Semiconductor

What Are Semiconductors?
Semiconductors are substances that conduct electricity under certain conditions i.e. they require a medium for the conduction of electricity. They have partial properties of both conductors and non-conductors. Semiconductors are used in microprocessor chips, production of radio waves etc. Semiconductors are pure materials but sometime impurities are added to increase their degree of conductivity.

History of Semiconductors
In 1879, E.H. Hall discovered an effort, which was essential for the development of methods to characterize semiconductors. The Hall Effect allows the determination of the carrier density as well as the type of the charge carriers. J.J. Thomson’s discovery of the electron in 1897 stimulated the development of theoretical descriptions of the conduction in metals. Three years later, P. Drude developed a model, which describes the electrical and thermal conduction of solids. However, a quantitative description of carrier transport in semiconductors was only possible after quantum mechanics was available and applied to solids. In 1926, F. Bloch formulated a quantum mechanical theorem, now referred to as Bloch’s theorem, which describes the electron wave function taking into account the crystal structure of the solid. Another important discovery was made by A.H. Wilson in 1931, who demonstrated that semiconductors are insulators with narrow energy gaps. He also introduced the concept of holes. The energy gap is also important for understanding the reverse breakdown of semiconductors and insulators the large electric fields. In 1934, C. Zener showed that it originates from inter band tunneling, i.e., tunneling through the energy gap.

Fifty years ago, Bardeen, Brattain, and Shockley developed the first transistor opening the way for semiconductor devices. The first semiconductor lasers were fabricated in the early 1960s. A vast amount of research on III-V and II-VI semiconductors in the following decades was stimulated by the development of epitaxial growth techniques in the late 1960’s to early 1970’s, in particular for heterostructures, which are composed of two or more semiconductors with different energy gaps. A new wave of research on low-dimensional semiconductor heterostructures was triggered by the discovery of the quantum Hall Effect in 1980 by K.Von Klitzing in the inversion layer of a field effect transistor, which forms a quasi two dimensional layer. This research is geared today towards the realization of quantum wire and dot lasers as well as a single electron transistor. A missing link for optoelectronic applications was discovered in 1992, the blue laser based on II-VI semiconductors. However, presently its lifetime is still rather short. In 1993, the blue light emitting diode based on Gallium nitride (GaN) with an estimated lifetime of more than 10,000 hours was reported. These are only a few highlights of semiconductor physics.

General View:
In general, the difference between semiconductors and insulators lies in the band gaps. In insulators, the distance between the conduction band and valence band is much larger when compared to the semi conductors. Therefore, electrons obtain thermal energy at room temperature and sweep from covalent band to valence band thus conducting current in the case of semi conductors. Owing to this property, semiconductors acts like insulators in the absence of electric field.

What really distinguishes metals from semiconductors is the temperature dependence of the conductivity. While metals and semimetals retain their metallic conductivity even at low temperatures, semiconductors are transformed into insulators at very low temperatures. In this sense semiconductors and insulators are actually one class of materials, which differs from metals and semimetals, which form another class. This classification is directly connected to the existence of a gap between occupied and empty states i.e., an energy gap, in semiconductors and insulators.

The border line between semiconductors and insulators is rather arbitrary. In particular, the value of the energy gap separating the semi conducting materials from the insulating ones is not well-defined. For example, diamond was considered for a long time an insulator, but today it is possible to prepare it in such a way that it has semi conducting properties even at room temperature. The important distinction between these two systems originates historically from their different conductivities at room temperature. However, an insulator at room temperature can become a semiconductor at higher temperatures. Therefore, wide energy gap materials are currently under investigation for high temperature electronics.

Types:
The basic types of semiconductors are intrinsic and extrinsic semiconductors.

Intrinsic semiconductors are pure semiconductors that contain no impurities. When temperature increases, the conduction property of the intrinsic type also increases. This is because, at high temperatures, electrons are excited to higher energy levels and create holes. These holes are positively charged and flow in the direction opposite to that of electrons thus causing electricity. In an intrinsic semiconductor, the number of holes and electrons are equal. Other causal agent of electricity in this type is crystal defects.

