p-n-transition(n - negative - negative, electronic, p - positive - positive, hole), or electron-hole junction - a type of homojunction, Zone p-n transition called the region of a semiconductor in which there is a spatial change in the type of conductivity from electronic n to the hole p.
An electron-hole transition can be created in various ways:
- in the volume of the same semiconductor material, doped in one part with a donor impurity ( n-region), and in the other - acceptor ( p-region);
- at the interface of two different semiconductors with different types of conductivity.
If p-n- the transition is obtained by fusing impurities into a single-crystal semiconductor, then the transition from n- To R-area occurs abruptly (sharp transition). If diffusion of impurities is used, a smooth transition is formed.
Energy diagram p-n-transition. a) Equilibrium state b) With forward voltage applied c) With reverse voltage applied
When two areas come into contact n- And p- type, due to the concentration gradient of charge carriers, diffusion of the latter occurs in areas with the opposite type of electrical conductivity. IN p- the region near the contact after the diffusion of holes from it, uncompensated ionized acceptors (negative stationary charges) remain, and in n-regions - uncompensated ionized donors (positive stationary charges). Formed space charge region(SCR), consisting of two oppositely charged layers. Between uncompensated opposite charges of ionized impurities, an electric field appears, directed from n-areas to p-region and called the diffusion electric field. This field prevents further diffusion of the majority carriers through the contact - an equilibrium state is established (in this case, there is a small current of the majority carriers due to diffusion, and a current of minority carriers under the influence of the contact field, these currents compensate each other). Between n- And p-regions, there is a potential difference called the contact potential difference. The n-region potential is positive with respect to the potential p-regions Typically the contact potential difference is in this case is tenths of a volt.
An external electric field changes the height of the barrier and disrupts the balance of current carrier flows through the barrier. If a positive potential is applied to p-region, then the potential barrier decreases (direct displacement), and the SCR narrows. In this case, with increasing applied voltage, the number of majority carriers capable of overcoming the barrier increases exponentially. Once these carriers have passed p - n-transition, they become non-essential. Therefore, the concentration of minority carriers on both sides of the junction increases (injection of minority carriers). Simultaneously in p- And n-areas through contacts enter equal quantities main carriers causing compensation of charges of injected carriers. As a result, the recombination rate increases and a non-zero current appears through the junction, which increases exponentially with increasing voltage.
Application of negative potential to p-region (reverse bias) leads to an increase in the potential barrier. The diffusion of majority carriers through the junction becomes negligible. At the same time, the flows of minority carriers do not change (there is no barrier for them). Minority charge carriers are attracted by the electric field into p-n-junction and pass through it to the neighboring region (extraction of minority carriers). Minority carrier fluxes are determined by the rate of thermal generation of electron-hole pairs. These pairs diffuse to the barrier and are separated by its field, resulting in p-n- junction current flows I s(saturation current), which is usually small and almost independent of voltage. Thus, the current-voltage characteristic of the p-n junction has a pronounced nonlinearity. When changing sign U the value of the current through the junction can change by 10 5 - 10 6 times. Thereby p-n- the junction can be used to rectify alternating currents (diode).
Volt-ampere characteristics
To derive the dependence of the current value through p-n-transition from external bias voltage V, we must consider electron and hole currents separately. In what follows we will denote by the symbol J particle flux density, and symbol j- electric current density; Then j e = −eJ e , j h = eJ h.
Volt-ampere characteristics p-n-transition. I s- saturation current, U pr- breakdown voltage.
At V= 0 both J e and J h vanish. This does not mean, of course, that there is no movement of individual carriers through the junction, but only that equal numbers of electrons (or holes) move in both directions. At V≠ 0 the balance is disrupted. Consider, for example, a hole current through a depletion layer. It includes the following two components:
- Generation current n-regions in p-transition area. As the name suggests, this current is caused by holes generated directly in n-depletion layer region during thermal excitation of electrons from valence band levels. Although the concentration of such holes (minority carriers) in n-areas are extremely small compared to the concentration of electrons (majority carriers) they play important role in current transfer through the junction. This happens because every hole entering the depletion layer is immediately transferred to p-area under strong influence electric field, which exists inside the layer. As a result, the magnitude of the resulting generation current does not depend on the value of the potential change in the depletion layer, since any hole found in the layer is transferred from n-regions in p-region.
