What are semiconductors? Resistance of semiconductors

What is the semiconductor material? What are its features? What is the physics of semiconductors? How are they built? What is conductivity of semiconductors? What physical indicators do they have?

What are called semiconductors?

So designate crystalline materials that do not conduct electricity as well as metals do. But still this indicator is better than the insulators have. Such characteristics are due to the number of mobile carriers. If we consider in general, then 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 position will be corrected. When indium is used, elements with a positive charge are obtained. 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 NPN-type element, then we can note a significant amplifying role, which is especially important in the transmission of weak signals.

Constructive features of electrical semiconductors

Conductors have many free electrons. Insulators do not practically have them at all. Semiconductors also contain a certain number 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 a single possible semiconductor element. So, there are PNP-transistors, as well as diodes.

Speaking about the latter briefly, it is such an element that it can transmit signals only in one direction. Also, the diode can convert an alternating current into a constant one. What is the mechanism of such a transformation? And why does he move only in one direction? Depending on where the current comes from, electrons and omissions can either diverge or meet halfway. In the first case, due to the increase in distance, the supply of supply is interrupted, and therefore the transfer of negative voltage carriers is carried out only in one direction, that is, the conductivity of semiconductors is one-sided. After all, the current can only be transmitted if the constituent particles are near. And this is possible only when power is supplied from one side. These types of semiconductors exist and are in use at the moment.

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 a forbidden band. What are its features? The fact is that there are no energy levels in the forbidden zone. With the help of impurities and structural defects, this can be changed. The highest completely filled zone is called the valence band. Then follows the resolved, but empty. It is called the conduction band. The physics of semiconductors is a rather interesting topic, and within the framework of the article it will be well covered.

The state of electrons

For this, concepts such as the number of the allowed band and the quasimomentum are used. The structure of the first is determined by the dispersion law. He says that the dependence of energy on quasimomentum influences it. 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, it means that there is a positively charged quasiparticle - a pass or a hole. They are charge carriers in semiconductors in the valence band.

Degenerate zones

The valence band in a typical conductor is sixfold degenerate. This is without taking into account the spin-orbit interaction and only when the quasimomentum is zero. It can be split under the same condition into doubly and quadruply degenerate zones. The energy distance between them is called the spin-orbit splitting energy.

Impurities and defects in semiconductors

They can be electrically inactive or active. The use of the former makes it possible to obtain a plus or minus charge in semiconductors, which can be compensated by the appearance of a hole in the valence band or an electron in the conducted zone. Inactive impurities are neutral, and they have relatively little effect on electronic properties. And often it can matter what the valence has atoms that take part in the transfer of charge, and the structure of the crystal lattice.

Depending on the type and amount of impurities, the ratio between the number of holes and electrons can vary. Therefore, semiconductor materials must always be carefully selected in order to obtain the desired result. This is preceded by a significant number of calculations, and subsequent experiments. Particles, which most call the main charge carriers, are non-basic.

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 differently to impurities than crystalline conductors. The absence of a rigid structure eventually results in the fact that the displaced atom receives another valence. It will be different from the one with which it initially saturates its connections. Atom it becomes unprofitable 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 one can not change the conductivity type by doping and create, for example, a pn junction.

Some amorphous semiconductors can change their electronic properties under the influence of doping. But this applies to them to a much lesser extent than to crystalline ones. The sensitivity of amorphous elements to alloying can be enhanced by processing. In the end I want to note that due to the long and hard work impurity semiconductors are still represented by a number of results with good characteristics.

The statistics of electrons in a semiconductor

When there is thermodynamic equilibrium, the number of holes and electrons is determined solely by the temperature, the parameters of the band structure and the concentration of electrically active impurities. When the ratio is calculated, it is assumed that part of the particles will be in the conduction band (at the acceptor or donor level). It also takes into account the fact that a part can leave the valent territory, and there are gaps.

Electrical Conductivity

In semiconductors, in addition to electrons, ions can act as charge carriers. But their electrical conductivity is negligible in most cases. As an exception, only ionic superconductors can be cited. In semiconductors there are three main mechanisms of electronic transfer:

1. The main zone zone. In this case, the electron comes into motion due to a change in its energy within one allowed area.
2. Hopping transfer over localized states.
3. Polaronic.

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 bond value. With a sufficient light intensity , a considerable amount of excitons can form in semiconductors. As their concentration increases, condensation takes place, and an electron-hole liquid is formed.

Surface of semiconductor

These words denote several atomic layers that are located near the boundary of the device. Surface properties differ from bulk ones. The presence of these layers disrupts the translational symmetry of the crystal. This leads to so-called surface states and polaritons. Developing the topic of the latter, we should also inform about spin and vibrational waves. Because of its chemical activity, the surface is covered by a microscopic layer of foreign molecules or atoms that have been adsorbed from the environment. They determine the properties of those several atomic layers. Fortunately, the creation of ultrahigh vacuum technology, in which semiconductor elements are created, allows us 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. The point here is that the electrical conductivity of any material (and this characteristic is inversely proportional to the resistance) depends on the charge of the current carriers, the speed of their movement in the electric field and their number in one unit of material volume.

In semiconductor elements, as the temperature rises, 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 young physics and the necessary material - silicon or germanium, you can also take a semiconductor made of them. Increasing the temperature will reduce their resistance. To make sure of this, you need to stock up with measuring devices that will allow you to see all the changes. This is in the general case. Let's look at a couple of particular options.

Resistance and electrostatic ionization

This is due to the tunneling of electrons passing through a very narrow barrier, which supplies about one-hundredth of a micrometer. It is located between the edges of the energy zones. Its appearance is possible only when the energy bands are tilted, 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, both in both the conductivity and valence bands. If we develop the process of electrostatic ionization, then a tunnel breakdown of the semiconductor can occur. During this process, the resistance of semiconductors will change. It is reversible, and as soon as the electric field is turned off, all processes will be restored.

Resistance and impact ionization

In this case, the holes and electrons are accelerated as long as the mean free path under the influence of a strong electric field reaches values that contribute to the ionization of atoms and the rupture of one of the covalent bonds (the main atom or impurity). Shock ionization occurs avalanche-like, and charge carriers multiply avalanche-like in it. In this case, newly created holes and electrons are accelerated by electric current. The value of the current in the final result is multiplied by the impact ionization coefficient, which is equal to the number of electron-hole pairs, which 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 resistance of semiconductors also varies, but, as in the case of tunnel breakdown, is reversible.

Application of semiconductors in practice

The special 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 it were not for the desire to independently assemble an object with their use. It is impossible to imagine the work of modern refrigerators, televisions, computer monitors without semiconductors. Not without them and advanced automobile development. They are also used in aerospace engineering. Do you understand what semiconductors are, how important they are? Of course, we can not 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 the wide prevalence of the materials from which they are made, and the ease of processing and obtaining the desired result, and other technical features that have made the choice of scientists who have developed electronic technology.

Conclusion

We have examined in detail what semiconductors are, how they work. Their resistance is based on complex physicochemical processes. And we can notify you that the facts described in the article do not fully understand what semiconductors are, for the simple reason that even science has not studied the specifics of their work to the end. But we know their basic properties and characteristics, which allow us to apply them in practice. Therefore, you can search for semiconductor materials and experiment with them yourself, being careful. Who knows, maybe a great researcher is slumbering in you ?!