The difference between semiconductors and metals. Electrical materials: semiconductors, dielectrics, conductors, superconductors

Solids are metals, semiconductors and dielectrics. They differ from each other in their electronic properties. The electrical conductivity of solids is determined by the properties of electrons.

What are semiconductors and metals

Semiconductors related to metals and solids. These include germanium, silicon, arsenic, etc., as well as various alloys and chemical compounds.
Metals are solids that have a certain structure.

Comparison of semiconductors and metals

What is the difference between semiconductors and metals?
Semiconductors differ from metals by the mechanism electric current.
Consider how an electric current arises in semiconductors.
Germanium atoms have four weakly bound valence electrons in their outer shell. V crystal lattice there are four more around each atom. Atoms in a semiconductor crystal are bound by pairs of valence electrons. Each valence electron belongs to two atoms. If there is an increase in temperature, some of the valence electrons will receive energy that is sufficient to break covalent bonds. Free electrons, called conduction electrons, will appear in the crystal. At the same time, vacancies and holes are formed in place of the departed electrons. The vacant place can be occupied by the valence electrons of the neighboring pair, then the hole will be in a new place in the crystal. At a certain temperature, a certain number of electron-hole pairs exist in a semiconductor. A free electron meeting a hole restores electronic communication. Holes are like positively charged particles. If electric field no, holes and conduction electrons move randomly. If we place a semiconductor in an electric field, then holes and free electrons will begin to move in an orderly manner. Therefore, the current in a semiconductor is the sum of the electron and hole currents. The number of free charge carriers varies, does not remain constant and depends on temperature. As it increases, the resistance of semiconductors increases.
Metals have a crystalline structure. They are made up of molecules and atoms that occupy a specific, ordered position. A metal is represented as a crystal lattice, at the nodes of which there are atoms, or ions, or molecules that vibrate around their location. Between them in space are free electrons that randomly move in different directions. But when an electric field appears, they begin to move in an orderly direction towards the positive pole, an electric current appears in the metals. The number of electrons is constant. As the temperature decreases, the speed of electrons slows down, and the resistance of metals decreases.

ImGist determined that the difference between semiconductors and metals is as follows:

Semiconductors differ from metals in the mechanism of electric current.
Electric current in metals is the directed movement of electrons.
Pure semiconductors have an electron-hole mechanism of conduction.
The resistivity of semiconductors and metals depends on temperature in different ways.

Metal	Biological cycle on land	Mass of metal, 1 0 6 t/year
river runoff	Transfer - with dust from the continents to the water area	Transfer from water to land with precipitation	The biological cycle of ocean photosynthetics
soluble forms	suspended
Fe	34,0	27,4	963,0	65,0	0,132	47,3
Mn	35,0	0,41	20,5	4,0	0,176	0,99
Zn	5,2	0,82	5,86	0,90	0,240	4,40
Cu	1,3	0,28	1,51	0,11	0,141	0,77
Ni	0,34	0,12	1,58	0,18	0,057	0,33
SG	0,31	0,041	2,46	0,19	-	0,16
V	0,26	0,040	2,30	0,25	-	0,33
Рb	0,21	0,041	2,87	0,040	0,44	0,011
So	0,086	0,011	1,51	0,038	-	0,110
Mo	0,085	0,037	0,057	0,004	-	0,220
CD	0,008	0,009	0,013	0,0006	-	0,055
hg.	0,002	0,003	-	0,0008	-	0,017

Nai large quantity metals migrates in the system of a large biological cycle, which occurs due to the photosynthesis of land vegetation and the destruction of dying organic matter invertebrates and microorganisms of the pedosphere. Significant masses of metals are carried out as part of river suspensions, but this material almost completely goes into sediments when it enters. fresh water to the ocean system.

The involvement of heavy metals in the biological cycle on land is accompanied by a selective differentiation of their masses. There is no proportionality between the amount of metals in the earth's crust and the relative intensity of their absorption by vegetation. Biological absorption coefficient K 6 land vegetation for most metals is from 1 to 9, for zinc, molybdenum and silver - more than 9, for iron, vanadium and chromium - less than 1. As a result of the selective absorption of metals in the biomass of vegetation, the ratios of metals that exist in the earth's crust change markedly. The ratio of iron to other metals decreases especially strongly. The biological cycle and differentiation of metals carried out by ocean photosynthetics have their own characteristics. The masses of metals passing through biological cycles on land and in the ocean during the year are comparable, but their ratio is not the same. The vegetation of the world land captures more manganese and lead, the photosynthetic organisms of the ocean - more molybdenum and cobalt.

