Mon transition electrical engineering. Pn junction operating principle

According to ability to conduct electricity Solids were initially divided into conductors and dielectrics. Later it was noticed that some substances conduct electric current worse than conductors, but they also cannot be classified as dielectrics. They were separated into a separate group of semiconductors. Characteristic differences between semiconductors and conductors:

1. Significant dependence of the conductivity of semiconductors on temperature.
2. Even a small amount of impurities has a strong influence on the conductivity of semiconductors.
3. The influence of various radiations (light, radiation, etc.) on their conductivity. According to these features, semiconductors are closer to dielectrics than to conductors.

For the production of semiconductor devices, germanium, silicon, and gallium arsenide are mainly used. Germanium is a rare element scattered in nature, while silicon, on the contrary, is very common. However, it is not found in pure form, but only in the form of compounds with other elements, mainly oxygen. Gallium arsenide is a compound of arsenic and gallium. It began to be used relatively recently. Compared to germanium and silicon, gallium arsenide is less susceptible to temperature and radiation.

To understand the mechanism of operation of semiconductor devices, you must first become familiar with conductivity in semiconductors and the mechanism of formation of p

-n transitions.

The most widely used semiconductors are germanium and silicon. They belong to group IV of the Mendeleev periodic system. The outer shell of a germanium (or silicon) atom contains 4 valence electrons. Each of them forms covalent bonds with the neighboring four atoms. They are formed by two electrons, each of which belongs to one of the neighboring atoms. Pair-electron bonds are very stable, therefore each electron pair is firmly bound to its atomic pair and cannot move freely in the volume of the semiconductor. This is true for a chemically pure semiconductor located at a temperature close to 0 K

(absolute zero). As the temperature increases, the atoms of the semiconductor begin to undergo thermal vibrational motion. The energy of this movement is transferred to electrons, and for some of them it is sufficient to break away from their atoms. These atoms turn into positive ions, and the detached electrons can move freely, i.e. become current carriers. More precisely, the departure of an electron leads to partial ionization of 2 neighboring atoms. The single positive charge that appears in this case should be attributed not to one or another atom, but to the violation of the pair-electron bond left by the electron. The absence of an electron in a bond is called a hole. The hole has a positive charge equal to absolute value electron charge. The hole can be occupied by one of the electrons of the neighboring bond, and a hole is formed in the neighboring bond. The transition of an electron from one bond to another corresponds to the movement of a hole in the opposite direction. In practice, it is more convenient to consider the continuous movement of a positive charge than the sequential movement of electrons from bond to bond. Conductivity, which occurs in the volume of a semiconductor due to the disruption of bonds, is called own conductivity. There are two types of conductivity: n - type and p - type (from the words negative - negative, positive - positive). n-type conductivity is called electronic, and p-type conductivity is called hole conductivity.

Note that the violation of valence bonds can occur not only due to thermal energy, but also due to light energy or electric field energy.

Everything we have considered applies to pure semiconductors, i.e. to semiconductors without impurities. The introduction of impurities changes the electrical properties of the semiconductor. Impurity atoms in the crystal lattice occupy the places of the main atoms and form pair-electron bonds with neighboring atoms. If an atom of a substance belonging to group V of the periodic system of elements (for example, an arsenic atom) is introduced into the structure of a pure semiconductor (germanium), then this atom will also form bonds with neighboring germanium atoms. But group V atoms have 5 valence electrons on their outer shell. Four of them form stable pair-electronic bonds, and the fifth will be superfluous. This excess electron is bound to its atom much more weakly, and to tear it away from the atom requires less energy than to release an electron from a pair-electron bond. In addition, the transformation of such an electron into a free charge carrier is not associated with the simultaneous formation of a hole. The loss of an electron from the outer shell of an arsenic atom turns it into a positive ion. Then we can already talk about the ionization of this atom; this positive charge will not move, i.e. is not a hole.

With increasing arsenic content in a germanium crystal, the number of free electrons increases without increasing the number of holes, as was the case with intrinsic conductivity. If the electron concentration significantly exceeds the hole concentration, then the main current carriers will be electrons. In this case, the semiconductor is called an n-type semiconductor. Now let’s introduce a group III atom, for example, an indium atom, into the germanium crystal. It has three valence electrons. It forms stable bonds with three germanium atoms. The fourth bond remains empty, but does not carry a charge, so the indium atom and the adjacent germanium atom remain electrically neutral. Even with a slight thermal excitation, an electron from one of the neighboring pair-electronic bonds can move into this fourth bond.