- Recombination current, that is, the hole current flowing from p-regions in n-region. The electric field in the depletion layer opposes this current, and only those holes that reach the depletion layer boundary with sufficient kinetic energy to overcome the potential barrier contribute to the recombination current. The number of such holes is proportional to e −eΔФ/kT and therefore
Unlike the generation current, the recombination current is extremely sensitive to the magnitude of the applied voltage V. We can compare the magnitudes of these two currents by noting that when V= 0 there is no total current through the transition: J h rec (V = 0) = J h gen It follows that J h rec = J h gen e eV/kT. Total hole current flowing from p-regions in n-region represents the difference between the recombination and generation currents:
Jh= J h rec − J h gen = J h gen(e eV/kT − 1).
A similar consideration is applicable to the components of the electron current with the only change that the generation and recombination currents of electrons are directed opposite to the corresponding hole currents. Since electrons have opposite charges, the electric currents of generation and recombination of electrons coincide in direction with the electric currents of generation and recombination of holes. Therefore, the total electric current density is j = e(J h gen + J e gen)(e eV/kT − 1).
Capacity p-n-transition and frequency characteristics
p-n-junction can be considered as a flat capacitor, the plates of which are the regions n- And p-type outside the transition, and the insulator is the space charge region, depleted of charge carriers and having high resistance. This capacity is called barrier. It depends on the external applied voltage, since external voltage changes the space charge. Indeed, an increase in the potential barrier during reverse bias means an increase in the potential difference between n- And p-regions of the semiconductor, and, hence, an increase in their volumetric charges. Since space charges are stationary and associated with donor and acceptor ions, an increase volumetric charge can only be caused by an expansion of its region and, consequently, a decrease in the electrical capacitance of the junction. Depending on the junction area, dopant concentration and reverse voltage, the barrier capacitance can take values from units to hundreds of picofarads. Barrier capacitance appears at reverse voltage; with direct voltage it is shunted with low resistance p-n-transition. Varicaps work due to the barrier capacitance.
In addition to barrier capacity p-n- the transition has the so-called diffusion capacity. Diffusion capacity is associated with the processes of accumulation and resorption of nonequilibrium charge in the base and characterizes the inertia of the movement of nonequilibrium charges in the base area. The diffusion capacity is due to the fact that an increase in voltage by p-n-transition leads to an increase in the concentration of majority and minority carriers, that is, to a change in charge. The magnitude of the diffusion capacitance is proportional to the current through p-n-transition. When forward bias is applied, the diffusion capacitance can reach tens of thousands of picofarads.
Equivalent circuit p-n-transition. C b- barrier capacity, C d - diffusion capacity, R a- differential resistance p-n-transition, r- volumetric resistance of the base.
Total capacity p-n-transition is determined by the sum of the barrier and diffusion capacitances. Equivalent circuit p-n- transition to alternating current presented in the figure. In the equivalent circuit, parallel to the differential resistance p-n-transition R and included diffusion capacitance C d and barrier capacity WITH b; the base volume resistance is connected in series with them r. With increasing frequency AC voltage, filed on p-n-transition, capacitive properties become more and more pronounced, R a is shunted by capacitance, and the total resistance p-n-transition is determined by the volume resistance of the base. Thus, at high frequencies p-n- the transition loses its linear properties.
Breakdown p-n-transition
Diode breakdown- this is the phenomenon of a sharp increase in the reverse current through the diode when the reverse voltage reaches a certain critical value for a given diode. Depending on the physical phenomena, leading to breakdown, distinguish between avalanche, tunnel, surface and thermal breakdown.
- Avalanche breakdown(impact ionization) is the most important breakdown mechanism p-n-transition. The avalanche breakdown voltage determines upper limit reverse voltage of most diodes. Breakdown is associated with the formation of an avalanche of charge carriers under the influence of a strong electric field, in which the carriers acquire energies sufficient for the formation of new electron-hole pairs as a result of impact ionization of semiconductor atoms.