Large masses of water-soluble and suspended metal forms are carried from land to the ocean with river runoff. Water migration coefficient values K B metals indicate that soluble forms of silver, mercury, zinc are most actively involved in water migration. (K B > 10), as well as molybdenum, cadmium and copper, K B which are from 2 to 9. Forms of iron, manganese, chromium, vanadium, lead, cobalt fixed in suspensions are carried out in the amount of 97-98% of the total mass of metals carried out with river runoff. In addition, significant masses of metals fixed on dust particles are carried into the ocean by the wind.

In turn, water-soluble forms of metals are transported from the water area by air masses. This process is not well understood, and there are no data on the mass transfer of individual metals. Nevertheless, it is obvious that the migration flow of masses of heavy metals from the ocean to land is much less than in the opposite direction. For this reason, the annual cycles of metals in the land-ocean system are strongly open. Significant masses of metals accumulate in the water of the seas and oceans and go into precipitation. Re-engagement of metals from sedimentary strata into mass transfer cycles occurs as tectonic cycles develop. At the same time, the mobilization of metals from sedimentary rocks is often more difficult than from deep crystalline rocks.

From the surface of the ocean, gaseous organic metal compounds are released into the atmosphere. As noted in ch. 3, higher plants emit volatile organic compounds (terpenes, isoprenes) containing metals. Even larger masses of metals are released into the air as part of the gaseous metabolites of bacteria. especially important role play the processes of biomethylation of metals. The wind captures small soil particles, also containing metals, into the troposphere. All of the listed forms of metals are part of aerosols and are washed out by atmospheric precipitation.

In the system of mass transfer in the biosphere, the pedosphere plays the role of a global regulator of the movement of masses of heavy metals. In the process of transformation of organic matter, the metals that enter the soil are included in the composition of easily mobile complex compounds and are simultaneously firmly fixed in the stable components of soil humus. The most firmly fixed mercury, which forms very stable complexes with the functional groups of humic acids. Lead is strongly bound, copper is less firmly bound, and zinc and cadmium are weaker.

The close conjugation of the migration cycles of heavy metals, as well as the regulatory role of the pedosphere, ensures the high stability of the biosphere in relation to the influx of additional masses of metals of natural or technogenic origin.

Literature:

1. Fundamentals of biogeochemistry - V.V. Dobrovolsky, 2003

How are semiconductors different from metals?

Solids are metals, semiconductors and dielectrics. They differ from each other in their electronic properties. The electrical conductivity of solids is determined by the properties of electrons.

What are semiconductors and metals
Semiconductors are metals, solids. These include germanium, silicon, arsenic, etc., as well as various alloys and chemical compounds.

Metals are solids that have a specific structure.

Comparison of semiconductors and metals
What is the difference between semiconductors and metals?

Consider how an electric current arises in semiconductors.

Germanium atoms have four weakly bound valence electrons in their outer shell. There are four more atoms in the crystal lattice near each atom. Atoms in a semiconductor crystal are bound by pairs of valence electrons. Each valence electron belongs to two atoms. If there is an increase in temperature, some of the valence electrons will receive energy that is sufficient to break covalent bonds. Free electrons, called conduction electrons, will appear in the crystal. At the same time, vacancies and holes are formed in place of the departed electrons. The vacant place can be occupied by the valence electrons of the neighboring pair, then the hole will be in a new place in the crystal. At a certain temperature, a certain number of electron-hole pairs exist in a semiconductor. A free electron, meeting with a hole, restores the electronic bond. Holes are like positively charged particles. If there is no electric field, holes and conduction electrons move randomly. If we place a semiconductor in an electric field, then holes and free electrons will begin to move in an orderly manner. Therefore, the current in a semiconductor is the sum of the electron and hole currents. The number of free charge carriers varies, does not remain constant and depends on temperature. As it increases, the resistance of semiconductors increases.

Metals have a crystalline structure. They are made up of molecules and atoms that occupy a specific, ordered position. A metal is represented as a crystal lattice, at the nodes of which there are atoms, or ions, or molecules that vibrate around their location. Between them in space are free electrons that randomly move in different directions. But when an electric field appears, they begin to move in an orderly direction towards the positive pole, an electric current appears in the metals. The number of electrons is constant. As the temperature decreases, the speed of electrons slows down, and the resistance of metals decreases.

