Semiconductor Materials, Physics tutorial

Introduction:

Semiconductors and polymers and are both highly visible engineering materials by a major impact on the contemporary society and while the application of semiconductor technology has clearly revolutionized society, solid-state electronics is revolutionizing technology itself.

A comparatively small group of elements and compounds encompass the significant electrical property of semi-conduction in which they are neither good electrical conductors nor good electrical insulators however rather; their capability to conduct electricity is intermediate. Semiconductors in common don't fit into any of the four structural materials classes based on the atomic bonding. Metals are innately good electrical conductors. Ceramics and polymers (that is, non-metals) are usually poor conductors however good insulators.

The three semiconducting elements (that is, Si, Ge and Sn) from column IV-A of the periodic table serve up as a boundary between the metallic and non-metallic elements.

Semiconductor Materials:

A semiconductor is a material having electrical conductivity due to the electron flow that is intermediate in magnitude between that of a conductor and an insulator that signifies conductivity around in the range of 103 to 10-8 Siemens per centimeter. Semiconductor materials are the base of modern electronics, comprising radio, computers, telephones and most of the other devices. These devices comprise transistors, solar panels; different diodes like light-emitting diode, the silicon controlled rectifier and digital and analog integrated circuits. Likewise, semiconductor solar photovoltaic panels directly transform light energy into the electrical energy.

In a metallic conduction, current is taken out by the flow of electrons. In semiconductors, current is frequently schematized as being taken out either by the flow of electrons or through the flow of positively charged 'holes' in the electron structure of the material. In both the cases, just electron movements are comprised.  

General semiconducting materials are crystalline solids, however amorphous and liquid semiconductors are recognized. These comprise hydrogenated amorphous silicon and mixtures of arsenic, selenium and tellurium in a diversity of proportions. These compounds share with better known semiconductors intermediate conductivity and a fast variant ion of conductivity with temperature, and also occasional negative resistance.

These disordered materials lack the rigid crystalline structure of the conventional semiconductors like silicon and are usually employed in thin film structures that are less demanding for as concerns the electronic quality of the material and therefore are relatively insensitive to the impurities and radiation damage.

You may find out it strange that there are organic semiconductors, however - it is true. Organic semiconductors are organic materials having properties resembling the conventional semiconductors. Silicon is employed to make most semiconductors commercially and most of the other materials are employed, comprising germanium, gallium arsenide and silicon carbide. A pure semiconductor is frequently termed as an 'intrinsic' semiconductor. The electronic properties and the conductivity of a semiconductor can be modified in a controlled way through adding extremely small quantities of other elements, termed as dopants, to the intrinsic material. This is acquired in crystalline silicon through adding impurities of boron or phosphorus to the molten silicon and then letting it to solidify into the crystal. This procedure is termed as doping.

Semiconductor materials are the insulators at absolute zero temperature however conduct electricity at room temperature. The defining property of a semiconductor material is that it can be doped by impurities that modify its electronic properties in a controllable manner.

Due to their application in devices such as transistors and lasers, the search for latest semiconductor materials and the enhancement of existing materials is a significant field of study in materials science. Most generally employed semiconductor materials are crystalline inorganic solids. Such materials are categorized according to the periodic table groups of their constituent atoms.

Various semiconductor materials vary in their properties. Therefore, in comparison by the silicon, compound semiconductors encompass both merits and demerits. For instance, gallium arsenide (GaAs) consists of six times higher electron mobility than the silicon, that lets faster operation; broader band gap, that lets operation of power tools at higher temperatures and provides lower thermal noise to low power devices at room temperature; its direct band gap provides it more favorable optoelectronic properties than the indirect band gap of silicon; it can be alloyed to ternary and quaternary compositions, having adjustable band gap width, allowing light emission at selected wavelengths, and allowing example: matching to wavelengths  by means of lowest losses in the optical fibers. GaAs can be as well grown in a semi insulating form that is appropriate as a lattice-matching insulating substrate for GaAs devices. On the other hand, silicon is healthy, cheap and simple to process, while GaAs is brittle and costly, and insulation layers can't be made by just growing an oxide layer; GaAs is thus employed only where silicon is not adequate.