When impurities are added to intrinsic semiconductors, extrinsic semiconductors are formed. The process of adding impurities to the conductor is called doping.

Doping:
In the process of doping, impure atoms are added to the intrinsic semiconductors. These impure atoms are nothing but atoms of elements that differ from the actual semiconductor element. They change the electron and hole concentrations acting as a donor or acceptor.

The acceptor atom has less valence electron than the semiconductors, thus attracting electrons from the valence bond creating holes. The whole carrier concentration of the semiconductor increases and this type is called as p-type semiconductors. P-type refers to positive charge. Here electrons are the minority charge carriers. The Fermi energy level is less than that of the intrinsic semiconductors and it lies close to the valence band.

The donor atom has excess valence electrons than the semiconductors, and hence it donates these valence electrons to it increasing the concentration of the electrons in the semi conductor. This type of semiconductor is called n-type semiconductor. N-type refers to the negative charge. In this, the minority charge carriers are the holes and the Fermi energy level lies close to the conduction band than the valence band and it is greater than that of the intrinsic semiconductors.

Classification of Semiconductors
There is a large variety of semiconductors available today, although for applications Si completely dominates the market. Nevertheless, other semiconductors can have quite different properties. For example, Si (Silicon) cannot be used for light emitting diodes, or lasers. Semiconductors will be classified according to their chemical composition. There are elementary semiconductors such as Si (Silicon), Ge (Germanium), and gray tin (α-Sn), which all belong to group IV in the periodic table. Therefore, these systems are usually referred to as group IV semiconductors. Another group IV element is carbon, which solidifies into two structures, diamond and graphite. Diamond is an insulator and has the same crystal structure as Si, Ge, and α-Sn. Graphite is a semimetal and exhibits a hexagonal structure. Sn also exists in two phases, white tin ( -Sn), which is metallic, and semiconducting α-Sn. The last element of the group IV is lead, which is metallic.

Group IV elements are exceptional in the periodic table in the sense that the outer shell of the individual atoms is exactly half filled. By sharing one of the four electrons of the outer shell with another Si atom, a three-dimensional crystal structure with no preferential direction can be realized. One can also combine two different group IV semiconductors to obtain a compound material such as SiC (Silicon Carbide). SiC is a material close to the border line between semiconductors and insulators with a lattice constant of 0.436 nm and an energy gap of 3.0 eV (413 nm).

The elementary group IV semiconductors can also be understood as a special case of the 8N rule, i.e., completing the outer shell by sharing electrons with other atoms. There are many other ways to fulfill this 8N rule. However, these materials are all compound semiconductors. Elements from group III can be combined with group V elements. Group I elements in conjunction with group VII elements lead to wide energy gap insulators, since these materials are formed by ionic bonds and not covalent bonds as III-V and most II-VI semiconductors. Most of the III-V semiconductors exist in the so-called zincblende structure, which is a cubic lattice. Some exist in the wurtzite structure, which corresponds to a hexagonal lattice. Gallium arsenide (GaAs) is the best known III-V compound semiconductor, while Gallium nitride (GaN), although known for a long time. In contrast to Si, GaAs, and many other compound semiconductors are direct semiconductors so that optical applications of these systems are very common. The most common III-V semiconductors are compiled together with their respective lattice constant, energy gap, and lattice structure.

Semiconductor Materials:
Semi conductors play a variety of roles in humans’ life. All the electric and electronic components contain semi conductor materials. It replaces the functions of the vacuum tube in volume and reliability which is used in making electronic components. A microprocessor chip replaces a set of vacuum tubes in the generation of electricity.

A diode is a form of semiconductor that allows current in one direction and not letting out in other direction. It is used in batteries to prevent the flow of current when it is fixed in reverse by mistake. During forward biasing at the voltage of 0.7 volts, electron-hole carrier concentration starts, thus causing flow of current. During reverse biasing, reverse voltage is applied thus breaking down the junction.

A transistor is another form of semiconductor device which contain two diodes attached back to back. It is made up of three layers, NPN or PNP sandwich. The transistor has a switching behavior i.e. when a small amount of current is applied to the middle layer, current flows through the whole.

Another semiconductor device called silicon chip contains lot of transistors.