Semiconductors occupy an intermediate place in electrical conductivity between conductors and non-conductors of electric current. The group of semiconductors includes many more substances than the groups of conductors and non-conductors taken together. The most characteristic representatives of semiconductors that have found practical use in technology, are germanium, silicon, selenium, tellurium, arsenic, cuprous oxide and a huge number of alloys and chemical compounds. Almost all inorganic substances the world around us are 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 primarily in the dependence of resistivity on temperature. With decreasing temperature, the resistance of metals decreases. In semiconductors, on the contrary, with decreasing temperature, the resistance increases and near absolute zero they practically become insulators.
In semiconductors, the concentration of free charge carriers increases with increasing temperature. The mechanism of electric current in semiconductors cannot be explained within the free electron gas model.
Germanium atoms have four loosely bound electrons in their outer shell. They are called valence electrons. In a crystal lattice, each atom is surrounded by four nearest neighbors. The bond between atoms in a germanium crystal is covalent, that is, it is carried out by pairs of valence electrons. Each valence electron belongs to two atoms. The valence electrons in a germanium crystal are much more strongly bonded to atoms than in metals; therefore, the concentration of conduction electrons at room temperature in semiconductors is many orders of magnitude smaller than in metals. Near absolute zero temperature in a germanium crystal, all electrons are engaged in the formation of bonds. Such a crystal does not conduct electricity.
As the temperature rises, some of the valence electrons can gain enough energy to break covalent bonds. Then free electrons (conduction electrons) will appear in the crystal. At the same time, vacancies that are not occupied by electrons are formed at the sites of bond breaking. These vacancies are called "holes".
At a given semiconductor temperature, a certain number of electron-hole pairs are formed per unit time. At the same time, the reverse process is going on - when a free electron meets a hole, the electronic bond between germanium atoms is restored. This process is called recombination. Electron-hole pairs can also be produced when a semiconductor is illuminated due to the energy of electromagnetic radiation.
If a semiconductor is placed in electric field, then not only free electrons are involved in the ordered motion, but also holes, which behave like positively charged particles. Therefore, the current I in a semiconductor is the sum of the electronic I n and hole I p currents: I = I n + I p.
The concentration of conduction electrons in a semiconductor is equal to the concentration of holes: n n = n p . The electron-hole mechanism of conduction manifests itself only in pure (i.e., without impurities) semiconductors. It is called intrinsic electrical conductivity of semiconductors.
In the presence of impurities, the electrical conductivity of semiconductors changes greatly. For example, adding impurities phosphorus into crystal silicon in the amount of 0.001 atomic percent reduces the resistivity by more than five orders of magnitude.
A semiconductor in which an impurity is introduced (i.e., part of the atoms of one type is replaced by atoms of another type) is called doped or doped.
There are two types of impurity conduction, electron and hole conduction.
Thus, when doping a four-valent germanium (Ge) or silicon (Si) pentavalent - phosphorus (P), antimony (Sb), arsenic (As) an extra free electron appears at the location of the impurity atom. In this case, the impurity is called donor .
When doping four valent germanium (Ge) or silicon (Si) trivalent - aluminum (Al), indium (Jn), boron (B), gallium (Ga)
- there is a line hole. Such impurities are called acceptor
.
In the same sample of a semiconductor material, one section may have p-conductivity, and the other n-conductivity. Such a device is called a semiconductor diode.
The prefix "di" in the word "diode" means "two", it indicates that the device has two main "details", two semiconductor crystals closely adjacent to each other: one with p-conductivity (this is the zone R), the other - with n - conductivity (this is the zone P). In fact, a semiconductor diode is one crystal, in one part of which a donor impurity is introduced (zone P), into another - acceptor (zone R).
If the battery is connected to the diode constant pressure"plus" to the zone R and "minus" to the zone P, then free charges - electrons and holes - will rush to the boundary, rush to the pn junction. Here they will neutralize each other, new charges will approach the boundary, and a constant current will be established in the diode circuit. This is the so-called direct connection of the diode - the charges move intensively through it, a relatively large forward current flows in the circuit.
Now we will change the polarity of the voltage on the diode, we will carry out, as they say, its reverse inclusion - we will connect the “plus” of the battery to the zone P,"minus" - to the zone R. Free charges will be pulled away from the boundary, electrons will go to the "plus", holes - to the "minus" and, as a result, the pn - junction will turn into a zone without free charges, into a pure insulator. This means that the circuit will break, the current in it will stop.