TheDifference.ru determined that the difference between semiconductors and metals is as follows:
Semiconductors differ from metals in the mechanism of electric current.
Electric current in metals is the directed movement of electrons.
Pure semiconductors have an electron-hole mechanism of conduction.
The resistivity of semiconductors and metals depends on temperature in different ways.

What is the difference between a dielectric and a conductor? In conductors, unlike dielectrics, a high concentration of free electric charges. In metals, these are free electrons that are able to move throughout the volume of the substance. The appearance of free electrons is due to the fact that valence electrons in metal atoms interact very poorly with nuclei and easily lose contact with them.

In dielectrics, on the contrary, electrons are tightly bound to atoms and are not able to move freely under the influence of an electric field. And since the number of free charged carriers in dielectrics is negligible, it follows that there is no electrostatic induction in them, and the electric field strength inside the dielectrics does not turn to zero, but only decreases.

The tension cannot be increased indefinitely, because at a certain value, all charges can shift so much that a change in the structure of the material occurs, in other words, a breakdown of the dielectric occurs. In this case, it will lose its insulating properties.

TheDifference.ru determined that the difference between a dielectric and a conductor is as follows:
In a conductor, free electrons, subjected to the influence of electric field forces, move throughout the volume.
Unlike a conductor, there are no free charges in a dielectric (insulator). Insulators are composed of neutral molecules or atoms. The charges in a neutral atom are strongly bound to each other and cannot move under the influence of an electric field throughout the entire volume of the dielectric.

According to their structure, PMs are divided into crystalline, amorphous, and liquid. A number of organic substances also exhibits semiconductor properties and constitutes an extensive group of organic semiconductors. The most important are inorganic. crystalline P. m., to-rye on chemical. composition are divided into elementary, double, triple and quadruple chemical. compounds, solutions and alloys. Semiconductor compounds are classified according to the numbers of periodic groups. tab. elements, to which the elements included in their composition belong.

The main groups of crystalline semiconductor materials (see Table 1):

1. Elementary diamonds: Ge, Si, C (diamond ) , V, Sn, Te, Se, etc. The most important representatives of this group are Ge and Si - main. materials of semiconductor electronics. Possessing 4 valence electrons, Ge and Si atoms form crystalline. a diamond-type lattice, where each atom has 4 nearest neighbors, with each of which is associated covalent bond(coordination of neighbors - tetrahedral). They form among themselves a continuous series of solid solutions, which are also important molecular weights.

2. Connections of the type Have in the main. crystalline sphalerite structure. Communication of atoms in a crystal. the lattice wears preim. covalent character with a certain proportion (5-15%) of the ionic component (see Chemical bond). The most important representatives of this group: GaAs, InP, InAs, InSb, GaP. Mn. P. m.

form among themselves a continuous series of solid solutions of triple p more complex (n etc.), which are also important P. m. (see. heterojunction, heterostrtsk-tura).

3. Compounds of elements VI g r y and -p y (O, S, Se, Te) with elements of groups I - V, as well as with transition and rare earth metals. Among these P. m. Naib, compounds of the type are of interest. They have crystalline structure like sphalerite or wurtzite, less often - like NaCl. The bond between atoms has a covalent nature (the share of the ionic component is about 45-60%). Polymorphism and the presence of polytypes of cubic and hexagonal modifications are characteristic of the polymorphic type. The most important representatives: CdTe, CdS, ZnTe, ZnSe, ZnO, ZnS. Mn. P. m. type form a continuous series of solid solutions among themselves; the most important of them:

Phys. properties in a mean, the extent determined by the concentration of its own point defects of the structure, showing electric. activity (centers of scattering and recombination).

Compounds of the type have crystalline. structure like NaCl or orthorhombic. The bond between atoms is covalent-ionic. Typical representatives: PbS, PbTe, SnTe. They form a continuous series of solid solutions among themselves, among them the most important are Int. point structure defects in have a low ionization energy and exhibit electric. activity.

