p-n (pe-en) junction is a region of space at the junction of two p- and n-type semiconductors, in which a transition from one type of conductivity to another occurs, such a transition is also called an electron-hole transition.
There are two types of semiconductors: p and n types. In the n type, the main charge carriers are electrons , and in the p - type the main ones are positively charged holes. A positive hole appears after an electron is removed from an atom and a positive hole is formed in its place.
To understand how a p-n junction works, you need to study its components, that is, a p-type and n-type semiconductor.
P and n type semiconductors are made on the basis of monocrystalline silicon, which has a very high degree purity, therefore the slightest impurities (less than 0.001%) significantly change it electrical properties.
In an n-type semiconductor, the main charge carriers are electrons . To obtain them they use donor impurities, which are introduced into silicon,- phosphorus, antimony, arsenic.
In a p-type semiconductor, the main charge carriers are positively charged holes . To obtain them they use acceptor impurities — aluminum, boron
Semiconductor n - type (electronic conductivity)
An impurity phosphorus atom usually replaces the main atom at the sites of the crystal lattice. In this case, the four valence electrons of the phosphorus atom come into contact with the four valence electrons of the neighboring four silicon atoms, forming a stable shell of eight electrons. The fifth valence electron of the phosphorus atom turns out to be weakly bound to its atom and under the influence external forces(thermal vibrations of the lattice, external electric field) easily becomes free, creating increased concentration of free electrons . Crystal acquires electronic conductivity or n-type conductivity . In this case, the phosphorus atom, devoid of an electron, is rigidly bound to the silicon crystal lattice with a positive charge, and the electron is a mobile negative charge. In the absence of external forces, they compensate each other, i.e. in silicon n-typethe number of free conduction electrons is determined the number of introduced donor impurity atoms.
Semiconductor p - type (hole conductivity)
An aluminum atom, which has only three valence electrons, cannot independently create a stable eight-electron shell with neighboring silicon atoms, since for this it needs another electron, which it takes away from one of the silicon atoms located nearby. An electron-less silicon atom has a positive charge and, since it can grab an electron from a neighboring silicon atom, it can be considered a mobile positive charge not associated with the crystal lattice, called a hole. An aluminum atom that has captured an electron becomes a negatively charged center, rigidly bound to the crystal lattice. The electrical conductivity of such a semiconductor is due to the movement of holes, which is why it is called a p-type hole semiconductor. The hole concentration corresponds to the number of introduced acceptor impurity atoms.
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 distinct 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, the main and mobile ones are negative media charge, electrons. 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 transition becomes negative charge due to attracted electrons. Since the electrons have left the N-type region, it acquires a local positive charge. Thin layer 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 exposure to external source voltage causing the junction to conduct electricity. The formation of a transition and a potential barrier occurs during production 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 biased reverse direction, then only the leakage current of the intrinsic 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 actual practice, this predictable current is only a fraction of the leakage current. Most of The leakage current is due to surface conductivity associated with 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 from the transition.
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.
Strongly depends on the concentration of impurities. Semiconductors, the electrical properties of which depend on the impurities of other chemical elements, are called impurity semiconductors. There are two types of impurities: donor and acceptor.
Donor is called an impurity, the atoms of which give the semiconductor free electrons, and the resulting electrical conductivity associated with the movement of free electrons is electronic. A semiconductor with electronic conductivity is called an electronic semiconductor and is conventionally denoted Latin letter n is the first letter of the word “negative”.
Let us consider the process of formation of electronic conductivity in a semiconductor. Let's take silicon as the main semiconductor material (silicon semiconductors are the most common). Silicon (Si) has four electrons in the outer orbit of the atom, which determine its electrical properties (i.e., they move under the influence of voltage to create an electric current). When arsenic (As) impurity atoms are introduced into silicon, which has five electrons in its outer orbit, four electrons interact with four electrons of silicon, forming a covalent bond, and the fifth electron of arsenic remains free. Under these conditions, it is easily separated from the atom and is able to move in the substance.
Acceptor is an impurity whose atoms accept electrons from the atoms of the host semiconductor. The resulting electrical conductivity associated with the movement positive charges- holes, called hole. A semiconductor with hole electrical conductivity is called a hole semiconductor and is conventionally denoted by the Latin letter p - the first letter of the word “positive”.
