What is semiconductor material? What are its features? What is the physics of semiconductors? How are they built? What is the conductivity of semiconductors? What physical indicators do they have?
What are called semiconductors?
This is the meaning of crystalline materials that do not conduct electricity as well as metals do. But still this figure is better than the insulators have. Such characteristics are due to the number of mobile carriers. Generally speaking, there is a strong attachment to the nuclei. But when several atoms, say antimony, which has an excess of electrons, are introduced into the conductor, this situation will be corrected. When using indium receive elements with a positive charge. All these properties are widely used in transistors - special devices that can amplify, block or pass current in only one direction. If we consider an element of the NPN type, we can note a significant reinforcing role, which is especially important when transmitting weak signals.
Design Features of Electrical Semiconductors
Conductors have many free electrons. Insulators practically do not possess them. Semiconductors also contain a certain amount of free electrons, and gaps with a positive charge, which are ready to accept the released particles. And most importantly - they all conduct electric current. The type of NPN transistor considered earlier is not the only possible semiconductor element. So, there are also PNP transistors, as well as diodes.
If we talk about the latter briefly, then this is such an element that can transmit signals only in one direction. Also, the diode can turn alternating current into direct current. What is the mechanism of such a transformation? And why is he moving in only one direction? Depending on where the current comes from, the electrons and the gaps can either diverge or go in the opposite direction. In the first case, due to the increase in the distance, the supply is interrupted, therefore, negative voltage carriers are transferred only in one direction, that is, the conductivity of the semiconductors is one-sided. After all, the current can only be transmitted if the constituent particles are nearby. And this is possible only when applying current from one side. These types of semiconductors exist and are currently in use.
Zone structure
The electrical and optical properties of conductors are related to the fact that when electrons fill energy levels, they are separated from possible states by the band gap. What are her features? The fact is that in the forbidden zone there are no energy levels. With the help of impurities and structural defects, this can be changed. The highest fully filled zone is called the valence. Then comes the allowed but empty. It is called the conduction band. Semiconductor physics is a rather interesting topic, and it will be well covered in the article.
Electron state
For this, concepts such as the number of the allowed zone and quasi-momentum are used. The structure of the first is determined by the law of dispersion. He says that it is affected by the dependence of energy on the quasimomentum. So, if the valence band is completely filled with electrons (which carry charge in semiconductors), then they say that there are no elementary excitations in it. If for some reason there is no particle, then this means that a positively charged quasiparticle has appeared here - a gap or a hole. They are charge carriers in semiconductors in the valence band.
Degenerate zones
The valence band in a typical conductor is six times degenerate. This is without taking into account the spin-orbit interaction and only when the quasimomentum is equal to zero. Under the same condition, it can be split into twofold and fourfold degenerate zones. The energy distance between them is called the energy of spin-orbit splitting.
Impurities and defects in semiconductors
They may be electrically inactive or active. The use of the former makes it possible to obtain a positive or negative charge in semiconductors, which can be compensated by the appearance of a hole in the valence band or of an electron in the conductive band. Inactive impurities are neutral, and they relatively weakly affect electronic properties. Moreover, the valency of the atoms that take part in the process of charge transfer and the structure of the crystal lattice can often matter .
Depending on the type and amount of impurities, the ratio between the number of holes and electrons can also change. Therefore, semiconductor materials should always be carefully selected to obtain the desired result. This is preceded by a significant number of calculations, and subsequently experiments. Particles, which most call primary charge carriers, are non-basic.
The dosed introduction of impurities into semiconductors makes it possible to obtain devices with the required properties. Defects in semiconductors can also be in an inactive or active electrical state. Important here are dislocation, interstitial atom and vacancy. Liquid and non-crystalline conductors react to impurities in a different way than crystalline ones. The absence of a rigid structure ultimately results in the fact that the displaced atom receives a different valency. It will be different from the one with which he initially saturates his ties. It becomes unprofitable for an atom to give or attach an electron. In this case, it becomes inactive, and therefore impurity semiconductors have a great chance of failure. This leads to the fact that it is impossible to change the type of conductivity with the help of doping and create, for example, a pn junction.
Some amorphous semiconductors can change their electronic properties under the influence of alloying. But this applies to them to a much lesser extent than crystalline ones. The sensitivity of amorphous elements to alloying can be increased by processing. In the end, I want to note that due to the long and hard work, impurity semiconductors are nevertheless represented by a number of results with good characteristics.
Statistics of electrons in a semiconductor
When there is a thermodynamic equilibrium, the number of holes and electrons is determined solely by temperature, parameters of the band structure, and the concentration of electrically active impurities. When the ratio is calculated, it is believed that part of the particles will be in the conduction band (at the acceptor or donor level). Also taken into account is the fact that part can leave the valence territory, and gaps are formed there.