What will happen? An extra electron will appear in the outer shell of indium, and the atom will turn into a negative ion. The electrical neutrality in the pair-electronic connection from which the electron came will be disrupted. A positive charge will appear - a hole in this broken connection. As the indium content increases, the number of holes will increase and they will become the main charge carriers. In this case, the semiconductor is called a p-type semiconductor.

Electron-hole transition (p – n junction).

A p–n junction is a region located at the interface between the hole and electron regions of one crystal. The transition is not created by simple contact of p and n type semiconductor wafers. It is created in one crystal by introducing two different impurities, creating electron and hole regions in it.

Fig.1. Mechanism of formation and action of p – n junction.

a – majority and minority carriers in semiconductor regions.

b – formation of a p–n junction.

c – direction of flow of diffusion current and conduction current.

d – p–n junction under the influence of external reverse voltage.

1 – electrons; 2 – holes; 3 – interface; 4 – immobile ions.

Let's consider a semiconductor in which there are two regions: electron and hole. In the first there is a high concentration of electrons, in the second there is a high concentration of holes. According to the law of concentration equalization, electrons tend to move (diffuse) from the n - region, where their concentration is higher, to the p - region, while holes do the opposite. This movement of charges is called diffusion. The current that arises in this case is diffusion. Equalization of concentrations would occur until holes and electrons are distributed evenly, but this is prevented by the forces of the emerging internal electric field. Holes leaving the p-region leave negatively ionized atoms in it, and electrons leaving the n region leave positively ionized atoms. As a result, the hole region becomes negatively charged, and the electron region becomes positively charged. An electric field created by two layers of charges arises between the regions.

Thus, near the interface between the electron and hole regions of the semiconductor, a region appears consisting of two layers of charges of opposite sign, which form the so-called p–n junction. A potential barrier is established between the p and n regions. In the case under consideration, inside the formed p–n junction there is an electric field E created

two layers of opposite charges. If the direction of the electrons entering the electric field coincides with it, then the electrons are slowed down. For holes it’s the opposite. Thus, thanks to the resulting electric field, the diffusion process stops. FIGURE 1 shows that in both the n- and p-regions there are both majority and minority charge carriers. Minority carriers are formed due to intrinsic conductivity. Electrons of the p-region, performing thermal chaotic movement, enter the electric field of the p-n junction and are transferred to the n region. The same thing happens with holes in the n-region. The current formed by the majority carriers is called the diffusion current, and the minority carriers are called the conduction current. These currents are directed towards each other, and since in an insulated conductor the total current is zero, they are equal. Let us now apply an external voltage to the junction with a plus to the n - region, and a minus to the p - region. The field created by the external source will enhance the action of the internal field of the p–n junction. The diffusion current will decrease to zero, as electrons from the n - region and holes from the p - region are carried away from the p - n junction to the external contacts, as a result of which the p - n junction expands. Only conduction current, which is called reverse current, passes through the junction. It consists of electron and hole conduction currents. The voltage applied in this way is called reverse voltage. The dependence of current on voltage is shown in the figure.

Rice. Volt-ampere characteristics p-n junction A. 2 – direct branch; 1 – reverse branch.

If an external voltage is applied with a plus to the p-region and a minus to the n-region, then the electric field of the source will be directed towards the field of the p-n junction and weaken its effect. In this case, the diffusion (direct) current (2) will increase. This phenomenon is the basis for the operation of a semiconductor diode.

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

In reality, an electron-hole transition cannot be created by simple contact of n- and p-type plates, since in this case an intermediate layer of air, oxides or surface contaminants is inevitable, a perfect match of crystal lattices is impossible, etc. These junctions are obtained by fusing or diffusion of appropriate impurities into the plates of a semiconductor single crystal, or by growing a p-n junction from a semiconductor melt with a controlled amount of impurities, etc. Depending on the method manufacturing р-n transitions can be alloyed, 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 conductivities of different types (n and p types) with the same concentration of donor and acceptor impurities and with an ideal surface and crystal lattice. Let us consider the phenomena that arise at their boundary.