Compounds of the type have crystalline. a sphalerite-type structure with unfilled cation sites. In terms of their properties, they occupy an intermediate position between and. They are characterized by low lattice thermal conductivity and carrier mobility charge . Typical representatives:

In semiconductors, when the temperature changes, not only the sub-

mobility, but also the concentration of charge carriers. If you raise the temperature

pure semiconductor, then some of the atoms are ionized, as a result of which

there are free electrons and holes in the same number. Addiction

concentration of electrons and holes on temperature is determined by the formula:

Hole[edit | edit source]

Main article: Hole

When the bond between the electron and the nucleus is broken, a free space appears in the electron shell of the atom. This causes the transfer of an electron from another atom to an atom with free space. The atom, from which the electron has passed, enters another electron from another atom, etc. This process is determined by the covalent bonds of atoms. Thus, there is a movement of a positive charge without moving the atom itself. This conditional positive charge is called a hole.

Usually the mobility of holes in a semiconductor is lower than the mobility of electrons.

Free electrons and holes are called charge carriers, since their directed movement leads to the appearance of a current in the semiconductor. The process of appearance in the semiconductor of free electrons in the conduction band and holes in the valence band, caused by heating of the semiconductor, is called thermal generation of charge carriers. The process of return of free electrons from the conduction band to the valence band, associated with the disappearance of charge carriers, is called recombination. In semiconductor materials, a dynamic equilibrium is established between the processes of thermal generation and recombination of charge carriers, at which the concentration of charge carriers, i.e., the number of free electrons in the conduction band and holes in the valence band per 1 cm3 of the semiconductor, remains unchanged at a constant temperature of the semiconductor.

The process of formation of a pair "conduction electron - conduction hole" is called the generation of a pair of charge carriers (1 to 16.6). We can say that the intrinsic electrical conductivity of a semiconductor is the electrical conductivity caused by the generation of "conduction electron - conduction hole" pairs. The resulting electron-hole pairs can disappear if the hole is filled with an electron: the electron will become non-free and lose the ability to move, and the excess positive charge of the ion of the atom will be neutralized. In this case, both the hole and the electron disappear simultaneously. The process of reuniting an electron and a hole is called recombination (2 by 16.6). Recombination, in accordance with the band theory, can be considered as the transition of electrons from the conduction band to free places in the valence band. Note that the transition of electrons from a higher energy level to a lower one is accompanied by the release of energy, which is either emitted in the form of light quanta (photons) or transferred to the crystal lattice in the form of thermal vibrations (phonons).

Impurity conductivity of semiconductors - electrical conductivity due to the presence of donor or acceptor impurities in the semiconductor.

Impurity conductivity, as a rule, greatly exceeds its own, and therefore the electrical properties of semiconductors are determined by the type and amount of dopants introduced into it.

The intrinsic conductivity of semiconductors is usually low, since the number of free electrons, for example, in germanium at room temperature about 3 10 13 / cm 3. At the same time, the number of germanium atoms in 1 cm 3 is ~ 10 23 . The conductivity of semiconductors increases with the introduction of impurities, when, along with intrinsic conductivity, an additional impurity conductivity arises.

Impurity conductivity of semiconductors is the conductivity due to the presence of impurities in the semiconductor.

Impurity centers can be:

1. atoms or ions chemical elements embedded in the semiconductor lattice;

2. excess atoms or ions embedded in lattice interstices;

3. various other defects and distortions in the crystal lattice: empty nodes, cracks, shifts that occur during crystal deformations, etc.

By changing the concentration of impurities, one can significantly increase the number of charge carriers of one sign or another and create semiconductors with a predominant concentration of either negatively or positively charged carriers.

Impurities can be divided into donor (donating) and acceptor (receiving).

Let us consider the mechanism of electrical conductivity of a semiconductor with a donor pentavalent arsenic impurity AS 5+, which is introduced into the crystal, for example, silicon. A pentavalent arsenic atom donates four valence electrons to form covalent bonds, and the fifth electron is unoccupied in these bonds.

The detachment energy (ionization energy) of the fifth valence electron of arsenic in silicon is 0.05 eV = 0.08·10 -19 J, which is 20 times less than the energy of detachment of an electron from a silicon atom. Therefore, already at room temperature, almost all arsenic atoms lose one of their electrons and become positive ions. Positive arsenic ions cannot capture the electrons of neighboring atoms, since all four of their bonds are already equipped with electrons. In this case, there is no movement of the electron vacancy - "hole" and the hole conductivity is very low, i.e. practically absent. A small part of the semiconductor's own atoms is ionized, and part of the current is generated by holes, i.e. donor impurities are impurities that supply conduction electrons without the appearance of an equal number of mobile holes. As a result, we get a semiconductor with predominantly electronic conductivity, called an n-type semiconductor.