Not a large reverse current through the diode will still go. Because, in addition to the main free charges (charge carriers) - electrons, in the zone P, and holes in the p zone - in each of the zones there is also an insignificant amount of charges of the opposite sign. These are their own minor charge carriers, they exist in any semiconductor, appear in it due to the thermal movements of atoms, and it is they who create the reverse current through the diode. There are relatively few of these charges, and the reverse current is many times less than the direct one. The magnitude of the reverse current is highly dependent on: temperature environment, semiconductor material and area pn transition. 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. Often CVC, for clarity, is presented in the form of graphs.
Lesson #41-169 Electricity in semiconductors. semiconductor diode. Semiconductors.
A semiconductor is a substance whose resistivity can change with wide range and decreases very rapidly with increasing temperature., which means that the electrical conductivity increases. It is observed in silicon, germanium, selenium and in some compounds.
Conduction mechanism in semiconductors
Semiconductor crystals have an atomic crystal lattice, where outer electrons are bound to neighboring atoms by covalent bonds. At low temperatures Pure semiconductors do not have free electrons and behave like a dielectric. If the semiconductor is pure (without impurities), then it has its own conductivity (small).
There are two types of intrinsic conduction:
1) electronic (conductivity " P"-type) At low temperatures in semiconductors, all electrons are associated with nuclei and the resistance is large; As the temperature increases, the kinetic energy of the particles increases, bonds break and free electrons appear - the resistance decreases.
Free electrons move opposite to the intensity vector electric field. The electronic conductivity of semiconductors is due to the presence of free electrons.
2) hole (p-type conductivity). With an increase in temperature, covalent bonds between atoms are destroyed, carried out by valence electrons, and places with a missing electron are formed - a “hole”. It can move throughout the crystal, because. its place can be replaced by valence electrons. Moving the "hole" is equivalent to moving positive charge. The hole moves in the direction of the electric field strength vector.
The rupture of covalent bonds and the appearance of intrinsic conductivity of semiconductors can be caused by heating, lighting (photoconductivity) and the action of strong electric fields.
Dependence of R on illumination: Photoresistor - photorelay - emergency switches |
The total conductivity of a pure semiconductor is the sum of the "p" and "n" -type conductivities and is called electron-hole conductivity.
Semiconductors in the presence of impurities
They have their own and impurity conductivity. The presence of impurities greatly increases the conductivity. When the impurity concentration changes, the number of electric current carriers—electrons and holes—changes. The ability to control the current underlies the widespread use of semiconductors. There are the following impurities:
1) donor impurities (donating) - are additional
suppliers of electrons to semiconductor crystals, easily donate electrons and increase the number of free electrons in the semiconductor. These are conductors "n" - type, i.e. semiconductors with donor impurities, where the main charge carrier is electrons, and the minority is holes. Such a semiconductor has electronic impurity conductivity (an example is arsenic).
2) acceptor impurities (receiving) create "holes", taking electrons into themselves. These are semiconductors "p" - type, i.e. semiconductors with acceptor impurities, where the main charge carrier is
holes, and the minority electrons. Such a semiconductor has
hole impurity conductivity (an example is indium).
Electrical properties "p-n » transitions.
"pn" transition (or electron-hole transition) - the contact area of two semiconductors, where the conductivity changes from electronic to hole (or vice versa).
In a semiconductor crystal, such regions can be created by introducing impurities. In the contact zone of two semiconductors with different conductivities, mutual diffusion of electrons and holes will take place and a blocking barrier will form.
electrical layer. The electric field of the barrier layer prevents
further transition of electrons and holes through the boundary. The barrier layer has an increased resistance compared to other areas of the semiconductor.
An external electric field affects the resistance of the barrier layer. In the direct (transmission) direction of the external electric field, the current passes through the boundary of two semiconductors. Because electrons and holes move towards each other to the interface, then the electrons,
crossing the border, fill the holes. The thickness of the barrier layer and its resistance are continuously decreasing.
With a blocking (reverse direction of the external electric field), the current will not pass through the contact area of the two semiconductors. Because electrons and holes move from the boundary in opposite directions, then the blocking layer
thickens, its resistance increases.
Thus, the electron-hole transition has one-sided conduction.
semiconductor diode- a semiconductor with one "rn" junction.