By alloying multiple compounds, several semiconductor materials are tuneable in band gap or lattice constant. The outcome is ternary, quaternary or even quinary compositions. Ternary compositions let adjusting the band gap in the range of the comprised binary compounds; though, in case of combination of direct and indirect band gap materials there is a ratio where  indirect band gap prevails, limiting the range utilizable for optoelectronics. Lattice constants of the compounds as well tend to be dissimilar and the lattice mismatch against the substrate, based on the mixing ratio, causes defects in the amounts based on the mismatch magnitude; this affects the ratio of achievable Radiative or non Radiative recombination and finds out the luminous efficiency of the device.

Quaternary and higher compositions let adjusting concurrently the band gap and the lattice constant, allowing rising radiant efficiency at broader range of wavelengths; for instance AlGaInP is employed for LEDs. Materials transparent to the produced wavelength of light are beneficial that facilitates proficient extraction of photons from the material. In such transparent materials, light production is not restricted to just the surface. Index of refraction is as well composition-dependent and affects the extraction efficiency of photons from the material.

Electrical Conduction:

In crystalline semiconductors, electrons can encompass energies only in some bands that are positioned between the ground state, corresponding to electrons tightly bound to the atomic nuclei of the material and free electron energy. The free electron energy being the energy needed for an electron to escape completely from the material. Each energy bands correspond to a large number of discrete quantum states of electrons and most of the states by low energy that is closer to the nucleus are full up to a specific band termed as the valence band.  

Insulators and Semiconductors are differentiated from metals as their valence band is almost filled by electrons beneath usual operating conditions, whereas very few in the case of semiconductor or virtually none in case of insulator are available in the conduction band; the conduction band being the band instantly above the valence band.  

The band gap between the bands finds out the ease by which electrons in a semiconductor can be excited from the valence band to the conduction band.

By covalent bonds, an electron moves by hopping to a neighboring bond. The Pauli Exclusion Principle needs the electron to be lifted into the higher anti-bonding state of that bond.

For delocalized states, for instance in one dimension, for each and every energy there is a state by the electrons flowing in one direction and the other state by the electrons flowing in the other. For a total current to flow, more states for one direction than for the other direction should be occupied. For this to take place energy is needed, as in the semiconductor the subsequent higher states lay above the band gap.

This can be stated as: Full bands don't contribute to the electrical conductivity. Though, as the temperature of a semiconductor increases above absolute zero, there is more energy in the semiconductor to spend on lattice vibration and on lifting some electrons to an energy state of the conduction band. 

Electrons excited to the conduction band as well leave electron holes behind that are unoccupied states in the valence band. Both the conduction band electrons and the valence band holes contribute to the electrical conductivity.

The holes themselves do not in reality move, however a neighboring electron can move to fill the hole, leaving a hole at the place it has just come from, and in this manner the holes appear to move, and the holes behave as if they were in reality positively charged particles. As one covalent bond between neighboring atoms in solid is 10 times stronger than the binding of a single electron to the atom, freeing an electron doesn't mean destruction of the crystal structure.

Holes:

The concept of holes can as well be applied to metals too where the Fermi level lies in the conduction band. By means of most metals the Hall Effect points out electrons are the charge carriers. Though, a few metals encompass a mostly filled conduction band. In such, the Hall Effect reveals positive charge carriers that are not the ion-cores, however holes. In case of a metal, just a small amount of energy is required for the electrons to determine unoccupied states to move into, and therefore for current to flow.

Fermi-Dirac distribution:

The States having energy beneath the Fermi energy contain higher probability of being occupied, and those above are less probable to be occupied and the energy distribution of the electrons finds out which of the states are filled and which are empty. This distribution is explained by Fermi-Dirac statistics. The distribution is featured through the temperature of the electrons, and the Fermi energy or Fermi level. Beneath absolute zero conditions the Fermi energy can be thought of as the energy up to which available electron states are occupied. At higher temperatures, the Fermi energy is the energy at which the probability of a state being occupied has drop to 0.5. The dependence of the electron energy distributed ion on temperature as well describes why the conductivity of a semiconductor consists of a strong temperature dependency, as a semiconductor operating at lower temperatures will encompass fewer available free electrons and holes.