Let us consider the process of formation of hole conductivity. When indium (In) impurity atoms are introduced into silicon, which has three electrons in the outer orbit, they enter into communication with three electrons of silicon, but this connection turns out to be incomplete: one more electron is missing to connect with the fourth electron of silicon. The impurity atom acquires the missing electron from one of the nearby atoms of the host semiconductor, after which it becomes associated with all four neighboring atoms. Thanks to the addition of an electron, it acquires an excess negative charge, that is, it turns into a negative ion. At the same time, the semiconductor atom from which the fourth electron has gone to the impurity atom turns out to be connected with neighboring atoms by only three electrons. thus, an excess of positive charge arises and an unfilled bond appears, that is hole.
One of important properties A semiconductor is that if there are holes, current can pass through it even if there are no free electrons in it. This is explained by the ability of holes to move from one semiconductor atom to another.
Movement of "holes" in a semiconductor
By introducing a donor impurity into part of a semiconductor and an acceptor impurity into another part, it is possible to obtain regions with electron and hole conductivity in it. At the boundary of the regions of electronic and hole conductivity, a so-called electron-hole transition is formed.
P-N junction
Let us consider the processes occurring when current passes through electron-hole transition. The left layer, designated n, has electronic conductivity. The current in it is associated with the movement of free electrons, which are conventionally indicated by circles with a minus sign. The right layer, designated p, has hole conductivity. The current in this layer is associated with the movement of holes, which are indicated in the figure by circles with a “plus”.
Movement of electrons and holes in direct conduction mode
Movement of electrons and holes in reverse conduction mode.
When semiconductors come into contact with various types conduction electrons due to diffusion will begin to move to the p-region, and holes - to the n-region, as a result of which boundary layer The n-region is charged positively, and the boundary layer of the p-region is negatively charged. An electric field arises between the regions, which acts as a barrier for the main current carriers, due to which a region with a reduced charge concentration is formed in the p-n junction. The electric field in a pn junction is called a potential barrier, and the pn junction is called a blocking layer. If the direction of the external electric field is opposite to the direction p-n fields transition (“+” on the p-region, “-” on the n-region), then the potential barrier decreases, the concentration of charges in the p-n junction increases, the width and, consequently, the resistance of the junction decreases. When the polarity of the source changes, the external electric field coincides with the direction of the field of the pn junction, the width and resistance of the junction increases. Therefore, the pn junction has gate properties.
Semiconductor diode
Diode called an electrical converting semiconductor device with one or more p-n junctions and two terminals. Depending on the main purpose and the phenomenon used in the p-n junction, there are several main functional types semiconductor diodes: rectifier, high-frequency, pulse, tunnel, zener diodes, varicaps.
Basic characteristics of semiconductor diodes is the current-voltage characteristic (VAC). For each type of semiconductor diode, the current-voltage characteristic has its own form, but they are all based on the current-voltage characteristic of a planar rectifier diode, which has the form:
Current-voltage characteristic (CVC) of the diode: 1 - direct current-voltage characteristic; 2 - reverse current-voltage characteristic; 3 — breakdown area; 4 - rectilinear approximation of the direct current-voltage characteristic; Upor—threshold voltage; rdin—dynamic resistance; Uprob - breakdown voltage
Y-axis scale for negative values currents chosen are many times larger than for positive ones.
The current-voltage characteristics of diodes pass through zero, but a sufficiently noticeable current appears only when threshold voltage(U pore), which for germanium diodes is equal to 0.1 - 0.2 V, and for silicon diodes is equal to 0.5 - 0.6 V. In the region of negative voltage values on the diode, at already relatively low voltages (U arr. ) arises reverse current(I arr.). This current is created by minority carriers: electrons of the p-region and holes of the n-region, the transition of which from one region to another is facilitated by a potential barrier near the interface. As the reverse voltage increases, the current does not increase, since the number of minority carriers appearing at the transition boundary per unit time does not depend on the externally applied voltage, unless it is very high. The reverse current for silicon diodes is several orders of magnitude less than for germanium diodes. Further increase in reverse voltage to breakdown voltage(U samples) leads to the fact that electrons from the valence band move to the conduction band, and a Zener effect. In this case, the reverse current increases sharply, which causes heating of the diode and a further increase in current leads to thermal breakdown and destruction of the p-n junction.