Semiconductor diodes main elements of rectifiers alternating current.
When an electric field is applied: in one direction, the resistance of the semiconductor is high, in the opposite direction, the resistance is low.
Transistors.(from English words transfer - transfer, resistor - resistance)
Consider one of the types of germanium or silicon transistors with donor and acceptor impurities introduced into them. The distribution of impurities is such that a very thin (of the order of several micrometers) n-type semiconductor layer is created between two p-type semiconductor layers (see Fig.).
This thin layer is called basis or base. The crystal has two R-n -transitions, the direct directions of which are opposite. Three pins from areas with various types conductivity allow you to include a transistor in the circuit shown in the figure. With this inclusion, the left R-n -jump is direct and separates the base from a p-type region called emitter. If there was no right R-n -transition, in the emitter - base circuit there would be a current depending on the voltage of the sources (batteries B1 and an AC voltage source) and circuit resistance, including the low resistance of the direct emitter-base junction.
Battery B2 turned on so that the right R-n -transition in the circuit (see fig.) is reverse. It separates the base from the right p-type region called collector. If there was no left R-n -junction, the current in the collector circuit would be close to zero, since
reversal resistance is very high. In the presence of a current in the left R-n -junction current also appears in the collector circuit, and the current in the collector is only slightly less than the current in the emitter (if a negative voltage is applied to the emitter, then the left R-n -transition will be reversed and the current in the emitter circuit and in the collector circuit will be practically absent). When a voltage is created between the emitter and the base, the main carriers of the p-type semiconductor - holes penetrate into the base, where they are already minor carriers. Since the thickness of the base is very small and the number of majority carriers (electrons) in it is small, the holes that have fallen into it hardly combine (do not recombine) with base electrons and penetrate into the collector due to diffusion. Right R-n -transition is closed for the main charge carriers of the base - electrons, but not for holes. In the collector, the holes are carried away by the electric field and close the circuit. The strength of the current branching into the emitter circuit from the base is very small, since the cross-sectional area of the base in the horizontal (see Fig. Above) plane is much smaller than the cross-section in the vertical plane.
Collector current, practically equal to strength current in the emitter, changes along with the current in the emitter. Resistor resistance R has little effect on the current in the collector, and this resistance can be made sufficiently large. By controlling the emitter current with an AC voltage source included in its circuit, we get a synchronous change in the voltage across the resistor R .
With a large resistance of the resistor, the voltage change across it can be tens of thousands of times greater than the signal voltage change in the emitter circuit. This means increased voltage. Therefore, on the load R it is possible to obtain electrical signals whose power is many times greater than the power entering the emitter circuit.
Application of transistors Properties R-n-junctions in semiconductors are used to amplify and generate electrical oscillations.
Semiconductors include many chemical elements (germanium, silicon, selenium, tellurium, arsenic, etc.), a huge number of alloys and chemical compounds. Almost all inorganic substances of the world around us are 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 temperature dependence of resistivity(fig.9.3)
Band model of electron-hole conductivity of semiconductors
At education solids a situation is possible when the energy band arising from the energy levels of the valence electrons of the initial atoms turns out to be completely filled with electrons, and the nearest energy levels available for filling with electrons are separated from valence band E V an interval of unresolved energy states - the so-called forbidden zone 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, while 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.
With an increase in the temperature of semiconductors and dielectrics, 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 action of an external electric field can move almost freely.
In this case, in a circuit with a semiconductor material, as the temperature of the semiconductor rises, an electric current will increase. This current is associated not only with the movement of electrons in the conduction band, but also with the appearance vacancies from electrons that have gone into the conduction band in the valence band, the so-called holes . A vacant place can be occupied by a valence electron from a neighboring pair, then the hole will move 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 is made up of an electronic I n and hole Ip currents: I= I n+ Ip.
The electron-hole mechanism of conduction manifests itself only in pure (i.e., without impurities) semiconductors. It is 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 forbidden zone(Fig. 9.4).
It is possible to significantly change the conductivity of semiconductors by introducing very small amounts of impurities into them. In metals, an impurity always reduces the conductivity. Thus, the addition of 3% phosphorus atoms to pure silicon increases the electrical conductivity of the crystal by a factor of 105.
Slight addition of dopant to the semiconductor called doping.
