Semiconductor

Semiconductors are solids whose electrical conductivity lies between that of electrical conductors (>104 S/cm) and that of non-conductors (<10-8 S/cm). Since the boundary regions of the three groups overlap, the negative temperature coefficient of resistivity is another important characteristic of semiconductors, i.e. their conductivity increases with increasing temperature, they are so-called hot conductors. The reason for this is the so-called band gap between the valence band and the conduction band. Close to the absolute temperature zero, these are full or unoccupied, and semiconductors are therefore non-conductors. In contrast to metals, there are primarily no free charge carriers; these must first be created, for example, by heating. However, the electrical conductivity of semiconductors increases steeply with temperature, so that they are more or less conductive at room temperature, depending on the material-specific distance between the conduction and valence bands. Furthermore, by introducing foreign atoms (doping) from another main chemical group, the conductivity and the conduction character (electron and hole conduction) can be specifically influenced within wide limits.

Semiconductors are divided into crystalline and amorphous semiconductors on the basis of their crystal structure; see section Classification. Furthermore, they can have different chemical structures. The best known are the elemental semiconductors silicon and germanium, which are made up of a single element, and compound semiconductors such as the III-V compound semiconductor gallium arsenide. In addition, organic semiconductors have gained importance and prominence in recent decades; they are used, for example, in organic light-emitting diodes (OLEDs). However, there are also other materials with semiconductor properties, such as metal-organic semiconductors and materials that acquire semiconductor properties through nanostructuring. Very new are ternary hydride compounds such as lithium barium hydride (LiBaH3).

Semiconductors are important for electrical engineering and especially for electronics, where the possibility to influence their electrical conductivity by doping plays a decisive role. The combination of differently doped regions, e. g. in the p-n junction, enables both electronic components with a direction-dependent conductivity (diode, rectifier) or a switch function (e. g. transistor, thyristor, photodiode), which can be controlled, e. g. by applying an electrical voltage or current (cf. working states in metal-insulator-semiconductor structure). Other applications besides the transistor are: Thermistors, varistors, radiation sensors (photoconductors, photoresistors, photodiodes or solar cells), thermoelectric generators, Peltier elements, and radiation or light sources (laser diode, light-emitting diode). The majority of all semiconductor components manufactured are silicon-based. Although silicon does not have the very best electrical properties (e.g. charge carrier mobility), in combination with its chemically stable oxide it has clear advantages in production (see also thermal oxidation of silicon).

History

Stephen Gray discovered the difference between conductors and non-conductors in 1727. After Georg Simon Ohm established Ohm's law in 1821, which describes the proportionality between current and voltage in an electrical conductor, it was also possible to determine the conductivity of an object.

Nobel Prize winner Ferdinand Braun discovered the rectifying effect of semiconductors in 1874. He wrote: "In a large number of natural and artificial sulphur metals [...] I found that the resistance of the same differed with direction, intensity and duration of the current. The differences amount to as much as 30 pCt. of the whole value." He thus described for the first time that resistance can be variable.

Greenleaf Whittier Pickard received the first patent for a silicon-based tip diode for demodulating the carrier signal in a detector receiver in 1906. Initially, the receiver of the same name ("Pickard Crystal Radio Kit") mostly used galena as the semiconductor, with more robust and powerful diodes based on copper sulfide-copper contacts emerging in the 1920s. The operation of the rectifier effect based on a semiconductor-metal junction remained unexplained for decades, despite its technical application. It was not until 1939 that Walter Schottky was able to lay the theoretical foundations for the description of the Schottky diode named after him.

The first patent on the principle of the transistor was filed in 1925 by Julius Edgar Lilienfeld (US physicist of Austrian-Hungarian descent). In his work, Lilienfeld described an electronic component which is comparable in the broadest sense to today's field-effect transistors; at the time, he lacked the necessary technologies to practically realize field-effect transistors.

When, in 1947, the scientists John Bardeen, William Bradford Shockley and Walter Houser Brattain plugged two metal wire tips onto a small germanium plate at Bell Laboratories and were thus able to control the p-conducting zone with the second wire tip with an electrical voltage, they thus realised the tip transistor (bipolar transistor). This earned them the 1956 Nobel Prize in Physics and established microelectronics.

The production of high-purity silicon was achieved in 1954 by Eberhard Spenke and his team at Siemens & Halske AG using the zone melting process. This, together with the availability of an insulating material (silicon dioxide) with favorable properties (not water-soluble like germanium oxide, easy to produce, etc.) in the mid-1950s, brought about the breakthrough of silicon as a semiconductor material for the electronics industry and, about 30 years later, for the first microsystem technology products. Today (2009), silicon produced more cheaply using the Czochralski process is used almost exclusively for the manufacture of integrated circuits.

Alan Heeger, Alan MacDiarmid and Hideki Shirakawa showed in 1976 that when polyacetylene - a polymer that is an insulator in the undoped state - is doped with oxidizing agents, the electrical resistivity can drop to 10-5 Ω-m (silver: ≈ 10-8 Ω-m). In 2000, they received the Nobel Prize in Chemistry for this (see section on organic semiconductors).

Division

The classical, i.e. crystalline electronic, semiconductors used in microelectronics can be divided into two groups: element semiconductors and compound semiconductors. Elemental semiconductors include elements with four valence electrons, such as silicon (Si) and germanium (Ge). The group of compound semiconductors includes chemical compounds that have an average of four valence electrons. These include compounds of elements of the IIIrd with the Vth main group of the periodic table (III-V compound semiconductors), such as gallium arsenide (GaAs) or indium antimonide (InSb), and of the IIth subgroup with the VIth main group (II-VI semiconductors), such as zinc selenide (ZnSe) or cadmium sulfide (CdS).

In addition to these commonly used semiconductors, there are also the I-VII semiconductors, such as copper(I) chloride. Materials that do not have four valence electrons on average can also be called semiconductors if they have a resistivity in the range greater than 10-4 Ω-m and less than ¹⁰⁶ Ω-m.

Another large class are the organic semiconductors. They are called organic because they are mainly composed of carbon atoms. They are subdivided into semiconducting polymers (chains of varying lengths of individual monomers) and small molecules (single, self-contained units). Although fullerenes, carbon nanotubes, and their derivatives are also, strictly speaking, small molecules, they are often perceived as a stand-alone subgroup. Classic examples of organic semiconductors are P3HT (poly-3-hexylthiophene, polymer), pentacene (small molecule), or PCBM (phenyl-C61-butyric acid methyl ester, fullerene derivative). Organic semiconductors are used in light-emitting diodes (OLEDs), solar cells (OPVs) and field-effect transistors.

Several semiconducting molecules or atoms combine to form a crystal or create a disordered (amorphous) solid. Roughly, most inorganic semiconductors can be classified as crystalline, most organic semiconductors as amorphous. However, whether a crystal or an amorphous solid is actually formed depends largely on the manufacturing process. For example, silicon can be crystalline (c-Si) or amorphous (a-Si), or form a polycrystalline hybrid (poly-Si). There are also single crystals of organic molecules.