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

In reality, an electron-hole transition cannot be created by simply contacting n-type and p-type plates, since an intermediate layer of air, oxides or surface contaminants is inevitable in this case, an ideal coincidence of crystal lattices, etc. is impossible. These junctions are obtained by fusing or diffusing appropriate impurities into the plates of a single crystal of a semiconductor, or by growing a p-n junction from a semiconductor melt with a controlled amount of impurities, etc. Depending on the manufacturing method, p-n junctions can be alloy, diffusion, etc. However, to simplify the analysis of the transition formation process, we will assume that we initially took and mechanically connected two impurity semiconductor crystals with different types of conductivity (n and p types) with the same concentration donor and acceptor impurities and with an ideal surface and crystal lattice. Let us consider the phenomena arising at their boundary.

Figure 1.3. Formation of p-n transition

Due to the fact that the electron concentration in the n region is higher than in the p region, and the hole concentration in the p region is higher than in the n region, there is a carrier concentration gradient at the boundary of these regions, causing a diffusion current of electrons from the n region to the p region and the diffusion current of holes from the p region to the n region. In addition to the current due to the movement of the majority charge carriers, a current of minority carriers (electrons from the p region to the n region and holes from the n region to the p region) is possible through the semiconductor interface. However, they are insignificant (due to a significant difference in the concentrations of major and minor carriers) and we will not take them into account.

If electrons and holes were neutral, then diffusion would ultimately lead to a complete equalization of their concentration throughout the entire volume of the crystal. In fact, the diffusion process is hindered by an electric field that arises in the near-contact region. The departure of electrons from the near-contact n region leads to the fact that their concentration here decreases and an uncompensated positive charge of donor impurity ions arises. Similarly, in the p region, due to the departure of holes, their concentration in the near-contact layer decreases, and here an uncompensated negative charge of acceptor impurity ions arises. Ions, on the other hand, cannot “leave” their places, because they are held by the strongest forces (bonds) of the crystal lattice. Thus, two layers of charges opposite in sign are formed at the boundary of the n and p type regions. There is an electric field directed from positively charged donor ions to negatively charged acceptor ions. The area formed by the space charges and the electric field itself is a p-n transition. Its width is on the order of hundredths to a few micrometers, which is a significant size compared to the size of the crystal lattice.

Thus, a contact potential difference is formed at the border of the p-n transition, numerically characterized by the height of the potential barrier ( Figure 1.3), which the main carriers of each region must overcome in order to get into another region. The contact potential difference is of the order of tenths of a volt.

The p-n transition field is retarding for the majority charge carriers and accelerating for minor ones. Any electron that passes from the electronic region to the hole region enters an electric field that tends to return it back to the electronic region. In the same way, holes, falling from the p region into the electric field of the p-n junction, will be returned by this field back to the p region. In a similar way, the field affects the charges formed for one reason or another inside the p-n transition. As a result of the action of the field on the charge carriers, the region of the pn junction turns out to be depleted, and its conductivity is close to the intrinsic conductivity of the original semiconductor.

The presence of its own electric field also determines the passage of current when an external voltage source is applied - the magnitude of the current turns out to be different depending on the polarity of the applied voltage. If the external voltage is opposite in sign of the contact potential difference, then this leads to a decrease in the height of the potential barrier. Therefore, the width of the p-n transition will decrease (Figure 1.3, b). The conditions for current passage are improved: the reduced potential barrier will be able to overcome the main carriers with the highest energy. As the external voltage increases, the current through the pn junction will increase. This polarity of the external voltage and current is called direct.

It is easy to see that charge carriers that have overcome the potential barrier enter the region of the semiconductor for which they are minor. They diffuse deep into the corresponding region of the semiconductor, recombining with the majority carriers in this region. Thus, as holes penetrate from the p region into the n region, they recombine with electrons. Similar processes occur with electrons injected into the p-region.

The process of introducing charge carriers through an electron-hole transition with a decrease in the height of the potential barrier into the region of the semiconductor, where these charge carriers are minor, is called injection (from the English word inject - to inject, enter).

If you change the polarity of the external voltage (apply a reverse external voltage), then the electric field created by the source coincides with the field of the p-n junction. The potential barrier between the p and n regions increases by the value of the external voltage. The number of basic carriers capable of overcoming the action of the resulting field decreases. The majority carriers 6 will be drawn away from the boundary layers into the interior of the semiconductor. The width of the p-n transition increases (Earley effect, Figure 1.3, c).