- Tunnel breakdown electron-hole transition is the electrical breakdown of a transition caused by quantum mechanical tunneling of charge carriers through the band gap of a semiconductor without changing their energy. Electron tunneling is possible provided that the width of the potential barrier that electrons need to overcome is sufficiently small. For the same band gap (for the same material), the width of the potential barrier is determined by the electric field strength, that is, the slope energy levels and zones. Consequently, conditions for tunneling arise only at a certain electric field strength or at a certain voltage at the electron-hole junction - at a breakdown voltage. The value of this critical electric field strength is approximately 8∙10 5 V/cm for silicon junctions and 3∙10 5 V/cm for germanium junctions. Since the probability of tunneling very strongly depends on the electric field strength, then externally tunnel effect manifests itself as a breakdown of the diode.
- Surface breakdown (leakage current). Real p-n-junctions have sections that extend to the surface of the semiconductor. Due to possible contamination and the presence of surface charges between the p- and n-regions, conductive films and conductive channels can be formed, through which a leakage current I current flows. This current increases with increasing reverse voltage and can exceed the thermal current I 0 and the generation current I gen. The current Iut weakly depends on temperature. To reduce I ut, protective film coatings are used.
- Thermal breakdown- this is a breakdown, the development of which is due to the release of heat in the rectifying electrical junction due to the passage of current through the junction. When reverse voltage is applied, almost all of it drops to p-n- a junction through which there is, albeit a small, reverse current. The released power causes heating p-n-junction and adjacent areas of the semiconductor. If there is insufficient heat removal, this power causes a further increase in current, which leads to breakdown. Thermal breakdown, unlike the previous ones, is irreversible.
A P-N junction is the point in a semiconductor device where an N-type material and a P-type material come into contact with each other. The N-type material is usually referred to as the cathode part of the semiconductor, and the P-type material is the anodic part.
When contact occurs between these two materials, electrons from the n-type material flow into the p-type material and connect to the holes present in it. A small area on each side of the line of physical contact between these materials is almost devoid of electrons and holes. This region in a semiconductor device is called the depletion region.
This depletion region is a key element in the operation of any device that has a P-N junction. The width of this depletion region determines the resistance to current flow through the P-N junction, therefore the resistance of a device having such a P-N junction depends on the size of this depletion region. Its width can change when any voltage passes through this P-N junction. Depending on the polarity of the applied potential P-N A junction can be either forward biased or reverse biased. The width of the depletion region, or resistance, of a semiconductor device depends on both the polarity and the magnitude of the bias voltage applied.
When the P-N junction is direct (with forward bias), then a positive potential is applied to the anode, and a negative potential is applied to the cathode. The result of this process is a narrowing of the depletion region, which reduces the resistance to current flow through the P-N junction.
If the potential increases, the depletion region will continue to decrease, thereby further reducing the resistance to current flow. Eventually, if the applied voltage is high enough, the depletion region will narrow to the point of minimum resistance and maximum current will flow through the P-N junction, and with it through the entire device. When a P-N junction is appropriately forward biased, it provides minimal resistance to the flow of current through it.
When the P-N junction is reverse (reverse biased), a negative potential is applied to the anode, and a positive potential is applied to the cathode.
This causes the depletion region to expand, which causes an increase in resistance to current flow. When a P-N junction is reverse biased, there is maximum resistance to current flow and the junction acts essentially as an open circuit.
At a certain critical value reverse bias voltage, the resistance to current flow that occurs in the depletion region is overcome and a rapid increase in current occurs. The value of the reverse bias voltage at which the current increases rapidly is called the breakdown voltage.
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 of purity, so 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 bonded to crystal lattice silicon has a positive charge, and the electron is 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. A silicon atom lacking an electron has positive charge and, since it can capture 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.
The vast majority of modern semiconductor devices operate thanks to phenomena that occur at the very boundaries of materials that have different types of electrical conductivity.
There are two types of semiconductors - n and p. A distinctive feature of n-type semiconductor materials is that they contain electric charge negatively charged electrons. IN semiconductor materials p-type, the same role is played by the so-called holes, which are positively charged. They appear after an atom is torn away electron, and that is why a positive charge is formed.
Silicon single crystals are used to produce n-type and p-type semiconductor materials. Their distinctive feature is extremely high degree chemical purity. It is possible to significantly change the electrical properties of this material by introducing into it impurities that are quite insignificant at first glance.
The symbol "n" used in semiconductors comes from the word " negative» (« negative"). The main charge carriers in n-type semiconductor materials are electrons. In order to obtain them, so-called donor impurities are introduced into silicon: arsenic, antimony, phosphorus.