Electrical conductivity
In semiconductors, in addition to electrons, ions can also act as charge carriers. But their electrical conductivity in most cases is negligible. As an exception, only ionic superconductors can be cited. In semiconductors, there are three main mechanisms of electron transfer:
- The main zone. In this case, the electron comes into motion due to a change in its energy within the same permitted territory.
- Hopping over localized states.
- Polaron.
Exciton
A hole and an electron can form a bound state. It is called the Wannier-Mott exciton. In this case, the photon energy, which corresponds to the absorption edge, decreases by the size of the coupling value. With sufficient light intensity , a significant number of excitons can form in semiconductors. With an increase in their concentration, condensation occurs and an electron-hole liquid forms.
Semiconductor surface
These words indicate several atomic layers that are located near the boundary of the device. Surface properties differ from bulk. The presence of these layers violates the translational symmetry of the crystal. This leads to the so-called surface states and polaritons. Developing the theme of the latter, one should also report about spin and vibrational waves. Due to its chemical activity, the surface is covered by a microscopic layer of external molecules or atoms that have been adsorbed from the environment. They determine the properties of those several atomic layers. Fortunately, the creation of ultra-high vacuum technology, which creates semiconductor elements, allows you to obtain and maintain a clean surface for several hours, which positively affects the quality of the products.
Semiconductor. Temperature affects resistance
When the temperature of metals increases, so does their resistance. With semiconductors, the opposite is true - under the same conditions, this parameter will decrease for them. The point here is that the conductivity of any material (and this characteristic is inversely proportional to resistance) depends on what kind of current charge the carriers have, on the speed of their movement in the electric field and on their number in one unit volume of the material.
In semiconductor elements with increasing temperature, the concentration of particles increases, due to this, the thermal conductivity increases, and the resistance decreases. You can check this if you have a simple set of a young physicist and the necessary material - silicon or germanium, you can also take a semiconductor made from them. An increase in temperature will reduce their resistance. To make sure of this, it is necessary to stock up with measuring instruments that will allow you to see all the changes. This is a general case. Let's look at a couple of private options.
Resistance and electrostatic ionization
This is due to the tunneling of electrons passing through a very narrow barrier, which delivers about one hundredth of a micrometer. It is located between the edges of energy zones. Its appearance is possible only when the energy zones tilt, which occurs only under the influence of a strong electric field. When tunneling occurs (which is a quantum-mechanical effect), the electrons pass through a narrow potential barrier, and their energy does not change. This entails an increase in the concentration of charge carriers, and in both zones: both conductivity and valence. If the process of electrostatic ionization is developed, then a tunneling breakdown of the semiconductor can occur. During this process, the semiconductor resistance changes. It is reversible, and as soon as the electric field is turned off, then all processes will be restored.
Resistance and impact ionization
In this case, holes and electrons are accelerated while they travel the mean free path under the influence of a strong electric field to values that contribute to the ionization of atoms and the breaking of one of the covalent bonds (the main atom or impurity). Impact ionization occurs in an avalanche-like manner, and charge carriers multiply in it in an avalanche-like manner. In this case, the newly created holes and electrons are accelerated by electric current. The value of the current in the final result is multiplied by the coefficient of impact ionization, which is equal to the number of electron-hole pairs that are formed by the charge carrier on one segment of the path. The development of this process ultimately leads to an avalanche breakdown of the semiconductor. The semiconductor resistance also changes, but, as in the case of tunneling breakdown, it is reversible.
The use of semiconductors in practice
The particular importance of these elements should be noted in computer technology. We have almost no doubt that you would not be interested in the question of what semiconductors are, if not for the desire to independently assemble an object using them. It is impossible to imagine the work of modern refrigerators, televisions, computer monitors without semiconductors. Advanced automobile developments are not complete without them. They are also used in aircraft and space technology. Understand what semiconductors are, how important are they? Of course, one cannot say that these are the only irreplaceable elements for our civilization, but they should not be underestimated either.
The use of semiconductors in practice is also due to a number of factors, among which are the widespread prevalence of the materials from which they are made, the ease of processing and obtaining the desired result, and other technical features due to which the choice of scientists who developed electronic equipment settled on them.
Conclusion
We examined in detail what semiconductors are, how they work. The basis of their resistance lies in complex physical and chemical processes. And we can notify you that the facts described in the article will not fully understand what semiconductors are, for the simple reason that even science has not studied the features of their work to the end. But we know their basic properties and characteristics, which allow us to put them into practice. Therefore, you can search for semiconductor materials and experiment with them yourself, being careful. Who knows, maybe a great explorer is dozing in you ?!