Necessary condition A sharp decrease in the resistivity of a semiconductor with the introduction of impurities is the difference in the valency 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 conduction – electronic and hole conductivity. Electronic conductivity occurs when pentavalent atoms (for example, arsenic, As) are introduced into a germanium crystal with tetravalent atoms (Fig. 9.5).
The four valence electrons of the arsenic atom are involved in the formation of covalent bonds with four neighboring germanium atoms. The fifth valence electron turned out to be redundant. It easily detaches from the arsenic atom and becomes free. An atom that has lost an electron turns into a positive ion located at a site in the crystal lattice.
An admixture of atoms with a valency greater than the valency of the main atoms of a semiconductor crystal is called donor impurity . 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 - by thousands and even millions of times.
The resistivity of a conductor with a high content of impurities can approach that of a metallic conductor. Such conductivity, due to free electrons, is called electronic, and a semiconductor with electronic conductivity is called n-type semiconductor.
hole conduction 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 an indium atom from a covalent bond of neighboring germanium atoms. In this case, the indium atom turns into a 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 in the crystal, many covalent bonds are broken and vacant sites (holes) are formed. Electrons can jump to these places from neighboring covalent bonds, which leads to random wandering of holes around 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 intrinsic electrical conductivity of the semiconductor: np>> n n. This type of conduction is called hole conductivity . An impurity semiconductor with hole conductivity is called p-type semiconductor . Major 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 electrovacuum devices.
Any semiconductor device has one or more electron-hole junctions. . Electron-hole transition (or n–p-transition) - is the contact area 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-transition to pass current in almost only one direction is used in devices called semiconductor diodes. Semiconductor diodes are made from silicon or germanium crystals. During their manufacture, an impurity is melted into a crystal with a certain type of conductivity, which provides a different type of conductivity.
Figure 9.8 shows a typical volt-ampere 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 transistor n–p–n-type basic germanium plate is conductive p-type, and the two regions created on it - by conductivity n-type (Figure 9.9).
in transistor p–n–p- it's kind of the opposite. The plate of a transistor is called base(B), one of the regions with the opposite type of conductivity - collector(K), and the second - emitter(E).
Semiconductors are substances that occupy an intermediate position in terms of electrical conductivity between good conductors and good insulators (dielectrics).
Semiconductors are also 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, charge carriers are ions; in others, the charge carriers are electrons.
Intrinsic conductivity of semiconductors
There are two types of intrinsic conduction in semiconductors: electronic conduction and hole conduction in semiconductors.
1. Electronic conductivity of semiconductors.
Electronic conductivity is carried out by 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 is carried out with the directed movement of valence electrons to vacant places in pair-electron 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 the creation of the current is the same.
The conductivity at which the same number charge carriers (electrons and holes) is called the intrinsic conductivity of semiconductors.
The intrinsic conductivity of semiconductors is usually small, 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 atoms of foreign chemical elements that are not contained in the main semiconductor.
Impurity conductivity- this is the conductivity of semiconductors, due to the introduction of impurities into their crystal lattices.
In some cases, the influence of impurities manifests itself in the fact that the "hole" mechanism of conduction 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 the Latin word negativus - negative). The main charge carriers are electrons, and not the main ones are holes. n-type semiconductors are semiconductors with donor impurities.
1. Donor impurities.
Donor impurities are those that easily donate electrons and, consequently, increase the number of free electrons. Donor impurities supply conduction electrons without the appearance of the same number of holes.
A typical example of a donor impurity 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, and not the main - electrons. . Semiconductors of the p-type 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 are trivalent gallium atoms Ga
Electric current through p-type and n-type semiconductor contact p-n junction- this is a contact layer of two impurity semiconductors of p-type and n-type; The p-n junction is a boundary separating regions with hole (p) conduction and electronic (n) conduction in the same single crystal.
direct p-n junction
If the 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 action of an electric field, the electrons in the n-semiconductor and the holes in the p-semiconductor will move towards each other to the semiconductor interface. Electrons, crossing the boundary, "fill" the holes, the current through the pn junction is carried out by the main charge carriers. As a result, the conductivity of the entire sample increases. With such a direct (throughput) direction of the external electric field, the thickness of the barrier 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 the n-semiconductor is connected to the positive pole of the power source, and the p-semiconductor is connected to the negative pole of the power source, then the electrons in the n-semiconductor and holes in the p-semiconductor under the action of an electric field will move from the interface in opposite directions, the current through p -n-transition is carried out by minor 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 of the external field, the electric current practically does not pass through the contact of the p- and n-semiconductors.