Doping:

The property of semiconductors makes them most helpful for constructing electronic devices is that their conductivity might simply be altered by introducing impurities into their crystal lattice. The procedure of adding controlled impurities to a semiconductor are termed as doping and the amount of impurity, or dopants, added to an intrinsic (pure) semiconductor fluctuates its level of conductivity. Doped semiconductors are frequently termed to as extrinsic semiconductors.

By adding impurity to the pure semiconductors, the electrical conductivity might be varied not merely by the number of impurity atoms however as well, by the kind of impurity atom and the changes might be thousand folds and million folds. For illustration, 1 cm3of a metal or semiconductor specimen consists of a number of atoms on the order of 1022. As each and every atom in metal donates at least one free electron for conduction in metal, 1 cm3 of metal includes free electrons on the order of 1022. At temperature close to 20 °C, 1 cm3of pure germanium includes around 4.2 x 1022 atoms and 2.5 x 1013 free electrons and 2.5×1013 holes (that is, empty spaces in crystal lattice containing positive charge) The addition of 0.001% of arsenic (an impurity) shares an extra 1017free electrons in the similar volume and the electrical conductivity rises around 10,000 times.

The materials selected as suitable dopants based on the atomic properties of both the dopants and the material to be doped. In common, dopants that generate the desired controlled changes are categorized as either electron acceptors or donors. A donor atom which activates (that is, becomes incorporated into the crystal lattice) shares weakly bound valence electrons to the material, making surplus negative charge carriers. Such weakly bound electrons can move about in the crystal lattice comparatively freely and can ease conduction in the presence of the electric field.

Electrons at such states can be simply excited to the conduction band, becoming free electrons, at room temperature. On the contrary, an activated acceptor generates a hole. Semiconductors doped by donor impurities are termed as n-type, as those doped by acceptor impurities are termed as p-type. The n and p type designations point out that charge carrier acts as the material's majority carrier. The opposite carrier is termed as the minority carrier that exists due to thermal excitation at a much lower concentration compared to the majority carrier.

Carrier concentration:

The concentration of dopants introduced to an intrinsic semiconductor finds out its concentration and indirectly influences most of its electrical properties. The most significant factor that doping directly affects is the material's carrier concentration. In an intrinsic semiconductor beneath thermal equilibrium, the concentration of electrons and holes is equivalent.

n = p = ni2

If we encompass a non-intrinsic semiconductor in thermal equilibrium, then the relation becomes:

no . po = ni2

Here, no is the concentration of conducting electrons, po is the electron hole concentration, and ni is the material's intrinsic carrier concentration. Intrinsic carrier concentration differs between materials and is dependent on temperature. Silicon's ni, for illustration, is roughly 1.08 x 101 cm-3 at 300 Kelvin (room temperature)

Semiconductors Material Preparation:

Semiconductors by means of reliably predictable electronic properties are needed for the production and level of chemical purity required is very high as the presence of impurities even in extremely small proportions can have huge effects on the properties of the material. High degree of crystalline perfection is as well needed in the semiconductor material as faults and imperfections in crystal structure interfere by the semiconducting properties of the material. Crystalline faults are a main cause of defective semiconductor devices. The bigger the crystal, the harder it is to accomplish the essential perfection and conventional mass production processes utilize crystal ingots between 100 mm and 300 mm (4 to 12 inches) in diameter that are grown as cylinders and sliced into wafers.

Due to the required level of chemical purity and the perfection of the crystal structure that are required to make semiconductor devices, special processes have been build up to generate the initial semiconductor material. A method for accomplishing high purity requires growing the crystal by using the Czochralski method. An extra step which can be employed to further raise the purity is termed as zone refining. In zone refining, part of a solid crystal is melted. The impurities tend to concentrate in the melted area, whereas the desired material re-crystallizes leaving the solid material more pure and by some crystalline faults.

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