conductors (copper, aluminum, silver, gold). In contrast to solid metallic objects, the conductivity of gases or liquid is due to the mobility of their ions, and hence depends on the mobility of the material itself.
1939 the physicists Walter Schottky and Eberhard Spenke, both pioneers in the semiconductor industry, published a scientific work about how crystal diodes with a metal/ semiconductor junction work. Their worked was based on intensive fundamental research which showed that the junctions described, exhibit a rectifying property, i.e. they offer different electrical resistance to an electrical current depending on its direction of flow.
The first rectifiers were manufactured using selenium and germanium.
A notable milestone in the history of semiconductors was the development of the transistor. William Shockley and his team constructed 1947 a so called diode out of polycrystalline germanium. The Team observed, by chance, that a change in the forward voltage across the first diode resulted in a change in the reverse current through the second diode. They gave this effect the name “transitor effect”, derived from “transit” and “resistor”.
Silicon’s victory parade in semiconductors history
Germanium has the advantage of a high conductivity so that it is particularly suitable for use at high frequencies. The disadvantages of germanium as a semiconductor, on the other hand, are that the crystal structure cannot withstand temperatures above about 75 oC without incurring damage. In addition, the reverse current causes interference, even at room temperatures. By contrast, silicon crystals will stand temperatures up to 150 oC, and its higher band gap together with the higher specific resistance result in a far lower reverse current. Apart from this, silicon occurs naturally to a practical unlimited extent. Therefore silicon is a widely-used semiconductor material.