One of the main properties of a p‑n‑junction is its ability to pass electric current in one (forward) direction thousands and millions of times better than in the opposite direction.
Semiconductors are a class of substances that occupy an intermediate position between substances that conduct electric current well (conductors, mainly metals) and substances that practically do not conduct electric current (insulators or dielectrics).
Semiconductors are characterized by a strong dependence of their properties and characteristics on the microscopic amounts of impurities they contain. By changing the amount of impurity in a semiconductor from ten millionths of a percent to 0.1–1%, you can change their conductivity by millions of times. Another important property of semiconductors is that electric current is carried into them not only by negative charges - electrons, but also by positive charges of equal magnitude - holes.
If we consider an idealized semiconductor crystal, absolutely free of any impurities, then its ability to conduct electric current will be determined by the so-called intrinsic electrical conductivity.
Atoms in a semiconductor crystal are connected to each other using electrons in the outer electron shell. During thermal vibrations of atoms, thermal energy is distributed unevenly between the electrons forming bonds. Individual electrons can receive enough thermal energy to “break away” from their atom and be able to move freely in the crystal, i.e., become potential current carriers (in other words, they move into the conduction band). Such electron departure violates the electrical neutrality of the atom; it acquires a positive charge equal in magnitude to the charge of the departed electron. This vacant space is called a hole.
Since the vacant place can be occupied by an electron from a neighboring bond, the hole can also move inside the crystal and become a positive current carrier. Naturally, electrons and holes under these conditions appear in equal quantities, and the electrical conductivity of such an ideal crystal will be equally determined by both positive and negative charges.
If in place of an atom of the main semiconductor we place an impurity atom, the outer electron shell of which contains one more electron than the atom of the main semiconductor, then such an electron will turn out to be superfluous, unnecessary for the formation of interatomic bonds in the crystal and weakly connected with its atom. Tens of times less energy is enough to tear it away from its atom and turn it into a free electron. Such impurities are called donor, i.e., donating an “extra” electron. The impurity atom is charged, of course, positively, but no hole appears, since a hole can only be an electron vacancy in an unfilled interatomic bond, and in in this case all connections are complete. This positive charge remains associated with its atom, motionless and, therefore, cannot take part in the process of electrical conductivity.
The introduction of impurities into a semiconductor, the outer electron shell of which contains fewer electrons than in the atoms of the main substance, leads to the appearance of unfilled bonds, i.e. holes. As mentioned above, this vacancy can be occupied by an electron from a neighboring bond, and the hole is able to move freely throughout the crystal. In other words, the movement of a hole is a sequential transition of electrons from one neighboring bond to another. Such impurities that “accept” an electron are called acceptor impurities.
If a voltage (as indicated in the polarity figure) is applied to the metal-dielectric semiconductor structure of the n-type, then an electric field arises in the near-surface layer of the semiconductor, repelling electrons. This layer turns out to be depleted.
In a p-type semiconductor, where the majority carriers are positive charges - holes, the polarity of the voltage that repelled electrons will attract holes and create an enriched layer with reduced resistance. A change in polarity in this case will lead to the repulsion of holes and the formation of a near-surface layer with increased resistance.
With an increase in the amount of impurities of one type or another, the electrical conductivity of the crystal begins to acquire an increasingly pronounced electronic or hole character. According to the first letters Latin words negativus and positivus electronic electrical conductivity is called n-type electrical conductivity, and hole conductivity is called p-type, indicating which type of mobile charge carriers for a given semiconductor is the main one and which is the minority one.