For minority carriers (holes in the n region and electrons in the p region), there is no potential barrier in the electron-hole transition and they will be drawn in by the field in the region of the p-n transition. This phenomenon is called extraction. The current of minority carriers, as well as carriers that have arisen in the region of the pn junction, will determine the reverse current through the pn junction. The magnitude of the reverse current is practically independent of the external reverse voltage. This can be explained by the fact that the number of generated electron–hole pairs per unit time remains unchanged at a constant temperature.

The analysis carried out allows us to consider the pn junction as a nonlinear element, the resistance of which varies depending on the value in the polarity of the applied voltage. With an increase in forward voltage, the resistance of the p-n junction decreases. With a change in polarity and the magnitude of the applied voltage, the resistance of the p-n junction increases sharply. Consequently, a direct (linear) relationship between voltage and current (Ohm's law) for p-n junctions is not observed.

As can be seen from Figure 1.3, the pn junction is a double layer of immobile space charges opposite in sign. It can be likened to the plates of a flat capacitor, the plates of which are the p- and p-regions, and the p-n junction, which has practically no mobile charges, serves as a dielectric. The value of the formed, so-called barrier (charging) capacitance is inversely proportional to the distance between the plates. With an increase in the blocking voltage applied to the junction, the region depleted of mobile charge carriers - electrons or holes - increases, which corresponds to an increase in the distance between the capacitor plates and a decrease in the capacitance. Therefore, the p-n junction can be used as a capacitance controlled by the magnitude of the reverse voltage. The barrier capacitance value ranges from tens to hundreds of picofarads; the change in this capacitance with a change in voltage can reach a tenfold value

When passing through the direct current transition, on both sides of the interface between the regions, an excess charge of minority carriers of the opposite sign accumulates, which cannot instantly recombine. It forms a capacitance, which is called a diffusion capacitance. The diffuse capacitance is connected in parallel with the barrier capacitance. Diffusion capacitance values ​​can be on the order of hundreds to thousands of picofarads. Therefore, at forward voltage, the capacitance of the pn junction is determined mainly by the diffusion capacitance, and at reverse voltage, by the barrier capacitance.

With a forward voltage, the diffusion capacitance does not significantly affect the operation of the p-n junction, since it is always shunted by a small direct junction resistance. Its negative effect is manifested during rapid switching of the p-n transition from the open state to the closed state.

When using a p-n junction in real semiconductor devices, an external voltage can be applied to it. The magnitude and polarity of this voltage determine the behavior of the junction and the electrical current flowing through it. If the positive pole of the power supply is connected to p- area, and negative - to n-area, then inclusion pn-transition is called direct. When the specified polarity is changed, the inclusion pn-transition is called reverse.

With direct connection pn-junction, the external voltage creates a field in the junction that is opposite in direction to the internal diffusion field, Figure 2. The resulting field strength decreases, which is accompanied by a narrowing of the blocking layer. As a result, a large number of majority charge carriers get the opportunity to diffusely pass into the neighboring region (the drift current does not change in this case, since it depends on the number of minority carriers that appear at the transition boundaries), i.e. the resulting current will flow through the junction, determined mainly by the diffusion component. The diffusion current depends on the height of the potential barrier and increases exponentially as it decreases.

Increased diffusion of charge carriers through the junction leads to an increase in the hole concentration in the region n-type and electrons in the area p-type. Such an increase in the concentration of minority carriers due to the influence of an external voltage applied to the junction is called minority carrier injection. Nonequilibrium minority carriers diffuse deep into the semiconductor and break its electrical neutrality. The restoration of the neutral state of the semiconductor occurs due to the supply of charge carriers from an external source. This is the cause of the current in the external circuit, called direct.

When turned on pn-transition in the opposite direction, the external reverse voltage creates an electric field that coincides in direction with the diffusion one, which leads to an increase in the potential barrier and an increase in the width of the barrier layer, Figure 3. All this reduces the diffusion currents of the main carriers. For non-primary carriers, the field in pn-transition remains accelerating, and therefore the drift current does not change.