In the case of an acceptor impurity, for example, trivalent indium In 3+ an impurity atom can donate its three electrons to carry out covalent bond with only three neighboring silicon atoms, and one electron is “missing”. One of the electrons of neighboring silicon atoms can fill this bond, then the In atom will become an immobile negative ion, and a hole will form in place of the electron that left one of the silicon atoms. Acceptor impurities, capturing electrons and thereby creating mobile holes, do not increase the number of conduction electrons. Major charge carriers in a semiconductor with an acceptor impurity are holes, and minority carriers are electrons.

Semiconductors are a wide class of substances characterized by electrical conductivity values ​​that lie in the range between the electrical conductivity of metals and good dielectrics, that is, these substances cannot be classified as dielectrics (since they are not good insulators) and metals (they are not good conductors of electricity). Semiconductors, for example, include substances such as germanium, silicon, selenium, tellurium, as well as some oxides, sulfides and metal alloys.

Properties:

1) With increasing temperature, the resistivity of semiconductors decreases, in contrast to metals, in which the resistivity increases with increasing temperature. Moreover, as a rule, in a wide temperature range, this increase occurs exponentially. The resistivity of semiconductor crystals can also decrease when exposed to light or strong electronic fields.

2) The property of one-sided conduction of the contact of two semiconductors. It is this property that is used to create a variety of semiconductor devices: diodes, transistors, thyristors, etc.

3) Contacts of various semiconductors under certain conditions, when illuminated or heated, are sources of photo-e. d.s. or, respectively, thermo-e. d.s.

Semiconductors differ from other classes of solids in many specific features, the most important of which are:

1) positive temperature coefficient of electrical conductivity, that is, with increasing temperature, the electrical conductivity of semiconductors increases;

2) the specific conductivity of semiconductors is less than that of metals, but more than that of insulators;

3) big values thermoelectromotive force compared to metals;

4) high sensitivity of semiconductor properties to ionizing radiation;

5) the ability to abrupt change physical properties under the influence of negligibly small concentrations of impurities;

6) the effect of current rectification or non-ohmic behavior on the contacts.

3. Physical processes in p-n - transition.

The main element of most semiconductor devices is the electron-hole junction ( district junction), which is a transition layer between two regions of a semiconductor, one of which has electronic electrical conductivity, and the other has hole conductivity.

Education pn transition. Pn equilibrium transition

Let's take a closer look at the education process pn transition. The equilibrium state is called such a transition state when there is no external voltage. Recall that in R- region there are two types of main charge carriers: immobile negatively charged ions of acceptor impurity atoms and free positively charged holes; and in n-region there are also two types of main charge carriers: immobile positively charged ions of acceptor impurity atoms and free negatively charged electrons.

Before touch p and n regions, electrons, holes, and impurity ions are uniformly distributed. On contact at the border p and n regions, a concentration gradient of free charge carriers and diffusion arise. Under the action of diffusion, electrons from n-area goes into p and recombines there with holes. holes from R-areas go to n region and recombine with electrons there. As a result of such a movement of free charge carriers in the boundary region, their concentration decreases almost to zero and, at the same time, R region, a negative space charge of acceptor impurity ions is formed, and in n-region positive space charge of donor impurity ions. Between these charges there is a contact potential difference φ to and electric field E to, which prevents the diffusion of free charge carriers from the depth R- and n- areas through p-n- transition. Thus, the region united by free charge carriers with its electric field is called p-n- transition.

Pn The transition is characterized by two main parameters:

1. Potential barrier height. It is equal to the contact potential difference φ to. This is the potential difference in the transition due to the concentration gradient of charge carriers. This is the energy that a free charge must have in order to overcome the potential barrier:

where k is the Boltzmann constant; e is the electron charge; T- temperature; N a and N D are the concentrations of acceptors and donors in the hole and electron regions, respectively; p p and p n are the concentrations of holes in R- and n- areas respectively; n i - own concentration of charge carriers in an undoped semiconductor,  t \u003d kT / e- temperature potential. At a temperature T\u003d 27 0 С  t=0.025V, for germanium transition  to=0.6V, for silicon junction  to\u003d 0.8V.

2. p-n junction width(Fig. 1) is a border region depleted in charge carriers, which is located in p and n areas: l p-n = l p + l n:

From here,

where ε is the relative permittivity of the semiconductor material; ε 0 is the dielectric constant of free space.