With electrical conductivity due to the presence of impurities (i.e., impurity), there are still 2 types of carriers left in the crystal: the main ones, which appear mainly due to the introduction of impurities into the semiconductor, and the minority ones, which owe their appearance to thermal excitation. The content in 1 cm 3 (concentration) of electrons n and holes p for a given semiconductor at a given temperature is a constant value: n − p = const. This means that, by increasing the concentration of carriers of a given type by several times due to the introduction of impurities, we reduce the concentration of carriers of another type by the same amount. The next important property of semiconductors is their strong sensitivity to temperature and radiation. As the temperature rises, the average vibration energy of the atoms in the crystal increases, and more and more bonds will be broken. More and more pairs of electrons and holes will appear. At sufficiently high temperatures, the intrinsic (thermal) conductivity can be equal to the impurity conductivity or even significantly exceed it. The higher the concentration of impurities, the higher temperatures this effect will occur.
Bonds can also be broken by irradiating the semiconductor, for example, with light, if the energy of light quanta is sufficient to break the bonds. The energy of breaking bonds is different for different semiconductors, so they react differently to certain parts of the irradiation spectrum.
Silicon and germanium crystals are used as the main semiconductor materials, and boron, phosphorus, indium, arsenic, antimony and many other elements that impart the necessary properties to semiconductors are used as impurities. Obtaining semiconductor crystals with a given impurity content is a very difficult task. process, carried out in especially clean conditions using equipment of high precision and complexity.
All of the listed most important properties of semiconductors are used to create semiconductor devices that are very diverse in their purposes and areas of application. Diodes, transistors, thyristors and many other semiconductor devices are widely used in technology. The use of semiconductors began relatively recently, and today it is difficult to list all their “professions.” They transform light and thermal energy into electric energy and, conversely, with the help of electricity they create heat and cold (see Solar energy). Semiconductor devices can be found in a conventional radio receiver and in a quantum generator - a laser, in a tiny atomic battery and in miniature blocks of an electronic computer. Engineers today cannot do without semiconductor rectifiers, switches and amplifiers. Replacing tube equipment with semiconductor equipment has made it possible to reduce the size and weight of electronic devices tenfold, reduce their power consumption and dramatically increase reliability.
You can read about this in the article Microelectronics.
What are its features? What is the physics of semiconductors? How are they built? What is conductivity of semiconductors? What physical characteristics do they have?
What are semiconductors called?
This refers to crystalline materials that do not conduct electricity as well as metals do. But still this indicator is better than that of insulators. Such characteristics are due to the number of moving carriers. Generally speaking, there is a strong attachment to cores. But when several atoms, say antimony, which has an excess of electrons, are introduced into the conductor, this situation 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, we can note a significant amplifying role, which is especially important when transmitting weak signals.
Design features of electrical semiconductors
Conductors have many free electrons. Insulators practically do not have them at all. Semiconductors contain both a certain number of free electrons and gaps with a positive charge that are ready to accept released particles. And most importantly, they all conduct. The type of NPN transistor discussed 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 an element that can transmit signals only in one direction. A diode can also turn alternating current into direct current. What is the mechanism of this transformation? And why does it only move in one direction? Depending on where the current is coming from, electrons and gaps can either diverge or go towards each other. In the first case, due to an increase in distance, the supply supply is interrupted, and therefore negative voltage carriers are transmitted in only one direction, that is, the conductivity of semiconductors is one-way. After all, current can be transmitted only if the constituent particles are nearby. And this is only possible when current is supplied from one side. These are the types of semiconductors that exist and are currently in use.
Zone structure
The electrical and optical properties of conductors are due to the fact that when energy levels are filled with electrons, they are separated from possible states by a band gap. What are its features? The fact is that there are no energy levels in the band gap. This can be changed with the help of impurities and structural defects. The highest completely filled band is called the valence band. This is followed by one that is allowed, but empty. It is called the conduction band. Semiconductor physics - quite interesting topic, and it will be well covered within the article.