Thus, the resulting current will flow through the junction, determined mainly by the drift current of the minority carriers. Since the number of drifting minority carriers does not depend on the applied voltage (it only affects their speed), as the reverse voltage increases, the current through the junction tends to the limit value I S which is called the saturation current. The higher the concentration of donor and acceptor impurities, the lower the saturation current, and with increasing temperature, the saturation current grows exponentially.

1.3. Volt-ampere characteristic of p-n-junction

Dependence of current through pn-transition from the voltage applied to it I = f(U) called the current-voltage characteristic pn-transition, figure 4.

The current-voltage characteristic of the electron-hole transition is described by the equation Ebers Molla:

, (1)

where I– current through the junction at voltage U;

I S is the saturation current created by minority charge carriers. I S also called thermal current, since the concentration of minority carriers depends on temperature;

q e is the electron charge;

k is the Boltzmann constant;

T is the absolute temperature;

- temperature transition potential, approximately equal to 0.025 V = 25 mV at room temperature.

If district- the transition is switched on in the forward direction, the voltage U take with a plus sign, if in the opposite - with a minus sign.

With direct applied voltage
unity can be neglected in comparison with the term
, and the CVC will have a purely exponential character.

With reverse (negative) voltage
term
can be neglected compared to unity, and the current turns out to be equal to
.

However, the equation Ebers Molla very approximately coincides with the real current-voltage characteristics, since it does not take into account a number of physical processes occurring in semiconductors. These processes include: generation and recombination of carriers in the barrier layer, surface leakage currents, voltage drop across the resistance of neutral regions, and phenomena of thermal, avalanche, and tunnel breakdowns.

If the current flowing through the junction is negligible, then the voltage drop across the resistance of the neutral regions can be neglected. However, as the current increases, this process has an increasing effect on the CVC of the device, i.e. its real characteristic goes at a smaller angle and degenerates into a straight line when the voltage on the blocking layer becomes equal to the contact potential difference.

At a certain reverse voltage, a sharp increase in the reverse current is observed. This phenomenon is called junction breakdown. There are three types of breakdowns: tunnel, avalanche and thermal. Tunnel and avalanche breakdowns are types of electrical breakdown and are associated with an increase in the electric field strength in the junction. Thermal breakdown is determined by the overheating of the junction.

The tunnel effect (Zener effect) consists in the direct transition of valence electrons from one semiconductor to another (where they will already be free charge carriers), which becomes possible with a high electric field strength at the transition. Such a high electric field strength at the junction can be achieved at a high concentration of impurities in p- and n-areas where the transition thickness becomes very small.

In wide pn-junctions formed by semiconductors with medium or low impurity concentrations, the probability of tunneling electron leakage decreases and avalanche breakdown becomes more probable.

Avalanche breakdown occurs when the mean free path of an electron in a semiconductor is much less than the junction thickness. If, during the free path, electrons accumulate kinetic energy sufficient to ionize atoms in the transition, then impact ionization occurs, accompanied by an avalanche multiplication of charge carriers. The free charge carriers formed as a result of impact ionization increase the reverse junction current.

Thermal breakdown is caused by a significant increase in the number of charge carriers in pn-transition due to violation of the thermal regime. Transition power P arr = I arr U arr is spent on its heating. The heat released in the barrier layer is removed mainly due to the thermal conductivity of the crystal lattice. Under poor conditions for heat removal from the junction, as well as with an increase in the reverse voltage at the junction above the critical value, it can be heated to a temperature at which thermal ionization of atoms occurs. The charge carriers formed in this case increase the reverse current through the junction, which leads to its further heating. As a result of such an increasing process, the transition heats up unacceptably and a thermal breakdown occurs, which is characterized by the destruction of the crystal.

An increase in the number of charge carriers during heating of the junction leads to a decrease in its resistance and the voltage released on it. As a result, a section with a negative differential resistance appears on the reverse branch of the CVC during thermal breakdown.

Electron-hole transition ( p-n- transition, n-p-transition), the transition region of a semiconductor, in which there is a spatial change in the type of conductivity from electronic n to the hole p.Electronic-hole transition is the basis of a wide class of solid-state devices for non-linear conversion of electrical signals in various electronic devices.