Figure 1.3. Education r-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 caused by 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 across the semiconductor interface. However, they are insignificant (due to significant differences 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 hampered by the electric field that arises in the contact area. 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 appears. In the same way, in the p region, due to the departure of holes, their concentration in the near-contact layer decreases and an uncompensated negative charge of acceptor impurity ions appears here. Ions cannot “leave” from their places, because they are held back by the strongest forces (connections) crystal lattice. Thus, at the boundary of n- and p-type regions, two layers of charges of opposite sign are formed. An electric field arises, directed from the positively charged donor ions to the negatively charged acceptor ions. The region of the resulting space charges and the electric field actually represents a p-n junction. Its width ranges from hundredths to units of micrometers, which is a significant size compared to the dimensions of the crystal lattice.

Thus, at the boundary of the p-n junction, a contact potential difference is formed, 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 to another region. The contact potential difference is on the order of tenths of a volt.

The field of the pn transition is decelerating for the majority charge carriers and accelerating for the minority ones. Any electron passing from the electron region to the hole region enters an electric field, which tends to return it back to the electron region. In the same way, holes entering the electric field of the pn junction from the p region will be returned by this field back to the p region. In a similar way, the field affects charges formed for one reason or another inside the p-n junction. As a result of the influence of the field on charge carriers area p-n transition 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 to the contact potential difference, then this leads to a decrease in the height of the potential barrier. Therefore, the width of the p-n junction will decrease (Figure 1.3, b). Conditions for current passage are improved: the reduced potential barrier can be overcome by the majority carriers having the highest energy. As the external voltage increases, the current through the pn junction will increase. This polarity of 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 minority. They diffuse deep into the corresponding region of the semiconductor, recombining with the majority carriers of this region. Thus, as holes penetrate from the p-region to 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 junction when the height of the potential barrier is lowered into the region of the semiconductor, where these charge carriers are minority, is called injection (from the English word inject - inject, introduce).

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 p and n regions increases by the amount of external voltage. The number of primary carriers capable of overcoming the effect of the resulting field decreases. The majority carriers will be drawn away from the boundary layers into the interior of the semiconductor. The width of the p-n junction increases (Early 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 by the field into the region of the p-n junction. This phenomenon is called extraction. The current of minority carriers, as well as carriers arising 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 per unit time the number of electron–hole pairs generated at a constant temperature remains unchanged.

The analysis carried out allows us to consider the pn junction as a nonlinear element, the resistance of which varies depending on the polarity of the applied voltage. With increasing forward voltage p-n resistance transition decreases. With a change in polarity and the magnitude of the applied voltage, the resistance of the pn junction increases sharply. Therefore, the direct (linear) relationship between voltage and current (Ohm's law) for р-n transitions not complied with.

As can be seen from Figure 1.3, the pn junction is a double layer of stationary space charges of opposite sign. It can be likened to the plates of a flat capacitor, the plates of which are p- and n-regions, and the dielectric is a p-n junction, which has practically no moving charges. The size of the resulting so-called barrier (charging) capacitance is inversely proportional to the distance between the plates. As the blocking voltage applied to the junction increases, the area 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 value. Therefore, the pn 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 transition direct current 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 container, which is called diffusion. The diffuse capacitance is connected in parallel to the barrier capacitance. Diffusion capacitance values ​​can range from hundreds to thousands of picofarads. Therefore, at forward voltage, the capacitance of the pn junction is determined primarily by the diffusion capacitance, and at reverse voltage, by the barrier capacitance.

With forward voltage, the diffusion capacitance does not have a significant effect on the operation of the p-n junction, since it is always shunted by a small forward resistance of the junction. Its negative influence manifests itself during rapid switching of the p-n transition from an open state to a closed state.

The boundary between two adjacent regions of a semiconductor, one with n-type conductivity and the other with p-type conductivity, is called an electron-hole junction (p-n junction). It is the basis of most semiconductor devices. The most widely used are planar and point p-n junctions.

A planar p-n junction is a layered contact element in the bulk of a crystal at the boundary of two semiconductors with p- and n-type conductivities
(Fig. 1.2, a). In the production of semiconductor devices and integrated circuits, junctions of the type p+-n- or p-n+ junctions are used. The “+” index emphasizes the high electrical conductivity of this region of the single crystal.

Rice. 1.2 Planar (a) and point (b) p-n junctions

Let's consider the physical processes in a planar p-n junction (Fig. 1.3). Since the electron concentration in an n-type semiconductor is significantly greater than in a p-type semiconductor and, on the contrary, in a p-type semiconductor there is a high concentration of holes, a difference (gradient) in the concentration of holes dp/dx and electrons dn/dx is created at the interface between the semiconductors . This causes a diffusion movement of electrons from the n-region to the p-region and holes in the opposite direction. The densities of the hole and electron components of the diffusion current, caused by the movement of the majority carriers, are determined by the expressions:

where Dn and Dp are the diffusion coefficients of electrons and holes, respectively.

