Eryutkin Evgeniy Sergeevich
higher physics teacher qualification category GOU secondary school No. 1360, Moscow
If you make a direct connection, then the external field will neutralize the blocking field, and the current will be carried by the main charge carriers.
Rice. 9. p-n junction when connected directly ()
In this case, the minority carrier current is negligible, practically non-existent. Therefore, the p-n junction provides one-way conduction of electric current.
Rice. 10. Atomic structure of silicon with increasing temperature
The conductivity of semiconductors is electron-hole, and such conductivity is called intrinsic conductivity. And unlike conductor metals, as the temperature increases, the number of free charges increases (in the first case it does not change), therefore the conductivity of semiconductors increases with increasing temperature, and the resistance decreases
Very important issue in the study of semiconductors is the presence of impurities in them. And in the case of the presence of impurities, we should talk about impurity conductivity.
Small size and very high quality of transmitted signals have made semiconductor devices very common in modern electronic technology. The composition of such devices may include not only the aforementioned silicon with impurities, but also, for example, germanium.
One such device is a diode - a device that can pass current in one direction and prevent it from passing in the other. It is obtained by implanting a different type of semiconductor into a p- or n-type semiconductor crystal.
Rice. 11. Designation of the diode on the diagram and the diagram of its device, respectively
Another device, now with two p-n junctions, is called a transistor. It serves not only to select the direction of current transmission, but also to transform it.
Rice. 12. Diagram of the structure of the transistor and its designation on electrical diagram respectively ()
It should be noted that modern microcircuits use many combinations of diodes, transistors and other electrical devices.
In the next lesson we will look at the propagation of electric current in a vacuum.
- Tikhomirova S.A., Yavorsky B.M. Physics ( basic level) M.: Mnemosyne. 2012
- Gendenshtein L.E., Dick Yu.I. Physics 10th grade. M.: Ilexa. 2005
- Myakishev G.Ya., Sinyakov A.Z., Slobodskov B.A. Physics. Electrodynamics M.: 2010
- Principles of operation of devices ().
- Encyclopedia of Physics and Technology ().
- What causes conduction electrons to appear in a semiconductor?
- What is the intrinsic conductivity of a semiconductor?
- How does the conductivity of a semiconductor depend on temperature?
- How does a donor impurity differ from an acceptor impurity?
- *What is the conductivity of silicon with an admixture of a) gallium, b) indium, c) phosphorus, d) antimony?
Semiconductors are a class of substances whose conductivity increases with increasing temperature and decreases. electrical resistance. This is how semiconductors fundamentally differ from metals.
Typical semiconductors are crystals of germanium and silicon, in which the atoms are united by a covalent bond. At any temperature, semiconductors contain free electrons. Free electrons under the influence of external electric field can move in the crystal, creating electron current conductivity. Removing an electron from the outer shell of one of the atoms crystal lattice leads to the transformation of this atom into positive ion. This ion can neutralize itself by capturing an electron from one of its neighboring atoms. Further, as a result of the transition of electrons from atoms to positive ions, a process of chaotic movement of the place with the missing electron occurs in the crystal. Externally, this process is perceived as a movement of positive electric charge called hole.
When a crystal is placed in an electric field, an ordered movement of holes occurs - hole conduction current.
In an ideal semiconductor crystal, electric current is created by the movement of equal numbers of negatively charged electrons and positively charged holes. Conduction in ideal semiconductors is called intrinsic conductivity.
The properties of semiconductors are highly dependent on the impurity content. There are two types of impurities - donor and acceptor.
Impurities that donate electrons and create electronic conductivity are called donor(impurities having a valence greater than that of the main semiconductor). Semiconductors in which the concentration of electrons exceeds the concentration of holes are called n-type semiconductors.
Impurities that capture electrons and thereby create mobile holes without increasing the number of conduction electrons are called acceptor(impurities having a valency less than that of the main semiconductor).
At low temperatures The main current carriers in a semiconductor crystal with an acceptor impurity are holes, and not the main carriers are electrons. Semiconductors in which the concentration of holes exceeds the concentration of conduction electrons are called hole semiconductors or p-type semiconductors. Consider the contact of two semiconductors with various types conductivity.
Mutual diffusion of majority carriers occurs across the boundary of these semiconductors: electrons from the n-semiconductor diffuse into the p-semiconductor, and holes from the p-semiconductor into the n-semiconductor. As a result, the section of the n-semiconductor adjacent to the contact will be depleted of electrons, and an excess positive charge, due to the presence of bare impurity ions. The movement of holes from a p-semiconductor to an n-semiconductor leads to the appearance of excess negative charge in the boundary region of the p-semiconductor. As a result, an electric double layer is formed, and a contact electric field arises, which prevents further diffusion of the main charge carriers. This layer is called locking.