An external electric field changes the height of the potential barrier and upsets the balance of current carrier flows through it. If the power supply voltage is applied in such a way that the plus is connected to p- crystal area, and minus - to n- area, then this direction is called throughput. In this case, the external field is directed against the contact one, that is, the potential barrier is lowered (forward bias). As the applied voltage increases, the number of majority carriers that can overcome the potential barrier increases exponentially. The concentration of minority carriers on both sides of the electron-hole transition increases due to the injection of minority carriers, simultaneously in R- and n- through the contacts, equal amounts of the majority carriers enter, causing the neutralization of the charges of the injected carriers. As a result, the recombination rate increases and a nonzero current appears through the electron-hole transition. As the applied voltage increases, this current increases exponentially.

In reverse polarity (reverse bias), when the positive pole of the power supply is connected to n- area, and negative - to R- region, the potential in the transition region becomes equal to UD+U, where U- the magnitude of the applied voltage.

An increase in the potential barrier leads to the fact that the diffusion of the majority carriers through p-n- the transition becomes negligible. At the same time, minority carrier flows through the junction do not change, since there is no barrier for them. The fluxes of minority carriers are determined by the rate of thermal generation of electron-hole pairs. These vapors diffuse to the barrier and are separated by its field, as a result of which, through p-n- the junction flows a saturation current, which is usually small and almost independent of the applied voltage.

Thus, the dependence of the current through p-n- applied voltage transition U(current-voltage characteristic) has a pronounced non-linearity. When the sign of the voltage changes, the current through p-n- the transition can change by a factor of 10 5 -10 6 times. Thereby p-n- the transition is a valve device suitable for rectifying alternating currents (see Semiconductor Diode).

The nature of the current-voltage characteristic - the curvature of the ascending branch, the cutoff voltage, the absolute values ​​of the currents, the rectification factor (the ratio of forward and reverse currents at a voltage of 1 V), and other parameters are determined by the type of semiconductor, the concentration and type of impurity distribution near n-p-transition.

Change in voltage applied to p-n- transition, leads to the expansion or reduction of the space charge region. Space charges are immobile and associated with the crystal lattice ions of donors and acceptors, therefore, an increase in space charge can only be due to the expansion of its region and, consequently, a decrease in capacitance p-n- transition. With a forward bias, a diffusion capacitance is added to the capacitance of the space charge layer, which is also called the charging or barrier capacitance, due to the fact that an increase in voltage across p-n- the transition leads to an increase in the concentration of minority carriers, that is, to a change in charge. The dependence of the capacitance on the applied voltage makes it possible to use p-n- transition as an electric variable capacitor - varicap.

Resistance dependency p-n- the transition from the magnitude and sign of the applied voltage allows you to use it as an adjustable resistance - varistor.

When a sufficiently high reverse bias is applied to the electron-hole junction U = U pr an electrical breakdown occurs, in which a large reverse current flows. The state in which an electrical breakdown occurs p-n- transition, is the normal mode of operation of some semiconductor devices, such as zener diodes.

Depending on the physical processes that cause a sharp increase in the reverse current, there are three main breakdown mechanisms p-n- transitions: tunnel, avalanche, thermal.

Tunneling (Zener) breakdown occurs when carriers tunnel through a barrier (see Tunneling effect), when, for example, tunneling leakage of electrons from the valence band occurs p-areas in the conduction band n- region of the semiconductor. Tunneling of electrons occurs in that place pn- transition, in which, as a result of its inhomogeneity, the highest field strength occurs. Tunnel breakdown voltage p-n- transition depends not only on the dopant concentration and the critical field strength at which the tunneling current increases through p-n- transition, but also on the thickness p-n- transition. With increasing thickness p-n- transition, the probability of tunneling electron leakage decreases, and avalanche breakdown becomes more probable.

During an avalanche p-n- transition at the mean free path in the space charge region, the charge carrier acquires energy sufficient to ionize the crystal lattice, that is, it is based on impact ionization. With an increase in the electric field strength, the intensity of impact ionization increases strongly and the process of multiplication of free charge carriers (electrons and holes) acquires an avalanche character. As a result, the current in p-n- transition increases indefinitely up to thermal breakdown.

Thermal breakdown associated with insufficient heat removal, as a rule, is localized in separate areas where there is a heterogeneity of the structure p-n- transition, and, consequently, the inhomogeneity of the reverse current flowing through it. An increase in temperature causes a further increase in the reverse current, which in turn causes an increase in temperature. Thermal breakdown is an irreversible process that is predominant in semiconductors with a relatively narrow bandgap.