The total current density through the p-n junction is determined by the sum of the diffusion and drift components of the current densities, which are equal in the absence of external voltage. Since the diffusion and drift flows of charges through the p-n junction move in the opposite direction, they compensate each other. Therefore, in an equilibrium state, the total current density through the p-n junction is equal to

The presence of a double electrical layer causes the appearance of a contact potential difference in the p-n junction, which undergoes the greatest change at the boundary of n-p-type semiconductors and is called the potential barrier jк. The magnitude of the potential barrier is determined by the equation

where jT = kT/q – thermal potential (at normal temperature, i.e. at T = 300 K jT » » 0.026 V); рп and np – concentration of holes and electrons in n- and p-type semiconductors. For germanium junctions jT = (0.3 – 0.4) V, for silicon junctions jT = (0.7 – 0.8) V.

If you connect an external voltage source to the p-n junction in such a way that the plus is applied to the region of the n-type semiconductor, and the minus to the region of the p-type semiconductor (this connection is called reverse, Fig. 1.4), then the depletion layer expands, since under the influence of external voltage, electrons and holes are displaced from the p-n junction in different directions. In this case, the height of the potential barrier also increases and becomes equal to jк+ u (Fig. 1.5), since the external bias voltage is turned on according to the contact potential difference.

Fig 1.4 Reverse transition bias

Fig 1.5 Changing the potential barrier

Since the external source voltage is applied counter to the contact potential difference, the potential barrier is reduced by the amount u(cm.
rice. 1.7), and conditions are created for the injection of majority carriers - holes from the semiconductor p-type semiconductor n-type, and electrons - in the opposite direction. At the same time, through p—n-transition a large forward current flows due to the majority charge carriers. A further decrease in the potential barrier leads to an increase in the forward current with a constant value of the reverse drift current.

During the technological processing of a crystal, an impurity is introduced in such a way that its concentration, and therefore the concentration of the majority carriers in one of the regions of the crystal (usually in a p-type semiconductor) is two to three orders of magnitude higher than the impurity concentration in another region. The region with high impurity concentration (low-resistivity region) is the main source of mobile charge carriers through p—n-transition is called an emitter. The region with low impurity concentration is high-resistivity and is called the base. Therefore, the dominant component of the forward current flowing through p—n-transition and consisting of electron and hole components, will be the one that is determined by the main charge carriers of the region with their higher concentration

Ietc =Ip +In =I 0(e Uetc / j T — 1). (1.11)

When | U pr | >>j T the transition essentially disappears and the current is limited only by the resistance (units and even tens of ohms) of the base region r b .

Current-voltage characteristic (CVC) p—n-transition, constructed on the basis of expressions (1.10) and (1.11), has the form shown in Fig. 1.8. The region of the current-voltage characteristic lying in the first quadrant corresponds to direct connection p—n-transition, and the one lying in the third quadrant is in the opposite direction. As noted above, when the reverse voltage is sufficiently large, junction breakdown occurs. A breakdown is a sudden change in the operating mode of a junction under reverse voltage.

A characteristic feature of this change is a sharp decrease in the differential resistance of the junction r differential= du/di(u and i– transition voltage and transition current, respectively). After the breakdown begins, a slight increase in reverse voltage is accompanied by a sharp increase in reverse current. During the breakdown process, the current can increase while the reverse voltage remains unchanged and even decreases (in magnitude) (in the latter case, the differential resistance turns out to be negative). On the current-voltage characteristic of the transition (Fig. 1.9), the breakdown corresponds to the region of a sharp downward bend of the characteristic third quadrant.

Rice. 1.8 Current-voltage characteristic (a) and zener diode switching circuit (b)

There are three types of breakdown p-n-transitions: tunnel, avalanche and thermal. Both tunnel and avalanche breakdowns are commonly called electrical breakdowns.

Tunnel breakdown occurs when the geometric distance between the valence band and the conduction band (barrier width) is sufficiently small, then the tunnel effect occurs - the phenomenon of electrons passing through a potential barrier. Tunnel breakdown occurs in R—n- junctions with a base having a low resistivity value.