The thickness of electron-hole transitions is of the order of (0.1-10) µm. If , then and pn-transition is called symmetric, if , then and pn- transition is called asymmetric, and it is mainly located in the region of the semiconductor with a lower impurity concentration.

In the equilibrium state (without external voltage) through district transition, two counter currents of charges move (two currents flow). These are the drift current of minority charge carriers and the diffusion current, which is associated with the majority charge carriers. Since there is no external voltage, and there is no current in the external circuit, the drift current and diffusion current are mutually balanced and the resulting current zero

I dr + I diff = 0.

This relation is called the condition of dynamic equilibrium of diffusion and drift processes in an isolated (equilibrium) pn-transition.

The surface on which they are in contact p and n area is called the metallurgical boundary. In reality, it has a finite thickness - δ m. If δ m<< l p-n , then pn The transition is called a sharp one. If δ m >> lp-n, then pn The transition is called smooth.

Р-n transition at an external voltage applied to it

External voltage disturbs the dynamic balance of currents in pn-transition. Pn- the transition goes into a non-equilibrium state. Depending on the polarity of the voltage applied to the areas in pn-transition possible two modes of operation.

1) Forward biaspn transition. R-n- the junction is considered to be forward biased if the positive pole of the power supply is connected to R-region, and negative to n- areas (Fig. 1.2)

With forward bias, the voltages  to and U are directed oppositely, the resulting voltage on pn-transition decreases to the value  to - U. This leads to the fact that the electric field strength decreases and the process of diffusion of the main charge carriers resumes. In addition, forward offset reduces the width pn transition, because lp-n ≈( to - U) 1/2. The diffusion current, the current of the main charge carriers, becomes much larger than the drift current. Across pn-transition direct current flows

I p-n \u003d I pr \u003d I diff + I dr I differential .

When a direct current flows, the majority charge carriers in the p-region pass into the n-region, where they become minor. The diffusion process of introducing majority charge carriers into a region where they become minority is called injection, and direct current - diffusion current or injection current. To compensate for the minority charge carriers accumulated in the p and n regions, an electron current is generated in the external circuit from a voltage source, i.e. the principle of electroneutrality is preserved.

With an increase U the current increases sharply, - the temperature potential, and can reach large values. associated with the main carriers, the concentration of which is high.

2) reverse bias, occurs when R-area is applied a minus, and to n-area plus, an external voltage source (Fig. 1.3).

This external tension U included according to  to. It: increases the height of the potential barrier to a value  to + U; the electric field strength increases; width pn transition increases, because l p-n ≈( to + U) 1/2; the diffusion process stops completely and after pn transition flows drift current, minority carrier current. Such a current pn-transition is called reverse, and since it is associated with minor charge carriers that arise due to thermal generation, it is called thermal current and denoted - I 0, i.e.

I p-n \u003d I arr \u003d I diff + I dr I dr \u003d I 0.

This current is small in magnitude. associated with minority charge carriers, the concentration of which is low. In this way, pn transition has one-sided conductivity.

With a reverse bias, the concentration of minority charge carriers at the transition boundary somewhat decreases compared to the equilibrium one. This leads to the diffusion of minority charge carriers from the depth p and n-areas to the border pn transition. Having reached it, minority carriers fall into a strong electric field and are transferred through pn transition, where they become the majority charge carriers. Diffusion of minor charge carriers to the boundary pn transition and drift through it to the region where they become the main charge carriers is called extraction. Extraction and creates a reverse current pn transition is the current of minor charge carriers.

The magnitude of the reverse current is highly dependent on: temperature environment, semiconductor material and area pn transition.

The temperature dependence of the reverse current is determined by the expression , where is the nominal temperature, is the actual temperature, is the doubling temperature of the thermal current.

The thermal current of the silicon junction is much less than the thermal current of the germanium-based junction (by 3–4 orders of magnitude). It's connected with  to material.

With an increase in the transition area, its volume increases, and, consequently, the number of minority carriers appearing as a result of thermal generation and the thermal current increase.

So the main property pn-transition is its one-way conduction.

4. Current-voltage characteristic p-n - transition.

Get the volt-ampere characteristic p-n junction. To do this, we write the continuity equation in general view:

We will consider the stationary case dp/dt = 0.