An external electric field affects the electrical conductivity of the barrier layer. If the semiconductors are connected to the source as shown in Fig. 55, then under the influence of an external electric field the main charge carriers - free electrons in the p-semiconductor and holes in the p-semiconductor - will move towards each other towards the semiconductor interface, while the thickness of the p-n junction decreases, therefore, its resistance decreases. In this case, the current is limited by external resistance. This direction of the external electric field is called direct. Direct connection of the p-n junction corresponds to section 1 on the current-voltage characteristic (see Fig. 57).
Electric current carriers in different environments and current-voltage characteristics are summarized in table. 1.
If the semiconductors are connected to the source as shown in Fig. 56, then electrons in the n-semiconductor and holes in the p-semiconductor will move under the influence of an external electric field from the boundary to opposite sides. The thickness of the barrier layer and, therefore, its resistance increases. With this direction of the external electric field - reverse (blocking), only minority charge carriers pass through the interface, the concentration of which is much less than the majority ones, and the current is almost equal to zero. The reverse switching on of the pn junction corresponds to section 2 on the current-voltage characteristic (Fig. 57).
Semiconductors are substances that have electrical conductivity intermediate position between good conductors and good insulators (dielectrics).
Semiconductors are chemical elements(germanium Ge, silicon Si, selenium Se, tellurium Te), and compounds of chemical elements (PbS, CdS, etc.).
The nature of current carriers in different semiconductors is different. In some of them, the charge carriers are ions; in others, the charge carriers are electrons.
Intrinsic conductivity of semiconductors
There are two types of intrinsic conductivity of semiconductors: electronic conductivity and hole conductivity of semiconductors.
1. Electronic conductivity of semiconductors.
Electronic conductivity is carried out by the directed movement in the interatomic space of free electrons that have left the valence shell of the atom as a result of external influences.
2. Hole conductivity of semiconductors.
Hole conduction occurs with directed movement valence electrons for vacant places in pair-electronic bonds - holes. The valence electron of a neutral atom located in close proximity to a positive ion (hole) is attracted to the hole and jumps into it. In this case, a positive ion (hole) is formed in place of a neutral atom, and a neutral atom is formed in place of a positive ion (hole).
In an ideally pure semiconductor without any foreign impurities, each free electron corresponds to the formation of one hole, i.e. the number of electrons and holes involved in creating the current is the same.
Conductivity at which occurs same number charge carriers (electrons and holes) is called the intrinsic conductivity of semiconductors.
The intrinsic conductivity of semiconductors is usually low, since the number of free electrons is small. The slightest traces of impurities radically change the properties of semiconductors.
Electrical conductivity of semiconductors in the presence of impurities
Impurities in a semiconductor are considered to be atoms of foreign chemical elements that are not contained in the main semiconductor.
Impurity conductivity is the conductivity of semiconductors due to the introduction of impurities into their crystal lattices.
In some cases, the influence of impurities is manifested in the fact that the “hole” conduction mechanism becomes practically impossible, and the current in the semiconductor is carried out mainly by the movement of free electrons. Such semiconductors are called electronic semiconductors or n-type semiconductors(from Latin word negativus - negative). The majority charge carriers are electrons, and the non-majority charge carriers are holes. N-type semiconductors are semiconductors with donor impurities.
1. Donor impurities.
Impurities that easily donate electrons and, therefore, increase the number of free electrons are called donor impurities. Donor impurities supply conduction electrons without creating the same number of holes.
A typical example The donor impurities in tetravalent germanium Ge are pentavalent arsenic atoms As.
In other cases, the movement of free electrons becomes practically impossible, and the current is carried out only by the movement of holes. These semiconductors are called hole semiconductors or p-type semiconductors(from the Latin word positivus - positive). The main charge carriers are holes, not the main charge carriers - electrons. . P-type semiconductors are semiconductors with acceptor impurities.
Acceptor impurities are impurities in which there are not enough electrons to form normal pair-electron bonds.
An example of an acceptor impurity in germanium Ge is trivalent gallium atoms Ga
Electric current through the contact of p-type and n-type semiconductors, a p-n junction is a contact layer of two impurity semiconductors of p-type and n-type; A pn junction is the boundary separating regions with hole (p) conductivity and electron (n) conductivity in the same single crystal.
Direct p-n junction
If an n-semiconductor is connected to the negative pole of the power source, and the positive pole of the power source is connected to the p-semiconductor, then under the influence of an electric field, electrons in the n-semiconductor and holes in the p-semiconductor will move towards each other towards the interface of the semiconductors. Electrons, crossing the boundary, “fill” holes, the current through the p-n junction is carried out by the main charge carriers. As a result, the conductivity of the entire sample increases. With such a forward (through) direction of the external electric field, the thickness of the blocking layer and its resistance decrease.
In this direction, the current passes through the boundary of the two semiconductors.