State of electrons
For this purpose, concepts such as the number of the allowed zone and quasi-pulse are used. The structure of the first is determined by the dispersion law. He says that it is influenced by the dependence of energy on quasi-momentum. Thus, 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 sixfold degenerate. This is without taking into account the spin-orbit interaction and only when the quasi-momentum is zero. Under the same condition, it can split into doubly and quadruple 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 positive or negative charge in semiconductors, which can be compensated by the appearance of a hole in the valence band or an electron in the conductive band. Inactive impurities are neutral, and they have a relatively weak effect on the electronic properties. Moreover, what can often matter is the valency of the atoms that take part in the charge transfer process and the structure
Depending on the type and amount of impurities, the ratio between the number of holes and electrons may also change. Therefore, semiconductor materials must always be carefully selected to obtain the desired result. This is preceded by a significant number of calculations, and subsequently experiments. The particles that most people call majority charge carriers are minority ones.
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. The important ones here are dislocation, interstitial atom and vacancy. Liquid and non-crystalline conductors react to impurities differently than crystalline ones. The lack of a rigid structure ultimately results in the displaced atom receiving a different valence. It will be different from the one with which he initially saturates his connections. It becomes unprofitable for the atom to give or gain an electron. In this case, it becomes inactive, and therefore impurity semiconductors have a high chance of failure. This leads to the fact that it is impossible to change the type of conductivity by doping and create, for example, a pn junction.
Some amorphous semiconductors can change their electronic properties when exposed to doping. But this applies to them to a much lesser extent than to crystalline ones. The sensitivity of amorphous elements to alloying can be increased by technological processing. Ultimately, I would like to note that, thanks to long and hard work, impurity semiconductors are still represented by a number of results with good characteristics.
Statistics of electrons in a semiconductor
When the number of holes and electrons exists is determined solely by temperature, band structure parameters and the concentration of electrically active impurities. When the ratio is calculated, it is assumed that some of the particles will be in the conduction band (at the acceptor or donor level). Also taken into account is the fact that part may 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 is in most cases negligible. As an exception, only ionic superconductors can be cited. There are three main electron transfer mechanisms in semiconductors:
- Main zone. In this case, the electron begins to move due to a change in its energy within one allowed area.
- Jumping transfer across localized states.
- Polaronic.
Exciton
A hole and an electron can form a bound state. It is called a Wannier-Mott exciton. In this case, which corresponds to the absorption edge, decreases by the size of the coupling value. If sufficient, a significant number of excitons can be formed in semiconductors. As their concentration increases, condensation occurs and an electron-hole liquid is formed.
Semiconductor surface
These words denote several atomic layers that are located near the border of the device. Surface properties differ from volumetric ones. The presence of these layers breaks the translational symmetry of the crystal. This leads to so-called surface states and polaritons. Developing the theme of the latter, we should also talk about spin and vibrational waves. Due to its chemical activity, the surface is covered with a microscopic layer of foreign molecules or atoms that have been adsorbed from environment. They determine the properties of those several atomic layers. Fortunately, the creation of ultra-high vacuum technology, in which semiconductor elements are created, makes it possible to obtain and maintain a clean surface for several hours, which has a positive effect on the quality of the resulting product.
Semiconductor. Temperature affects resistance
When the temperature of metals increases, their resistance also increases. 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 inversely proportional to resistance) depends on what 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, as the temperature increases, the concentration of particles increases, due to which thermal conductivity increases and resistance decreases. You can check this with a simple set of young physicists and required material- silicon or germanium, you can also take a semiconductor made from them. Increasing the temperature will reduce their resistance. To make sure of this, you need to stock up on measuring instruments that will allow you to see all the changes. This is in the 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 that delivers approximately one-hundredth of a micrometer. It is located between the edges of energy zones. Its appearance is possible only when the energy zones are tilted, which occurs only under the influence of a strong electric field. When tunneling occurs (which is a quantum mechanical effect), electrons pass through a narrow potential barrier without changing their energy. This entails an increase in the concentration of charge carriers, in both bands: conductivity and valence. If the process of electrostatic ionization is developed, a tunnel breakdown of the semiconductor may occur. During this process, the resistance of the 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, holes and electrons are accelerated as they travel through the mean free path under the influence of a strong electric field to values that promote ionization of the atoms and breaking of one of the covalent bonds (main atom or impurity). Impact ionization occurs like an avalanche, and charge carriers multiply in it like an avalanche. In this case, the newly created holes and electrons are accelerated by an electric current. The final result of the current value is multiplied by the impact ionization coefficient, which is equal to the number of electron-hole pairs that are formed by the charge carrier along one path segment. The development of this process ultimately leads to an avalanche breakdown of the semiconductor. The resistance of semiconductors also changes, but, as in the case of tunnel breakdown, it is reversible.