V p-n- transitions, surface breakdown can also be observed. The surface breakdown voltage is determined by the magnitude of the charge localized on the surface of the semiconductor at the exit point p-n- transition outside. By its nature, surface breakdown can be tunneling, avalanche or thermal.

In addition to using the nonlinearity of the current-voltage characteristic and the dependence of capacitance on voltage, p-n- junctions find diverse applications based on the dependence of the contact potential difference and saturation current on the concentration of minority carriers. The concentration of minority carriers changes significantly under various external influences - thermal, mechanical, optical, etc. This is the basis for the principle of operation of various types of sensors: temperature, pressure, ionizing radiation, etc. p-n- transitions are also used to convert light energy into electrical energy in solar panels.

Electron-hole junctions are not only the basis of various kinds of semiconductor diodes, but are also included as components in more complex semiconductor devices - transistors, thyristors, etc. Injection and subsequent recombination of minority carriers in p-n- junctions are used in light emitting diodes and injection lasers.

If a P-type semiconductor block is connected to an N-type semiconductor block (Figure below (a)), the result will not matter. We will have two conductive blocks touching each other without exhibiting any unique properties. The problem lies in the two separate and distinct crystal structures. The number of electrons is balanced by the number of protons in both blocks. Thus, as a result, no block has any charge.

However, a single semiconductor chip made of a P-type material on one side and an N-type material on the other side (Figure below (b)) has unique properties. In a P-type material, the main ones are positive charge carriers, holes that move freely along the crystal lattice. In an N-type material, negative charge carriers, electrons, are basic and mobile. Near the junction, electrons in the N-type material diffuse through the junction, connecting with holes in the P-type material. The region of the P-type material near the junction acquires a negative charge due to the attracted electrons. Since the electrons have left the N-type region, it acquires a local positive charge. The thin layer of crystal lattice between these charges is now depleted in majority carriers, thus it is known as depleted region. This region becomes a non-conductive material from its own semiconductor. In fact, we have almost an insulator separating P and N type conductive doped regions.

(a) Blocks of P and N type semiconductors do not have usable properties when in contact.
(b) A single crystal doped with P and N type impurities creates a potential barrier.

This charge separation in the P-N junction is a potential barrier. This potential barrier can be overcome by applying an external voltage source to cause the junction to conduct electricity. The formation of the transition and potential barrier occurs during the manufacturing process. The magnitude of the potential barrier depends on the materials used in the production. Silicon P-N junctions have a higher potential barrier compared to germanium junctions.

In the figure below (a), the battery is connected so that the negative terminal of the source supplies electrons to the N-type material. These electrons diffuse towards the junction. A positive source terminal removes electrons from the P-type semiconductor, creating holes that diffuse to the junction. If the battery voltage is high enough to overcome the junction potential (0.6V for silicon), electrons from the N-type region and holes from the P-type region combine to destroy each other. This frees up space inside the lattice for more charge carriers to move towards the transition. Thus, the main charge currents of the N-type and P-type regions flow towards the junction. Recombination at the junction allows battery current to flow through the P-N junction of the diode. This inclusion is called direct displacement.

(a) Forward bias pushes charge carriers to a junction where recombination is reflected in battery current.
(b) Reverse bias pulls charge carriers to the battery terminals, away from the junction. The thickness of the depleted region increases. A steady current does not flow through the battery.

If the battery polarity is reversed, as shown in figure (b) above, the majority charge carriers are attracted from the junction to the battery terminals. The positive terminal of the battery pulls away from the transition of the main charge carriers in the N-type region, electrons. The negative terminal pulls away from the transition of the majority carriers in the P-type region, holes. This increases the thickness of the non-conductive depletion region. There is no recombination of the main carriers in it; and thus there is no conduction. This battery connection is called reverse bias.

The diode symbol shown in Figure (b) below corresponds to the doped semiconductor wafer in Figure (a). The diode is unidirectional device. The electron current flows in only one direction, against the arrow corresponding to the forward bias. The cathode, the stripe on the diode symbol, corresponds to an N-type semiconductor. The anode, arrow, corresponds to a P-type semiconductor.

Note: the original article proposes an algorithm for storing the location of semiconductor types in a diode. non-pointing ( N ot-pointing) part of the symbol (band) corresponds to the semiconductor N-type. Pointing ( P ointing) part of the symbol (arrow) corresponds to P-type.