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Reverse pn junction
If an n-semiconductor is connected to the positive pole of the power source, and a p-semiconductor is connected to the negative pole of the power source, then electrons in the n-semiconductor and holes in the p-semiconductor under the influence of the electric field will move from the interface in opposite directions, the current through p The -n junction is carried out by minority charge carriers. This leads to a thickening of the barrier layer and an increase in its resistance. As a result, the conductivity of the sample turns out to be insignificant, and the resistance is large.
A so-called barrier layer is formed. With this direction external field practically no electric current passes through the contact of p- and n-semiconductors.
Thus, the electron-hole transition has one-way conductivity.
Dependence of current on voltage - volt - ampere characteristic р-n junction shown in the figure (volt-ampere characteristic direct p-n transition is shown as a solid line, volt-ampere characteristic reverse p-n transition is shown with a dotted line).
Semiconductor devices:
Semiconductor diode - for rectification AC, it uses one p-n junction with different resistances: in forward direction The resistance of the p-n junction is significantly less than in the reverse.
Photoresistors - for recording and measuring weak light fluxes. With their help, they determine the quality of surfaces and control the dimensions of products.
Thermistors - for remote temperature measurement, fire alarms.
In semiconductors, this is the directed movement of holes and electrons, which is influenced by an electric field.
As a result of the experiments, it was noted that the electric current in semiconductors is not accompanied by the transfer of matter - no chemical changes. Thus, electrons can be considered current carriers in semiconductors.
The ability of a material to form an electric current in it can be determined by this indicator conductors occupy an intermediate position between conductors and dielectrics. Semiconductors are various types minerals, some metals, metal sulfides, etc. Electric current in semiconductors arises due to the concentration of free electrons, which can move directionally in the substance. Comparing metals and conductors, it can be noted that there is a difference between the influence of temperature on their conductivity. An increase in temperature leads to a decrease in the conductivity of semiconductors. If the temperature in a semiconductor increases, the movement of free electrons will be more chaotic. This is due to an increase in the number of collisions. However, in semiconductors, compared to metals, the concentration of free electrons increases significantly. These factors have the opposite effect on conductivity: the more collisions, the lower the conductivity; the higher the concentration, the higher it is. In metals there is no relationship between temperature and the concentration of free electrons, so that with a change in conductivity with increasing temperature, the possibility of orderly movement of free electrons only decreases. As for semiconductors, the effect of increasing concentration is higher. Thus, the more the temperature rises, the greater the conductivity will be.
There is a relationship between the movement of charge carriers and such a concept as electric current in semiconductors. In semiconductors, the appearance of charge carriers is characterized by various factors, among which temperature and purity of the material are especially important. Based on their purity, semiconductors are divided into impurity and intrinsic semiconductors.
As for the own conductor, the influence of impurities at a certain temperature cannot be considered significant for them. Since the band gap in semiconductors is small, own semiconductor When the temperature reaches the valence band is completely filled with electrons. But the conduction band is completely free: there is no electrical conductivity in it, and it functions as an ideal dielectric. At other temperatures, there is a possibility that, due to thermal fluctuations, certain electrons can overcome the potential barrier and end up in the conduction band.
Thomson effect
The principle of Thomson's thermoelectric effect: when an electric current is passed through semiconductors along which there is a temperature gradient, in addition to Joule heat, release or absorption will occur in them additional quantities heat depending on which direction the current will flow.
Insufficiently uniform heating of a sample having homogeneous structure, affects its properties, as a result of which the substance becomes heterogeneous. Thus, the Thomson phenomenon is a specific Pelte phenomenon. The only difference is that the different chemical composition sample, and the unusual temperature causes this heterogeneity.
Semiconductors include many chemical elements (germanium, silicon, selenium, tellurium, arsenic, etc.), huge amount alloys and chemical compounds. Almost everything inorganic substances the world around us - semiconductors. The most common semiconductor in nature is silicon, which makes up about 30% of the earth's crust.
The qualitative difference between semiconductors and metals is manifested in dependence of resistivity on temperature(Fig.9.3)
Band model of electron-hole conductivity of semiconductors
During education solids a situation is possible when energy zone, arising from the energy levels of the valence electrons of the original atoms, turns out to be completely filled with electrons, and the nearest ones available for filling with electrons energy levels separated from valence band E V interval of unresolved energy states- the so-called prohibited area E g.Above the band gap is the zone of energy states allowed for electrons - conduction band E c .
The conduction band at 0 K is completely free, and the valence band is completely occupied. Similar band structures are characteristic of silicon, germanium, gallium arsenide (GaAs), indium phosphide (InP) and many other semiconductor solids.
As the temperature of semiconductors and dielectrics increases, electrons are able to receive additional energy associated with thermal motion kT. For some electrons, the energy of thermal motion is sufficient for the transition from the valence band to the conduction band, where electrons under the influence of an external electric field can move almost freely.