Application 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 it were not for the desire to independently assemble an object using them. It is impossible to imagine the operation of modern refrigerators, televisions, and computer monitors without semiconductors. Advanced automotive developments cannot do without them. They are also used in aviation and space technology. Do you understand what semiconductors are and how important they are? Of course, we cannot say that these are the only irreplaceable elements for our civilization, but we should not underestimate them either.
The use of semiconductors in practice is also due to a number of factors, including the widespread availability of the materials from which they are made, the ease of processing and obtaining the desired result, and others technical features, thanks to which the choice of scientists who developed electronic equipment settled on them.
Conclusion
We looked in detail at what semiconductors are and how they work. Their resistance is based on 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 fully studied the features of their work. But we know their basic properties and characteristics, which allow us to use them in practice. Therefore, you can look for semiconductor materials and experiment with them yourself, being careful. Who knows, maybe there is a great explorer dormant within you?!
You, young friend, are a contemporary of the technical revolution in all areas of radio electronics. Its essence lies in the fact that vacuum tubes have been replaced by semiconductor devices, and they are now increasingly being replaced by microcircuits.
Ancestor of one of the most characteristic representatives The “army” of semiconductor devices - the transistor - was the so-called generating detector, invented back in 1922 by the Soviet radiophysicist O. V. Losev. This device, which is a semiconductor crystal with two wires adjacent to it - conductors, under certain conditions could generate and amplify electrical oscillations. But then, due to imperfections, it could not compete with an electron tube. A worthy semiconductor rival to the vacuum tube, called the transistor, was created in 1948 by American scientists Brattain, Bardeen and Shockley. In our country, a great contribution to the development of semiconductor devices was made by A.F. Ioffe, L.D. Landau, B.I. Davydova, V.E. Loshkarev and a number of other scientists and engineers, many scientific teams.
To understand the essence of the phenomena occurring in modern semiconductor devices, we will have to “look” into the structure of the semiconductor and understand the reasons for the formation of electric current in it. But before that, it would be good for you to remember that part of the first conversation where I talked about the structure of atoms.
SEMICONDUCTORS AND THEIR PROPERTIES
Let me remind you: in terms of electrical properties, semiconductors occupy a middle place between conductors and non-conductors of current. To what has been said, I will add that the group of semiconductors includes much more substances than the groups of conductors and non-conductors taken together. To semiconductors who have found practical application in technology, include germanium, silicon, selenium, cuprous oxide and some other substances. But for semiconductor devices, only germanium and silicon are mainly used.
What are the most characteristic properties semiconductors, distinguishing them from conductors and non-conductors of current? The electrical conductivity of semiconductors is highly dependent on the ambient temperature. At very low temperatures, close to absolute zero (-273°C), they behave as insulators in relation to electric current. Most conductors, on the contrary, at this temperature become superconducting, i.e. offer almost no resistance to current. As the temperature of conductors increases, their resistance to electric current increases, and the resistance of semiconductors decreases. The electrical conductivity of conductors does not change when exposed to light. The electrical conductivity of semiconductors under the influence of light, the so-called photoconductivity, increases. Semiconductors can convert light energy into electrical current. This is absolutely not typical for conductors. The electrical conductivity of semiconductors increases sharply when atoms of some other elements are introduced into them. The electrical conductivity of conductors decreases when impurities are introduced into them. These and some other properties of semiconductors have been known for a relatively long time, but they began to be widely used relatively recently.