Thus, the electron-hole transition has one-sided conduction.
Dependence of current strength on voltage - volt - ampere characteristic p-n transition is shown in the figure (voltage - current characteristic straight p-n transition is shown by a solid line, volt - ampere characteristic reverse p-n transition is shown as a dotted line).
Semiconductors:
Semiconductor diode - for rectifying alternating current, it uses one p - n - junction with different resistances: in the forward direction, the resistance of the p - n - junction is much less than in the reverse direction.
Photoresistors - for registration and measurement of weak light fluxes. With their help, determine the quality of surfaces, control the dimensions of products.
Thermistors - for remote temperature measurement, fire alarms.
Semiconductor- this is a substance in which the resistivity can vary over a wide range and decreases very quickly with increasing temperature., which means that the electrical conductivity (1 / R) increases.
- observed in silicon, germanium, selenium and in some compounds.
Conduction mechanism semiconductors
Semiconductor crystals have an atomic crystal lattice, where outer electrons are bound to neighboring atoms by covalent bonds.
At low temperatures, pure semiconductors have no free electrons and it behaves like a dielectric.
Semiconductors are pure (no impurities)
If the semiconductor is pure (without impurities), then it has own conductivity, which is small.
There are two types of intrinsic conduction:
1 electronic(conductivity "n" - type)
At low temperatures in semiconductors, all electrons are associated with nuclei and the resistance is large; as the temperature increases, the kinetic energy of the particles increases, the bonds break and free electrons appear - the resistance decreases.
Free electrons move opposite to the electric field strength vector.
The electronic conductivity of semiconductors is due to the presence of free electrons.
2. perforated(conductivity "p"-type)
With an increase in temperature, covalent bonds between atoms are destroyed, carried out by valence electrons, and places with a missing electron are formed - a "hole".
It can move throughout the crystal, because. its place can be replaced by valence electrons. Moving a "hole" is equivalent to moving a positive charge.
The hole moves in the direction of the electric field strength vector.
In addition to heating, the breaking of covalent bonds and the appearance of intrinsic conductivity of semiconductors can be caused by illumination (photoconductivity) and the action of strong electric fields.
The total conductivity of a pure semiconductor is the sum of the conductivities of the "p" and "n" types
and is called electron-hole conductivity.
Semiconductors in the presence of impurities
They have own + impurity conductivity
The presence of impurities greatly increases the conductivity.
When the concentration of impurities changes, the number of carriers of the electric current - electrons and holes - changes.
The ability to control the current underlies the widespread use of semiconductors.
Exists:
1)donor impurities (giving off)
They are additional suppliers of electrons to semiconductor crystals, easily donate electrons and increase the number of free electrons in a semiconductor.
These are conductors "n" - type, i.e. semiconductors with donor impurities, where the main charge carrier is electrons, and the minority is holes.
Such a semiconductor has electronic impurity conductivity.
For example, arsenic.
2. acceptor impurities (host)
They create "holes" by taking electrons into themselves.
These are semiconductors "p" - type, those. semiconductors with acceptor impurities, where the main charge carrier is holes, and the minority is electrons.
Such a semiconductor has hole impurity conductivity.
For example, indium.
Electrical properties of "p-n" junction
"p-n" transition(or electron-hole transition) - the contact area of two semiconductors, where the conductivity changes from electronic to hole (or vice versa).
In a semiconductor crystal, such regions can be created by introducing impurities. In the contact zone of two semiconductors with different conductivities, mutual diffusion will take place. electrons and holes, and a blocking electric layer is formed. The electric field of the blocking layer prevents the further transition of electrons and holes through the boundary. The barrier layer has an increased resistance compared to other areas of the semiconductor.
The external electric field affects the resistance of the barrier layer. With the direct (transmission) direction of the external electric field, the electric current passes through the boundary of two semiconductors.
Because electrons and holes move towards each other to the interface, then the electrons, crossing the interface, fill the holes. The thickness of the barrier layer and its resistance are continuously decreasing.
pass p-n mode transition:
With the blocking (reverse) direction of the external electric field, the electric current will not pass through the contact area of the two semiconductors.
Because electrons and holes move from the boundary in opposite directions, then the blocking layer thickens, its resistance increases.
locking mode p-n transition.