Designation and determination of the main electrical parameters of diodes
Semiconductor diode designation
As stated earlier, a diode conducts current in one direction (i.e., it is ideally just a conductor with low resistance), in the other direction it does not (i.e., it turns into a conductor with a very high resistance), in a word, it has one-way conductivity. Accordingly, it has only two conclusions. As has become the custom since the times of lamp technology, they are called anode(positive output) and cathode(negative).
All semiconductor diodes can be divided into two groups: rectifier and special. Rectifier diodes, as the name suggests, are intended for straightening alternating current. Depending on frequency and shape AC voltage They are divided into high-frequency, low-frequency and pulse. Special types of semiconductor diodes use different properties of p-n junctions; breakdown phenomenon, barrier capacitance, the presence of areas with negative resistance, etc.
Rectifier diodes
Structurally, rectifier diodes are divided into planar and point diodes, and according to manufacturing technology into alloy, diffusion and epitaxial. Planar diodes thanks to large area pn junctions are used for rectification high currents. Point diodes have small area transition and, accordingly, are intended for straightening low currents. To increase the avalanche breakdown voltage, rectifier columns are used, consisting of a series of diodes connected in series.
Rectifier diodes high power called by force. The material for such diodes is usually silicon or gallium arsenide. Silicon alloy diodes are used to rectify alternating current with a frequency of up to 5 kHz. Silicon diffusion diodes can operate at higher frequencies, up to 100 kHz. Silicon epitaxial diodes with a metal substrate (with a Schottky barrier) can be used at frequencies up to 500 kHz. Gallium arsenide diodes are capable of operating in the frequency range up to several MHz.
Power diodes are usually characterized by a set of static and dynamic parameters. TO static parameters diodes include:
- voltage drop U pr on the diode at a certain value of forward current;
- reverse current I rev at a certain value of reverse voltage;
- average value direct current I pr.sr. ;
- pulse reverse voltage U arr.i. ;
TO dynamic parameters diode include its time and frequency characteristics. These parameters include:
- recovery time treverse voltage;
- rise time direct current I outdoor ;
- limit frequency without reducing the diode modes f max.
Static parameters can be set using the current-voltage characteristic of the diode.
The diode reverse recovery time tres is the main parameter of rectifier diodes, characterizing their inertial properties. It is determined when the diode switches from a given forward current I pr to a given reverse voltage U arr. During switching, the voltage across the diode becomes reversed. Due to the inertia of the diffusion process, the current in the diode does not stop instantly, but over time t ext. Essentially, charge resorption occurs at the boundary of the p-n junction (i.e., discharge of equivalent capacity). It follows from this that the power loss in the diode increases sharply when it is turned on, especially when turned off. Hence, diode losses increase with increasing frequency of the rectified voltage.
When the temperature of the diode changes, its parameters change. The forward voltage on the diode and its reverse current depend most strongly on temperature. Approximately we can assume that TKN ( temperature coefficient voltage) Upr = -2 mV/K, and the reverse current of the diode has a positive coefficient. So, with every 10 °C increase in temperature, the reverse current of germanium diodes increases by 2 times, and of silicon diodes by 2.5 times.
Schottky barrier diodes
They are widely used for rectifying low voltages of high frequency. Schottky barrier diodes. These diodes use metal surface contact instead of a pn junction. At the point of contact, semiconductor layers depleted of charge carriers appear, which are called gate layers. Diodes with a Schottky barrier differ from diodes with a pn junction in the following parameters:
- more low straight voltage drop;
- have more low reverse voltage;
- more high current leaks;
- almost no charge reverse recovery.
Two main characteristics make these diodes indispensable: low forward voltage drop and short reverse voltage recovery time. In addition, the absence of non-primary media requiring recovery time means physical no losses to switch the diode itself.
The maximum voltage of modern Schottky diodes is about 1200 V. At this voltage, the forward voltage of the Schottky diode is 0.2...0.3 V less than the forward voltage of p-n junction diodes.
The advantages of a Schottky diode become especially noticeable when rectifying low voltages. For example, a 45-volt Schottky diode has a forward voltage of 0.4...0.6 V, and at the same current a p-n junction diode has a voltage drop of 0.5...1.0 V. When the reverse voltage drops to 15 V, the forward voltage decreases to 0.3...0.4 V. On average, the use of Schottky diodes in a rectifier can reduce losses by approximately 10...15%. The maximum operating frequency of Schottky diodes exceeds 200 kHz.
Theory is good, but without practical application these are just words.