Germanium and silicon, which are the starting materials of many modern semiconductor devices, each have four valence electrons in the outer layers of their shells. In total, there are 32 electrons in a germanium atom, and 14 in a silicon atom. But 28 electrons of a germanium atom and 10 electrons of a silicon atom, located in the inner layers of their shells, are firmly held by the nuclei and under no circumstances are separated from them. Only four valence electrons of the atoms of these semiconductors can, and even then not always, become free. Remember: four! A semiconductor atom that has lost at least one electron becomes a positive ion.
In a semiconductor, the atoms are arranged in a strict order: each atom is surrounded by four similar atoms. They are also located so close to each other that their valence electrons form single orbits passing around all neighboring atoms, binding them into a single substance. This relationship of atoms in a semiconductor crystal can be imagined in the form of a flat diagram, as shown in Fig. 72, a. Here, large balls with the “+” sign conventionally represent atomic nuclei with inner layers of electron shells (positive ions), and small balls represent valence electrons. Each atom, as you can see, is surrounded by four exactly the same atoms. Any of the atoms is connected to each neighboring one with two valence electrons, one of which is “its own”, and the second is borrowed from the “neighbor”. This is a two-electron, or valence, bond. The strongest connection!
Rice. 72. Diagram of the relationship of atoms in a semiconductor crystal (a) and a simplified diagram of its structure (b)
In turn, the outer layer of the electron shell of each atom contains eight electrons: four of its own and one each from four neighboring atoms. Here it is no longer possible to distinguish which of the valence electrons in the atom is “yours” and which is “foreign”, since they have become common. With such a connection of atoms throughout the entire mass of a germanium or silicon crystal, we can consider that the semiconductor crystal is one large molecule.
The diagram of the interconnection of atoms in a semiconductor can be simplified for clarity by depicting it as shown in Fig. 72, b. Here, the nuclei of atoms with inner electron shells are shown as circles with a plus sign, and interatomic bonds are shown as two lines symbolizing valence electrons.
What is a semiconductor and what is it eaten with?
Semiconductor- a material we can’t imagine without modern world technology and electronics. Semiconductors exhibit properties of metals and non-metals under certain conditions. According to the specific value electrical resistance semiconductors occupy an intermediate position between good conductors and dielectrics. Semiconductor differs from conductors in the strong dependence of specific conductivity on the presence of impurity elements (impurity elements) in the crystal lattice and the concentration of these elements, as well as on temperature and exposure to various types of radiation.
Basic property of a semiconductor- increase in electrical conductivity with increasing temperature.
Semiconductors are substances whose band gap is on the order of several electron volts (eV). For example, diamond can be classified as a wide-gap semiconductor, and indium arsenide can be classified as a narrow-gap semiconductor. The band gap is the width of the energy gap between the bottom of the conduction band and the top of the valence band, in which there are no allowed states for the electron.
The value of the band gap has important when generating light in LEDs and semiconductor lasers and determines the energy of emitted photons.
Semiconductors include many chemical elements: Si silicon, Ge germanium, As arsenic, Se selenium, Te tellurium and others, as well as all kinds of alloys and chemical compounds, for example: silicon iodide, gallium arsenide, mercury tellurite, etc.). In general, almost everything inorganic substances the world around us are semiconductors. The most common semiconductor in nature is silicon, which, according to rough estimates, makes up almost 30% of the earth's crust.
Depending on whether an atom of an impurity element gives up an electron or captures it, impurity atoms are called donor or acceptor atoms. The donor and acceptor properties of an atom of an impurity element also depend on which atom crystal lattice it replaces which crystallographic plane it is embedded in.
As mentioned above, the conductive properties of semiconductors strongly depend on temperature, and when the temperature reaches absolute zero (-273 ° C), semiconductors have the properties of dielectrics.