The symbol "p" used in semiconductors comes from the word " positive» (« positive"). The main charge carriers in them are holes. In order to obtain them, so-called acceptor impurities are introduced into silicon: boron, aluminum.
Number of free electrons and number holes in a pure semiconductor crystal is exactly the same. Therefore, when a semiconductor device is in an equilibrium state, each of its regions is electrically neutral.
Let us take as a starting point that the n-region is closely connected with the p-region. In such cases, a transition zone, that is, a certain space that is depleted of charges. It is also called " barrier layer", Where holes And electrons, undergo recombination. Thus, at the junction of two semiconductors that have different types of conductivity, a zone called p-n junction.
At the point of contact of semiconductors various types holes from the p-type region partially flow into the n-type region, and electrons, accordingly, into reverse direction. Therefore, a p-type semiconductor is charged negatively, and an n-type semiconductor is charged positively. This diffusion, however, lasts only until the electric field arising in the transition zone begins to interfere with it, resulting in movement and e electrons, And holes stops.
In industrially produced semiconductor devices for use p-n junction an external voltage must be applied to it. Depending on its polarity and magnitude, the behavior of the transition and the electric current passing directly through it depend. If the positive pole of the current source is connected to the p-region, and the negative pole is connected to the n-region, then direct connection takes place p-n junction. If the polarity is changed, a situation called reverse switching will occur. p-n junction.
Direct connection
When direct connection is performed p-n junction, then under the influence of external voltage a field is created in it. Its direction with respect to the direction of the internal diffusion electric field is opposite. As a result, the resulting field strength drops, and the blocking layer narrows.
As a result of this process, a considerable number of main charge carriers move into the neighboring region. This means that from region p to region n the resulting electricity will leak holes, and in the opposite direction – electrons.
Reverse switching
When reverse switching occurs p-n junction, then in the resulting circuit the current strength is significantly lower than with direct connection. The fact is that holes from region n will flow to region p, and electrons will flow from region p to region n. The low current strength is due to the fact that in the region p there is little electrons, and in the region n, respectively, – holes.
A pn junction is a thin region that forms where two semiconductors come into contact. different types conductivity. Each of these semiconductors is electrically neutral. The main condition is that in one semiconductor the main charge carriers are electrons and in the other they are holes.
When such semiconductors come into contact, as a result of charge diffusion, a hole from the p region enters the n region. It immediately recombines with one of the electrons in this region. As a result, an excess positive charge appears in the n region. And in the p region there is an excess negative charge.
In the same way, one of the electrons from the n region enters the p region, where it recombines with the nearest hole. This also results in the formation of excess charges. Positive in the n region and negative in the p region.
As a result of diffusion, the boundary region is filled with charges that create an electric field. It will be directed in such a way that it will repel holes located in the region p from the interface. And electrons from region n will also be repelled from this boundary.
In other words, an energy barrier is formed at the interface between two semiconductors. To overcome it, an electron from region n must have an energy greater than the energy of the barrier. Just like the hole from the p region.
Along with the movement of the majority charge carriers in such a transition, there is also the movement of minority charge carriers. These are holes from the n region and electrons from the p region. They also move to the opposite area through the transition. Although the resulting field contributes to this, the resulting current is negligible. Since the number of minority charge carriers is very small.
If an external potential difference is connected to the pn junction in the forward direction, that is, a high potential is supplied to the p region, and a low potential to the n region. That external field will lead to a decrease in the internal one. Thus, the barrier energy will decrease, and majority charge carriers can easily move through the semiconductors. In other words, both holes from the p region and electrons from the n region will move towards the interface. The recombination process will intensify and the current of the main charge carriers will increase.
Figure 1 - pn junction, forward biased
If the potential difference is applied in the opposite direction, that is, region p has a low potential, and region n has a high potential. That external electric field will add up to the internal one. Accordingly, the energy of the barrier will increase, preventing the majority of charge carriers from moving through the transition. In other words, electrons from region n and holes from region p will move from the transition to external parties semiconductors. And in the pn junction zone there will simply be no main charge carriers providing the current.
Figure 2 - pn junction, reverse biased
If the reverse potential difference is excessively high, the field strength in the junction region will increase until electrical breakdown occurs. That is, an electron accelerated by a field will not destroy covalent bond and will not knock out another electron, and so on.