(a) PN junction forward bias
(b) Corresponding diode symbol
(c) Silicon diode current versus voltage plot

If the diode is forward biased (as shown in (a) above), as the voltage increases from 0V, the current will slowly increase. In the case of a silicon diode, the current flowing can be measured when the voltage approaches 0.6V (figure (c) above). When the voltage increases above 0.6 V, the current after bending on the graph will begin to increase sharply. Increasing the voltage above 0.7V can result in a current large enough to destroy the diode. Forward voltage U pr is one of the characteristics of semiconductors: 0.6-0.7 V for silicon, 0.2 V for germanium, a few volts for light emitting diodes. The forward current can range from a few mA for point diodes to 100 mA for low current diodes and up to tens and thousands of amps for power diodes.

If the diode is reverse biased, then only the leakage current of its own semiconductor flows. This is depicted in the graph to the left of the origin (figure (c) above). For silicon diodes, this current under the most extreme conditions will be about 1 µA. This current increases imperceptibly with increasing reverse bias voltage until the diode is broken. During breakdown, the current increases so much that the diode fails unless a resistance is connected in series to limit this current. We usually choose a diode with a reverse voltage higher than the voltages that can be applied while the circuit is running, to prevent the diode from breaking down. Typically, silicon diodes are available with breakdown voltages of 50, 100, 200, 400, 800 volts and higher. It is also possible to manufacture diodes with a lower breakdown voltage (several volts) for use as voltage standards.

We mentioned earlier that the reverse leakage current up to the microampere in silicon diodes is due to the conductivity of the intrinsic semiconductor. This leak can be explained by theory. The thermal energy creates several electron-hole pairs that conduct the leakage current until recombination. In actual practice, this predictable current is only a fraction of the leakage current. Most of the leakage current is due to surface conduction due to the lack of cleanliness of the semiconductor surface. Both leakage current components increase with temperature, approaching the microamp for small silicon diodes.

For germanium, the leakage current is several orders of magnitude higher. Since germanium semiconductors are rarely used in practice today, this is not a big problem.

Summing up

P-N junctions are made from a single crystal piece of semiconductor with P and N type regions in close proximity to the junction.

The transfer of electrons through the junction from the N-type side to the holes to the P-type side, followed by mutual annihilation, creates a voltage drop across the junction of 0.6 to 0.7 volts for silicon and depends on the semiconductor.

Forward biasing a P-N junction when the forward voltage value is exceeded causes current to flow through the junction. An applied external potential difference causes the majority charge carriers to move towards the junction, where recombination occurs, allowing the flow of electric current.

Reverse biasing a P-N junction creates almost no current. The applied reverse bias pulls the majority charge carriers away from the junction. This increases the thickness of the non-conductive depletion region.

A reverse-biased P-N junction carries a reverse leakage current that depends on temperature. In small silicon diodes, it does not exceed microamperes.

The principle of operation of semiconductor devices is explained by the properties of the so-called electron-hole transition (p-n - transition) - the separation zone of semiconductor regions with different conduction mechanisms.

Electron-hole transition - this is the region of the semiconductor in which there is a spatial change in the type of conductivity (from electronic n-regions to perforated p-regions). Since the concentration of holes in the p-region of the electron-hole transition is much higher than in the n-region, holes from the n-region tend to diffuse into the electronic region. Electrons diffuse into the p-region.

To create n- or p-type conductivity in the original semiconductor (usually 4-valent germanium or silicon), atoms of 5-valent or 3-valent impurities, respectively, are added to it (phosphorus, arsenic or aluminum, indium, etc.)

Atoms of a pentavalent impurity (donors) easily donate one electron to the conduction band, creating an excess of electrons in the semiconductor that are not involved in the formation of covalent bonds; the conductor acquires n-type conductivity. The introduction of a 3-valent impurity (acceptors) leads to the fact that the latter, taking one electron from the atoms of the semiconductor to create the missing covalent bond, imparts p-type conductivity to it, since the holes formed in this case (vacant energy levels in the valence band) behave in electric or magnetic fields as carriers of positive charges. Holes in a p-type semiconductor and electrons in an n-type semiconductor are called majority carriers, in contrast to minor carriers (electrons in a p-type semiconductor and holes in an n-type semiconductor), which are generated due to thermal vibrations of crystal lattice atoms.