Consider the current in the quasi-neutral volume of an n-type semiconductor to the right of the depleted area p-n transition (x > 0). The generation rate G in a quasi-neutral volume is zero: G = 0. The electric field E is also zero: E = 0. The drift component of the current is also zero: I E = 0, therefore, the current is diffusion. The recombination rate R at a low injection level is described by the relation:

Let us use the following relationship relating the diffusion coefficient, diffusion length, and minority carrier lifetime: Dτ = L p 2 .

Taking into account the above assumptions, the continuity equation has the form:

The boundary conditions for the diffusion equation in the p-n junction are:

Solution differential equation(2.58) with boundary conditions (*) has the form:

Relation (2.59) describes the law of distribution of injected holes in the quasi-neutral volume of an n-type semiconductor for an electron-hole transition (Fig. 2.15). All carriers that have crossed the SCR boundary with a quasi-neutral volume of the p-n junction take part in the p-n junction current. Since the entire current is diffusion, substituting (2.59) into the expression for the current, we obtain (Fig. 2.16):

Relation (2.60) describes the diffusion component of the p-n junction hole current, which arises during the injection of minority carriers under forward bias. For the electronic component of the p-n junction current, we similarly obtain:

At V G = 0, the drift and diffusion components balance each other. Hence, .

Full p-n current junction is the sum of all four components of the p-n junction current:

The expression in brackets has the physical meaning of the reverse current of the p-n junction. Indeed, at negative voltages V G< 0 ток дрейфовый и обусловлен неосновными носителями. Все эти носители уходят из цилиндра длиной L n со скоростью L n /τ p . Тогда для дрейфовой компоненты тока получаем:

Rice. 2.15. Distribution of nonequilibrium carriers injected from the emitter over the quasi-neutral volume p-n bases transition

It is easy to see that this relation is equivalent to that obtained earlier in the analysis of the continuity equation.

If it is required to realize the condition of one-sided injection (for example, only injection of holes), then it follows from relation (2.61) that a small value of the concentration of minority carriers n p0 in the p-region should be chosen. It follows that the p-type semiconductor must be heavily doped compared to the n-type semiconductor: N A >> N D . In this case, the hole component will dominate in the p-n junction current (Fig. 2.16).

Rice. 2.16. Currents in a single-ended p-n junction with forward bias

Thus, the I–V characteristic of the p-n junction has the form:

The saturation current density J s is:

CVC p-n transition, described by relation (2.62), is shown in Figure 2.17.

Rice. 2.17. Volt-ampere characteristics perfect p-n transition

As follows from relation (2.16) and Figure 2.17, the current-voltage characteristic of an ideal p-n junction has a pronounced asymmetric form. In the region of direct voltages, the current of the p-n junction is diffusion and exponentially increases with increasing applied voltage. In the region of negative voltages, the p-n junction current is drift and does not depend on the applied voltage.

5. Capacitance p-n - junction.

Any system in which the electric charge Q changes when the potential φ changes has a capacitance. The capacitance value C is determined by the ratio: .

For the p-n junction, two types of charges can be distinguished: the charge in the region of the space charge of ionized donors and acceptors Q B and the charge of injected carriers into the base from the emitter Q p . With different biases on the p-n junction, one or another charge will dominate when calculating the capacitance. In this regard, for the capacitance of the p-n junction, barrier capacitance C B and diffusion capacitance C D are distinguished.

Barrier capacitance C B is capacitance p-n transition under reverse bias V G< 0, обусловленная изменением заряда ионизованных доноров в области пространственного заряда.

The charge value of ionized donors and acceptors Q B per unit area for an asymmetric p-n junction is:

Differentiating expression (2.65), we obtain:

It follows from equation (2.66) that the barrier capacitance C B is the capacitance of a flat capacitor, the distance between the plates of which is equal to the width of the space charge region W. Since the SCR width depends on the applied voltage V G, the barrier capacitance also depends on the applied voltage. Numerical estimates of the barrier capacitance show that its value is tens or hundreds of picofarads.

Diffusion capacitance C D is the capacitance of a p-n junction at a forward bias V G > 0, due to a change in the charge Q p of the injected carriers into the base from the emitter Q p .