In this case, in circuit with semiconductor material As the temperature of the semiconductor increases, the electric current will increase. This current is associated not only with the movement of electrons in the conduction band, but also with the appearance vacant places from electrons leaving the conduction band in the valence band, the so-called holes . The vacant place can be occupied by a valence electron from a neighboring pair, then the hole moves to a new place in the crystal.
If a semiconductor is placed in an electric field, then not only free electrons are involved in the ordered movement, but also holes, which behave like positively charged particles. Therefore the current I in a semiconductor it consists of electron I n and hole Ip currents: I= I n+ Ip.
The electron-hole conduction mechanism appears only in pure (i.e., without impurities) semiconductors. It's called own electrical conductivity semiconductors. Electrons are thrown into the conduction band with Fermi level, which turns out to be located in its own semiconductor in the middle of the bandgap(Fig. 9.4).
The conductivity of semiconductors can be significantly changed by introducing very small amounts of impurities into them. In metals, an impurity always reduces conductivity. Thus, adding 3% phosphorus atoms to pure silicon increases the electrical conductivity of the crystal by 10 5 times.
A small addition of a dopant to a semiconductor called doping.
A necessary condition A sharp decrease in the resistivity of a semiconductor with the introduction of impurities is the difference in the valence of the impurity atoms from the valence of the main atoms of the crystal. The conductivity of semiconductors in the presence of impurities is called impurity conductivity .
Distinguish two types of impurity conductivity – electronic And hole conductivity. Electronic conductivity occurs when pentavalent atoms (for example, arsenic atoms, As) are introduced into a germanium crystal with tetravalent atoms (Fig. 9.5).
The four valence electrons of the arsenic atom are included in the formation covalent bonds with four neighboring germanium atoms. The fifth valence electron turned out to be redundant. It easily breaks away from the arsenic atom and becomes free. An atom that has lost an electron becomes a positive ion located at a site in the crystal lattice.
An impurity of atoms with a valency exceeding the valence of the main atoms of a semiconductor crystal is called donor admixture . As a result of its introduction, a significant number of free electrons appear in the crystal. This leads to a sharp decrease in the resistivity of the semiconductor - thousands and even millions of times.
The resistivity of a conductor with a high content of impurities may approach that of a metal conductor. Such conductivity due to free electrons is called electronic, and a semiconductor with electronic conductivity is called n-type semiconductor.
Hole conductivity occurs when trivalent atoms are introduced into a germanium crystal, for example, indium atoms (Fig. 9.5)
Figure 6 shows an indium atom that has created covalent bonds with only three neighboring germanium atoms using its valence electrons. The indium atom does not have an electron to form a bond with the fourth germanium atom. This missing electron can be captured by the indium atom from the covalent bond of neighboring germanium atoms. In this case, the indium atom turns into negative ion, located at a site of the crystal lattice, and a vacancy is formed in the covalent bond of neighboring atoms.
An admixture of atoms capable of capturing electrons is called acceptor impurity . As a result of the introduction of an acceptor impurity, many covalent bonds are broken in the crystal and vacancies (holes) are formed. Electrons from neighboring covalent bonds can jump to these places, which leads to chaotic wandering of holes throughout the crystal.
The concentration of holes in a semiconductor with an acceptor impurity significantly exceeds the concentration of electrons that arose due to the mechanism of the semiconductor’s own electrical conductivity: n p>> n n. This type of conductivity is called hole conductivity . An impurity semiconductor with hole conductivity is called p-type semiconductor . The main free charge carriers in semiconductors p-type are holes.
Electron-hole transition. Diodes and transistors
In modern electronic technology, semiconductor devices play an exceptional role. Over the past three decades, they have almost completely replaced electric vacuum devices.
Any semiconductor device has one or more electron-hole junctions . Electron-hole transition (or n–p-transition) – this is the area of contact of two semiconductors with different types conductivity.
At the boundary of semiconductors (Fig. 9.7), a double electric layer is formed, the electric field of which prevents the process of diffusion of electrons and holes towards each other.
Ability n–p-transitions allow current to pass practically only in one direction, used in devices called semiconductor diodes. Semiconductor diodes made from silicon or germanium crystals. During their manufacture, an impurity is fused into a crystal with a certain type of conductivity, providing a different type of conductivity.
Figure 9.8 shows a typical current-voltage characteristic of a silicon diode.
Semiconductor devices with not one, but two n–p junctions are called transistors . Transistors are of two types: p–n–p-transistors and n–p–n-transistors. In a transistor n–p–n-type basic germanium plate has conductivity p-type, and the two areas created on it are conductive n-type (Fig.9.9).
In a transistor p–n–p– it’s kind of the other way around. The transistor plate is called base(B), one of the areas with opposite type conductivity – collector(K), and the second – emitter(E).