Based on the type of conductivity, semiconductors are divided into n-type and p-type
n-type semiconductor
Based on the type of conductivity, semiconductors are divided into n-type and p-type.
An n-type semiconductor has an impurity nature and conducts electric current like metals. Impurity elements that are added to semiconductors to produce n-type semiconductors are called donor elements. The term "n-type" comes from the word "negative", which refers to the negative charge carried by a free electron.
The theory of the charge transfer process is described as follows:
An impurity element, pentavalent As arsenic, is added to tetravalent Si silicon. During the interaction, each arsenic atom enters into a covalent bond with silicon atoms. But a fifth free arsenic atom remains, which has no place in saturated valence bonds, and it moves to a distant electron orbit, where less energy is needed to remove an electron from the atom. The electron breaks away and becomes free, capable of carrying charge. Thus, charge transfer is carried out by an electron, not a hole, that is, this type of semiconductor conducts electric current like metals.
Antimony Sb also improves the properties of one of the most important semiconductors - germanium Ge.
p-type semiconductor
A p-type semiconductor, in addition to the impurity base, is characterized by the hole nature of conductivity. The impurities that are added in this case are called acceptor impurities.
“p-type” comes from the word “positive,” which refers to the positive charge of the majority carriers.
For example, in a semiconductor, tetravalent Si silicon, not large number trivalent In indium atoms. In our case, indium will be an impurity element, the atoms of which establish a covalent bond with three neighboring silicon atoms. But silicon has one free bond while the indium atom does not have a valence electron, so it grabs a valence electron from covalent bond between neighboring silicon atoms and becomes a negatively charged ion, forming a so-called hole and, accordingly, a hole transition.
According to the same scheme, In ndium imparts hole conductivity to Ge germanium.
Investigating the properties of semiconductor elements and materials, studying the properties of contact between a conductor and a semiconductor, experimenting in the manufacture of semiconductor materials, O.V. Losev created the prototype of the modern LED in the 1920s.
In industry and energy microelectronics, various types semiconductors. With their help, one energy can be converted into another; without them, many electronic devices will not work normally. There are a large number of types of these elements, depending on the principle of their operation, purpose, material, and design features. In order to understand the mode of action of semiconductors, it is necessary to know their basic physical properties.
Properties and characteristics of semiconductors
The basic electrical properties of semiconductors allow them to be considered as a cross between standard conductors and materials that do not conduct electricity. The semiconductor group includes significantly more different substances than the total.
Widespread in electronics, semiconductors made from silicon, germanium, selenium and other materials were obtained. Their main characteristic is considered to be a pronounced dependence on the influence of temperature. At very low temperatures, comparable to absolute zero, semiconductors acquire the properties of insulators, and as the temperature rises, their resistance decreases while their conductivity increases. The properties of these materials can also change under the influence of light, when a significant increase in photoconductivity occurs.
Semiconductors convert light energy into electricity, unlike conductors, which do not have this property. In addition, the introduction of atoms of certain elements into the semiconductor contributes to an increase in electrical conductivity. All these specific properties allow the use of semiconductor materials in various fields electronics and electrical engineering.
Types and applications of semiconductors
Due to their qualities, all types of semiconductors are divided into several main groups.
Diodes. They include two crystals made of semiconductors with different conductivities. An electron-hole transition is formed between them. They are produced in various designs, mainly point and flat type. In planar cells, the germanium crystal is alloyed with indium. Point diodes consist of a silicon crystal and a metal needle.
Transistors. They consist of three crystalline semiconductors. Two crystals have the same conductivity, and in the third, the conductivity is opposite meaning. They are called collector, base and emitter. In electronics, amplifies electrical signals.
Thyristors. They are elements that convert electricity. They have three electron-hole junctions with gate properties. Their properties allow thyristors to be widely used in automation, computers, and control devices.
How does a semiconductor differ from insulators and conductors?