Rice. 1.9 I-V characteristics of the p-n junction

The mechanism of avalanche breakdown is similar to the mechanism of impact ionization in gases. An avalanche breakdown occurs if, when moving before the next collision with an atom, a hole (or electron) acquires energy sufficient to ionize the atom. As a result, the number of carriers increases sharply, and the current through the junction increases. The distance that a charge carrier travels before impact is called the mean free path. Avalanche breakdown occurs in junctions with a high-resistivity base (having a high resistivity). It is characteristic that in this case the breakdown voltage at the junction depends little on the current through it (steeply falling section in the third quadrant of the current-voltage characteristic, see Fig. 1.9).

During thermal breakdown, the increase in current is explained by the heating of the semiconductor in the region of the pn junction and the corresponding increase in specific conductivity. Thermal breakdown is characterized by negative differential resistance. If the semiconductor is silicon, then when the reverse voltage increases, thermal breakdown usually occurs after electrical breakdown (during electrical breakdown, the semiconductor heats up, and then thermal breakdown begins). After electrical breakdown, the pn junction does not change its properties. After thermal breakdown, if the semiconductor has managed to heat up sufficiently, the properties of the junction change irreversibly (the semiconductor device fails).

As already noted, due to the diffusion of electrons and holes through the p-n junction, uncompensated volumetric (spatial) charges of ionized impurity atoms arise in the transition region, which are fixed in the nodes of the crystal lattice of the semiconductor and therefore do not participate in the flow of electric current. However, space charges create an electric field, which, in turn, most significantly affects the movement of free carriers of electricity, i.e., the process of current flow.

Changing the external voltage applied to the pn junction changes the volumetric space charge of the depletion layer. Consequently, the p-n junction behaves like a flat capacitor, the capacitance of which, determined by the ratio of the change in space charge ¶Q to the change in voltage ¶U when the junction is switched back on, is called the barrier capacitance and can be found from the equation

where e0 – the dielectric constant vacuum; e – relative dielectric

permeability; S- square p- n-transition; d– thickness of the depleted layer (thickness p— n-transition).

Charge change in p- n-transition can also be caused by a change in the concentration of injected nonequilibrium carriers in the base during forward bias p— n-transition. The ratio of the magnitude of the change in the injected charge to the magnitude of the change in the forward voltage determines the diffusion capacitance p—n-transition:
With differential = d Q engineer/dU.Diffusion capacitance exceeds barrier capacitance under forward bias p—n-transition, however, has an insignificant value at reverse bias.

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

However, a single semiconductor chip made of P-type material on one side and N-type material on the other side (Figure below (b)) has unique properties. In a P-type material, the main carriers are positive charge carriers, holes, which move freely along the crystal lattice. In an N-type material, negative charge carriers, electrons, are the main and mobile ones. Near the junction, electrons from the N-type material diffuse through the junction, combining with holes in the P-type material. The region of 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 of majority carriers, thus it is known as depletion region. This area becomes a non-conducting material from its own semiconductor. In essence, we have almost an insulator separating the conductive doped regions of P and N types.

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

This separation of charges in a P-N junction represents a potential barrier. This potential barrier can be overcome by applying an external voltage source, causing the junction to conduct electrical current. 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 production. Silicon P-N junctions have a higher potential barrier compared to germanium junctions.

In the figure below (a), the battery is connected such that the negative terminal of the source supplies electrons to the N-type material. These electrons diffuse towards the junction. The positive terminal of the source removes electrons from the P-type semiconductor, creating holes that diffuse toward 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, canceling each other out. This frees up space inside the grid for movement towards the transition more charge carriers. Thus, the currents of the main charges of the N-type and P-type regions flow towards the transition. Recombination at the junction allows battery current to flow through the P-N junction of the diode. This inclusion is called forward bias.

(a) Forward bias pushes charge carriers toward the junction, where recombination is reflected in the battery current.
(b) Reverse bias attracts charge carriers to the battery terminals, away from the junction. The thickness of the depleted region increases. No steady current flows through the battery.

If the polarity of the battery 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-conducting depletion region. There is no recombination of the main carriers; and thus there is no conductivity. This battery connection is called reverse bias.