Operating principle semiconductor devices is explained by the properties of the so-called electron-hole junction (p-n junction) - the interface between regions of a semiconductor 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-area to hole 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 are added to it, respectively (phosphorus, arsenic or aluminum, indium, etc.)
Atoms of the 5-valent 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 semiconductor atoms to create the missing covalent bond, gives it p-type conductivity, since the holes formed in this case (vacant energy levels in the valence band) behave electrically 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 minority carriers (electrons in a p-type semiconductor and holes in an n-type semiconductor), which are generated due to thermal vibrations of the atoms in the crystal lattice.
If semiconductors with different types conductivity bring 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 a p-type semiconductor. will happen electron recombination with holes near the interface of different types of 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 into opposite direction. As a result of the departure of the main charge carriers, a layer depleted of mobile carriers is created at the interface of different types of 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 of mobile carriers and extending to fractions of a micron, is electron-hole transition.
Potential barrier in p-n junction.
If you apply to a semiconductor electrical voltage, then depending on the polarity of this voltage, the p-n junction exhibits completely different properties.
Properties of p-n junction when connected directly.
Properties of a p-n junction during reverse switching.
So, with a certain degree of approximation, we can assume that electric current flows through the p-n junction if the polarity of the power source voltage is straight, and, on the contrary, there is no current when the polarity is reversed.
However, in addition to the dependence of the resulting 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 intrinsic charge carriers sharply decreases, and therefore I OBR too. Thus, if the junction is exposed to external energy, then a pair appears free charges: electron – hole. Any charge carrier born in the space charge regionp–n transition, will be picked up electric field E VN and ejected: electron – inn– area, hole – in p– region. An electric current arises, which is proportional to the width of the space charge region. This is due to the fact that the more E VN , the wider the region where there is an electric field in which the creation 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 maximum 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 crystal resistance decreases, and the semiconductor is thermally destroyed.
Current-voltage characteristic of p-n junction.
Volt-ampere characteristics (volt-voltage characteristic) is a graphical dependence of the flow through р-n junction current from the external voltage applied to it I=f(U) . Current-voltage characteristic р-n junction for direct and reverse connection is given below.
It consists of straight(0-A) and reverse(0-B-C) branches; on vertical axis values deferred forward and reverse current
, and on the abscissa axis are the values forward and reverse voltage
.
Voltage from an external source applied to the crystal with r-p transition, focuses almost entirely on the carrier-depleted transition. Depending on the polarity, two switching options are possible DC voltage - direct and reverse.
At direct when turned 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 when 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 ( Rrev ).
The current-voltage characteristic of the p-n junction is described by the analytical function:
Where
U - external voltage of the corresponding sign applied to the transition;
Iо = Iт - reverse (thermal) current p-p transition;
- temperature potential, where k - Boltzmann constant, q- elementary charge(at T = 300K, 0.26 V).
At direct voltage ( U>0
) - the exponential term increases rapidly [ 
], the unit in brackets can be neglected and considered . With reverse voltage ( U<0
) the exponential term tends to zero, and the current through the junction is almost equal to the reverse current; Ip-n = -Io
.
Volt-ampere p-n characteristic-junction shows that even at relatively small forward voltages, the junction resistance drops, and the forward current increases sharply.
Breakdown of the p–n junction.
Breakthrough called a sharp change in the operating mode of a junction under reverse voltage.
A characteristic feature of this change is a sharp decrease differential transition resistance (Rdiff
). The corresponding section of the current-voltage characteristic is shown in the figure on the right (reverse branch). 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 with a constant and even decreasing (in absolute value) reverse voltage (in the latter case, the differential resistance Rdiff
turns out to be negative).
Breakdown happens avalanche, tunnel, thermal. Both tunnel and avalanche breakdowns are commonly called electrical breakdown.
Of particular importance are the contacts of semiconductors with different types of conductivity, the so-called p-n junctions. On their basis, semiconductor diodes, detectors, thermoelements, and transistors are created.
Figure 41 shows the circuit of a pn junction.
At the interface of p-n-type semiconductors, a so-called “blocking layer” is formed, which has a number of remarkable properties that have ensured the widespread use of p-n junctions in electronics.
Since the concentration of free electrons in an n-type semiconductor is very high, and in a p-type semiconductor it is many times lower, diffusion of free electrons from the n region to the p region occurs at the boundary.
The same can be said about holes; they diffuse inversely from p to n.
Because of this, intense recombination of electron-hole pairs occurs in the boundary region (in the “blocking layer”), the blocking layer is depleted of current carriers, and its resistance increases sharply.