The dependence of the barrier capacitance C B on the applied reverse voltage V G is used for instrumental implementation. A semiconductor diode that implements this dependence is called a varicap. Maximum value capacitance varicap has at zero voltage V G . As the reverse bias increases, the capacitance of the varicap decreases. The functional dependence of the varicap capacitance on voltage is determined by the doping profile of the varicap base. In the case of uniform doping, the capacitance is inversely proportional to the root of the applied voltage V G . By setting the doping profile in the varicap base N D (x), one can obtain various dependencies varicap capacitances on voltage C(V G) - linearly decreasing, exponentially decreasing.

6. Semiconductor diodes: classification, design features, conventions and marking.

semiconductor diode- a semiconductor device with one electrical junction and two leads (electrodes). Unlike other types of diodes, the principle of operation of a semiconductor diode is based on the phenomenon pn-transition.

It is known that in a substance placed in an electric field, under the influence of the forces of this field, the movement of free electrons or ions is formed in the direction of the field forces. In other words, an electric current occurs in the substance.

The property that determines the ability of a substance to conduct an electric current is called "electrical conductivity". The electrical conductivity is directly dependent on the concentration of charged particles: the higher the concentration, the higher the electrical conductivity.

According to this property, all substances are divided into 3 types:

1. Conductors.
2. Semiconductors.

Description of conductors

Conductors have highest electrical conductivity from all types of substances. All conductors are divided into two large subgroups:

• Metals(copper, aluminum, silver) and their alloys.
• electrolytes (water solution salts, acids).

In substances of the first subgroup, only electrons are able to move, since their connection with the nuclei of atoms is weak, and therefore, they are quite simply detached from them. Since the occurrence of current in metals is associated with the movement of free electrons, the type of electrical conductivity in them is called electronic.

Of the conductors of the first subgroup, they are used in the windings of electric machines, power lines, wires. It is important to note that the electrical conductivity of metals is affected by its purity and the absence of impurities.

In substances of the second subgroup, when exposed to a solution, the molecule breaks up into positive and negative ion. Ions move due to the action of an electric field. Then, when the current passes through the electrolyte, ions are deposited on the electrode, which is lowered into this electrolyte. The process when a substance is released from an electrolyte under the influence of an electric current is called electrolysis. The electrolysis process is usually used, for example, when a non-ferrous metal is extracted from a solution of its compound, or when coating a metal protective layer other metals.

Description of dielectrics

Dielectrics are also commonly referred to as electrical insulators.

All electrical insulating substances have the following classification:

• Depending on the state of aggregation, dielectrics can be liquid, solid and gaseous.
• Depending on the methods of obtaining - natural and synthetic.
• Depending on the chemical composition- organic and inorganic.
• Depending on the structure of the molecules - neutral and polar.

These include gas (air, nitrogen, SF6 gas), mineral oil, any rubber and ceramic substance. These substances are characterized by the ability to polarization in electric field . Polarization is the formation of charges with different signs on the surface of a substance.

Dielectrics contain a small number of free electrons, while the electrons have a strong bond with the nuclei of atoms and only in rare cases disconnect from them. This means that these substances do not have the ability to conduct current.

This property is very useful in the field of production of products used in protection against electric current: dielectric gloves, rugs, boots, insulators on electrical equipment etc.

About semiconductors

The semiconductor acts as intermediate substance between conductor and dielectric. by the most prominent representatives this type of substances are silicon, germanium, selenium. In addition, it is customary to refer to these substances the elements of the fourth group of the periodic table of Dmitry Ivanovich Mendeleev.

Semiconductors have additional "hole" conduction in addition to electronic conduction. This type of conductivity depends on a number of factors. external environment, including light, temperature, electric and magnetic fields.

These substances have weak covalent bonds. When exposed to one of external factors the bond is broken, after which free electrons are formed. At the same time, when an electron is detached, a free "hole" remains in the composition of the covalent bond. Free "holes" attract neighboring electrons, and so this action can be performed indefinitely.

It is possible to increase the conductivity of semiconductor substances by introducing various impurities into them. This technique is widely used in industrial electronics: in diodes, transistors, thyristors. Let us consider in more detail the main differences between conductors and semiconductors.

What is the difference between a conductor and a semiconductor?

The main difference between a conductor and a semiconductor is the ability to conduct electric current. At the conductor it is an order of magnitude higher.

When the temperature value rises, the conductivity of semiconductors also increases; the conductivity of the conductors decreases with increasing.

In pure conductors, under normal conditions, the passage of current releases a much larger number of electrons than in semiconductors. At the same time, the addition of impurities reduces the conductivity of conductors, but increases the conductivity of semiconductors.