If semiconductors with different types of conductivity are brought into contact (the contact is created technologically, but not mechanically), then the electrons in the n-type semiconductor get the opportunity to occupy free levels in the valence band of the p-type semiconductor. will happen electron recombination with holes near the boundary of heterogeneous semiconductors.

This process is similar to the diffusion of free electrons from an n-type semiconductor to a p-type semiconductor and the diffusion of holes in the opposite direction. As a result of the departure of the main charge carriers, a layer depleted in mobile carriers is created at the boundary of heterogeneous semiconductors, in which positive ions will be located in the n-region donor atoms; and in the p-region, negative ions acceptor atoms. This layer, depleted in mobile carriers, with a length of fractions of a micron, is electron-hole transition.

Potential barrier in the p-n junction.

If an electrical voltage is applied to a semiconductor, then, depending on the polarity of this voltage, the p-n junction exhibits completely different properties.

Properties of p-n junction with direct connection.

Properties of a p-n junction when switched back on.

So, with a certain degree of approximation, we can assume that an electric current flows through the p-n junction if the polarity of the power supply voltage is direct, and, conversely, there is no current when the polarity is reversed.

However, in addition to the dependence of the generated current on external energy, for example, a power source or photons of light, which is used in a number of semiconductor devices, there is thermal generation. In this case, the concentration of own charge carriers sharply decreases, and, consequently, I OBR too. Thus, if the transition is subjected to external energy, then a pair of free charges appears: an electron - a hole. Any charge carrier born in the space charge regionp–n transition, will be picked up by an electric field E VN and ejected: electron - inn– region, hole – in p- area. An electric current arises, which is proportional to the width of the space charge region. This is because the more E VN , the wider the region where there is an electric field in which the birth and separation of charge carriers occurs. As mentioned above, the rate of generation of charge carriers in a semiconductor depends on the concentration and energy position of deep impurities existing in the material.

For the same reason, the limiting operating temperature of the semiconductor is higher. For germanium it is 80º C, silicon: 150º C, gallium arsenide: 250º C (D E= 1.4 eV). At higher temperatures, the number of charge carriers increases, the resistance of the crystal decreases, and the semiconductor is thermally destroyed.

Current-voltage characteristic of the p-n junction.

Volt-ampere characteristics (CVC) is a graphical dependence of the current flowing through p-n transition current from the external voltage applied to it I=f(U) . The current-voltage characteristic of the p-n junction with forward and reverse switching is shown below.

It consists of straight(0-A) and reverse(0-B-C) branches; values ​​are plotted on the vertical axis forward and reverse current , and on the abscissa axis - the values forward and reverse voltage .

Voltage from an external source applied to the crystal with r-p junction is almost completely focused on the carrier-depleted junction. Depending on the polarity, two options for switching on direct voltage are possible - direct and reverse.

At direct switching on (fig. on the right - top), the external electric field is directed towards the internal one and partially or completely weakens it, reduces the height of the potential barrier ( Rpr ). At reverse turned on (fig. right - bottom), the electric field coincides in direction with the field r-p transition and leads to an increase in the potential barrier ( Robr ).

CVC p-n junction is described by an analytical function:

where

U - applied to the transition external voltage of the corresponding sign;

Iо = It - reverse (thermal) current p-p transition;

- temperature potential, where k is the Boltzmann constant, q- elementary charge (at T = 300K, 0.26V).

For direct voltage ( U>0 ) - the exponential term increases rapidly [ ], the unit in brackets can be neglected and considered as . With reverse voltage ( U<0 ) the exponential term tends to zero, and the current through the junction is practically equal to the reverse current; Ip-n = -Io .

The current-voltage characteristic of the p-n junction shows that even at relatively low forward voltages, the junction resistance drops, and the forward current increases sharply.

Breakdown p–n junction.

breakdown called a sharp change in the operating mode of the transition, which is under reverse voltage.

A characteristic feature of this change is a sharp decrease differential junction resistance (Rdif ). The corresponding section of the current-voltage characteristic is shown in the figure on the right (reverse branch). After the onset of breakdown, a slight increase in the reverse voltage is accompanied by a sharp increase in the reverse current. During the breakdown, the current can increase at a constant and even decreasing (in absolute value) reverse voltage (in the latter case, the differential resistance Rdif turns out to be negative).

Breakdown happens avalanche, tunnel, thermal. Both tunnel and avalanche breakdown are called electrical breakdown.