As a result of diffusion, a positive volume charge in the n region and a negative volume charge in the p region are formed on both sides of the boundary.
Thus, in the blocking layer an electric field arises with intensity , the field lines of which are directed from n to p, and hence the contact potential difference 
, where dk is the thickness of the barrier layer. Figure 37 shows a graph of the potential distribution in the pn junction.
The potential of the boundary of p and n regions is taken as zero potential.
It should be noted that the thickness of the barrier layer is very small and in Fig. 42 its scale is greatly distorted for clarity.
The greater the concentration of the main carriers, the greater the contact potential; in this case, the thickness of the barrier layer decreases. For example, for germanium at average concentrations of impurity atoms.
Uk = 0.3 – 0.4 (V)
dk = 10 -6 – 10 -7 (m)
The contact electric field inhibits the diffusion of electrons from n to p and holes from p to n, and very quickly a dynamic equilibrium is established in the blocking layer between electrons and holes moving due to diffusion (diffusion current) and their movement under the influence of the contact electric field in the opposite direction (drift current or conduction current).
In steady state, the diffusion current is equal and opposite to the conduction current, and since both electrons and holes participate in these currents, the total current through the blocking layer is zero.
Figure 43 shows graphs of the energy distribution of free electrons and holes in the p-n junction.
The graphs show that electrons from the n region need to overcome a high potential barrier to get into the p region. Consequently, it is available to very few of them, the most energetic ones.
At the same time, electrons from the p region freely pass into the n region, driven there by the contact field (roll into the “hole”).
But in the n-region the concentration of free electrons is negligible and in steady state a small, equal number of electrons move across the boundary in opposite directions.
Similar reasoning can be made about the movement of holes across the boundary of a pn junction. As a result, in the absence of an external electric field, the total current through the blocking layer is zero.
We connect the positive pole of the current source to the p-type semiconductor of the p-n junction, and the negative pole to the n-type semiconductor, as shown in Figure 44.
Then the electric field in this design, directed from the p-type semiconductor to the n-type semiconductor, promotes the directional movement of holes and electrons through the blocking layer, which leads to the enrichment of the blocking layer with majority current carriers and, consequently, to a decrease in its resistance. Diffusion currents significantly exceed conduction currents, both those generated by electrons and holes. Electric current flows through the pn junction due to the directional movement of the majority carriers.
In this case, the value of the contact potential (potential barrier) drops sharply, because the external field is directed opposite the contact one. This means that to create a current, it is enough to connect an external voltage of the order of only a few tenths of one volt to the pn junction.
The current arising here is called direct current. In a p-type semiconductor, forward current represents the directed movement of holes in the direction of the external field, and in an n-type semiconductor, free electrons in the opposite direction. Only electrons move in external (metal) wires. They move in the direction from the minus of the source and compensate for the loss of electrons leaving through the blocking layer into the p region. And from p electrons go through the metal to + the source. Towards the electrons, “holes” from the p-region move through the blocking layer into the n-region.
The potential distribution in this case is shown in Figure 45a
The dotted line shows the potential distribution in the pn junction in the absence of an external electric field. The change in potential outside the blocking layer is negligible.
In Fig. Figure 45b shows the distribution of electrons and holes under direct current conditions.
From Figure 40b it is clear that the potential barrier has dropped sharply, and it is easy for the main current carriers, electrons and holes, to penetrate through the barrier layer into regions that are “foreign” for them.
Now let's connect the positive pole to the n-type semiconductor, and the negative pole to the p-type. Under the influence of such reverse voltage through the p-n junction flows the so-called reverse current.
In this case, the strengths of the external electric and contact fields are co-directed, therefore, the strength of the resulting field increases and the potential barrier increases, which becomes practically insurmountable for the penetration of majority carriers through the blocking layer, and diffusion currents stop. The external field tends to drive holes and electrons away from each other, and the width of the blocking layer and its resistance increase. Only conduction currents, that is, currents caused by the directional movement of minority carriers, pass through the barrier layer. But since the concentration of minority carriers is much less than the majority, this reverse current is much less than the forward current.
Figure 45c shows the potential distribution in the pn junction in the case of reverse current.
A remarkable property of a pn junction is its one-way conductivity.
When the external field is directed directly from p to n, the current is large and the resistance is small.
In the opposite direction, the current is small and the resistance is high.