The diode symbol shown below in figure (b) corresponds to the doped semiconductor wafer in figure (a). The diode is unidirectional device. Electronic current flows in only one direction, against the arrow, corresponding to 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 remembering the location of semiconductor types in a diode. Non-indicating ( N ot-pointing) part symbol(band) corresponds to a 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) Current versus voltage plot of a silicon diode

If the diode is forward biased (as shown in figure (a) above), as the voltage increases from 0 V, the current will slowly increase. In the case of a silicon diode, the current flow can be measured when the voltage approaches 0.6 V (Figure (c) above). When the voltage increases above 0.6 V, the current after the bend in 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, several 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 amperes 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 is at its highest extreme conditions will be approximately 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 resistor is connected in series to limit this current. We typically select a diode with a reverse voltage greater than the voltages that can be applied during operation of the circuit to prevent breakdown of the diode. Typically, silicon diodes are available with breakdown voltages of 50, 100, 200, 400, 800 volts and higher. It is also possible to produce diodes with lower breakdown voltages (a few volts) for use as voltage standards.

We mentioned earlier that the microampere reverse leakage current in silicon diodes is due to the conductivity of the intrinsic semiconductor. This leak can be explained by theory. Thermal energy creates several electron-hole pairs that conduct leakage current before recombination. IN real practice this predictable current is only a fraction of the leakage current. Most of the leakage current is due to surface conductivity due to the lack of cleanliness of the semiconductor surface. Both components of leakage current increase with temperature, approaching microamps 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.

Let's sum it 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 across the junction from the N-type side to holes on the P-type side, followed by mutual annihilation, creates a voltage drop across the junction ranging from 0.6 to 0.7 volts for silicon, depending on the semiconductor.

Direct P-N offset transition 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 toward the junction, where recombination occurs, allowing electric current to flow.

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

A reverse leakage current flows through a P-N junction to which a reverse bias is applied, depending on the temperature. In small silicon diodes it does not exceed microamps.

-this is the region that separates the electron and hole conduction surfaces in a single crystal.

The electron-hole junction is made in a single single crystal, in which a fairly sharp boundary is obtained between the regions of electronic and hole conductivity.

The figure shows two adjacent regions of a semiconductor, one of which contains a donor impurity (the region of electronic, that is, n-conductivity), and the other an acceptor impurity (the region of hole conductivity, that is, p-conductivity). To understand how this or that type of semiconductor is formed, we recommend reading the article -Impurity semiconductors.

In the absence of an applied voltage, diffusion of the majority charge carriers from one region to another is observed. Since electrons are the main charge carriers, and in the n region their concentration is higher, they diffuse into the p-region, negatively charging the boundary layer of this region. But leaving their place, electrons create vacant places - holes, thereby charging the boundary layer of the n-region positively. Thus, after a fairly short period of time, space charges of opposite sign are formed on both sides of the interface.

The electric field created by space charges prevents further diffusion of holes and electrons. There is a so-called potential barrier, the height of which is characterized by the potential difference in the boundary layer.

The electron-hole junction, in its external design, is implemented in the form of a semiconductor diode.

If an external voltage is applied to the electron-hole junction so that the negative pole of the source is connected to the region with electronic conductivity, and the positive pole is connected to the region with hole conductivity, then the direction of the voltage of the external source will be opposite in sign to the electric one p-n field transition, this will cause an increase in current through the p-n junction. will arisedirect current,which will be caused by the movement of the main charge carriers, in our case this is the movement of holes from the p region to the n region, and the movement of electrons from the n region to p. You should know that holes move opposite to the movement of electrons, so in fact, the current flows in one direction. This connection is calleddirect. On the current-voltage characteristic, such a connection will correspond to the part of the graph in the first quadrant.

But if you change the polarity of the voltage applied to the p-n junction to the opposite, then electrons from the boundary layer will begin to move from the interface to the positive pole of the source, and holes to the negative. Consequently, free electrons and holes will move away from the boundary layer, thereby creating a layer in which there are practically no charge carriers. As a result, the current in the pn junction decreases by tens of thousands of times; it can be considered approximately equal to zero. Arises reverse current, which is not formed by the main charge carriers. This connection is called reverse. On the current-voltage characteristic, such a connection will correspond to the part of the graph in the third quadrant.

Volt-ampere characteristics

When connecting an electron-hole junction directly, the current increases with increasing voltage. When connected in reverse, the current reaches a value of I us, called the saturation current.If you continue to increase the voltage when turning it back on, a breakdown of the diode may occur. This property is also used in various zener diodes, etc.

The properties of pn junctions are widely used in electronics, namely in diodes, transistors and other semiconductors.