p-n-transition(n - negative - negative, electronic, p - positive - positive, hole), or electron-hole junction - a type of homojunction, Zone p-n junction A 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 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-regions through the contacts enter equal amounts of main carriers, causing compensation of the charges of the 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 the influence of a strong electric field that 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 junction: 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 density electric current There 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 in space charge can only be due to 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 on alternating current is shown 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 of alternating voltage applied to 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 the upper limit of the 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 much depends on the electric field strength, the tunneling effect externally 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.
Application
- Zener diodes (Zener diode)
- LEDs (Henry Round Diodes)
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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 its 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 free electrons . The crystal acquires electronic 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 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.
Electron-hole transition ( p–n-junction) is a transition layer between two regions of a semiconductor with different electrical conductivities, in which a diffusion electric field exists.
The regions are separated by a plane where the type of predominant impurities changes, called the metallurgical boundary. Near the metallurgical boundary, there is a layer depleted of mobile charge carriers, where immobile ionized impurity atoms are present (Fig. 3.1).
Rice. 3.1. Electron-hole transition
Stationary ions in the depletion layer create volumetric electric charges of positive and negative polarity. This creates a diffusion electric field of strength E diff and contact potential difference k. Contact value
the potential difference depends on the concentration of the acceptor impurity
N A, N D and temperatures:
.
The thickness of the depleted layer also depends on the concentration of impurities:
,
Where A– coefficient determined by the semiconductor material.
3.2. Current through p–n- transition
Through p–n-transition current flows, representing the sum of the diffusion and drift components. The diffusion current is formed by the main charge carriers, for whose movement the diffusion field is retarding. Increasing the diffusion current increases the field strength E diff, contact potential difference and potential barrier. This leads to a decrease in current. In this way, balance is established.
The drift current is formed by minority charge carriers, for which the diffusion field is accelerating.
In the equilibrium state, the sum of the diffusion and drift currents is zero:
I diff + I dr = 0.
3.3. Direct connection p–n-transition
A direct connection is one in which the field created by an external voltage is directed against the diffusion field (Fig. 3.2).
Rice. 3.2. Direct connection p–n-transition
As a result, the contact potential difference decreases, the potential barrier decreases, and the current of the majority charge carriers through the junction increases.
3.4. Reverse switching p – n-transition
Reverse switching p – n-transition is characterized by the fact that the field strength created by the external voltage coincides in direction with the diffusion field strength(Fig. 3.3).
Rice. 3.3. Reverse switching p–n-transition
As a result, the contact potential difference increases, the potential barrier increases, and the current of the majority charge carriers through the junction decreases.
3.5.
(volt-voltage characteristic) p–n- idealized
Volt-ampere characteristics p–n transition
-transition is the dependence of the current through the junction on the voltage applied to it. p–n Idealization
-transition consists of accepting the following
assumptions. p 1. Areas adjacent to the transition n And characterized by zero resistivity p–n. Therefore, external voltage is applied directly to
-transition. p–n-transition there are no processes of generation and recombination of free charge carriers. Then the current through the junction depending on the external voltage applied to the junction U external, i.e. The current-voltage characteristic can be described by the Shockley formula:
,
Where I 0 – thermal current, which is created by minority charge carriers and depends on three factors:
1) the concentration of minority charge carriers, inversely proportional to the concentration of impurities;
2) band gap Than more topics less-
she I 0 ;
3) temperature. With increasing temperature, the rate of generation of charge carriers increases and their concentration increases.
3.6. Zone (energy) diagram
p–n- idealized
At U ext = 0. Equilibrium state. The Fermi level has the same value for the entire structure (Fig. 3.4).
At U ext 0. Direct connection p–n-transition (Fig. 3.5).
Rice. 3.4. Band diagram of equilibrium p–n-transition
Rice. 3.5. Zone diagram with direct connection p-n-transition
At U ext 0. Reverse switching p–n-transition (Fig. 3.6).
Rice. 3.6. Zone diagram with reverse connection p–n-transition
3.7. Differences between the real current-voltage characteristics
and idealized p–n-transitions
Real p–n-transitions are, as a rule, asymmetrical. In this case, the concentration of impurities in one area exceeds the concentration of impurities in another. The region with a higher concentration is called the emitter, and the region with a lower concentration is called the base. A lower concentration of impurities means lower electrical conductivity and higher resistivity. Therefore, in real p–n- during transitions, neglect the specific
base resistance is not possible. Equivalent circuit of real
p–n-transition looks like (Fig. 3.7).
Rice. 3.7. Equivalent circuit of real p–n-transition
The second difference between real p–n-transition from the idealized one is the presence in the depletion layer of the processes of generation and recombination of charge carriers. Therefore, when turned back on, the current through the junction is not constant, but depends on the voltage applied to the junction (Fig. 3.8).
Rice. 3.8. The difference between the real current-voltage characteristic p–n-transition from idealized
The third difference is the presence of the breakdown phenomenon at
reverse switching p–n-transition.
3.8. Breakdown p–n-transition
The breakdown appears as sharp increase current through
p–n-transition with a slight change in the applied reverse voltage.
There are three types of breakdown.
Avalanche breakdown - occurs due to avalanche multiplication of minority charge carriers through impact ionization.
The voltage at which it appears increases with increasing temperature (Fig. 3.9). Rice. 3.9.
Tunnel breakdown occurs due to the transition of electrons from a bound state to a free state without imparting additional energy to them.
With increasing temperature, the breakdown voltage decreases (Fig. 3.10). Rice. 3.10. p–n-transition
CVC during tunnel breakdown
Thermal breakdown is a breakdown, the development of which is caused by the release of heat due to the passage of current through the junction. Unlike avalanche and tunnel, it is irreversible, i.e., as a result of a breakdown, the transition stops working. With increasing temperature, the breakdown voltage decreases (Fig. 3.11). p–n-transition
Rice. 3.11 p–n. CVC during thermal breakdown
3.9. VAC dependence
-transition p–n on temperature With increasing temperature, the current through- the transition increases when switched directly on due to an increase in carrier energy
electric charge p–n, which due to this more easily overcome the potential barrier.
When turned back on-junction, with increasing temperature, the current through it increases due to an increase in the rate of generation of charge carriers in the transition (Fig. 3.12). p–n Rice. 3.12.
VAC dependence p–n--transition from temperature
Volt-ampere characteristics p– n 3.10. VAC dependence
transition from semiconductor material I-transition depends on the band gap of the energy diagram of the semiconductor material.
The larger the band gap, the lower the rate of thermal generation and the lower the concentration of minority carriers creating the reverse current p– n 0 . Therefore, the reverse current is less. p– n When connected directly
.
-transition current through it will be greater, the smaller the bandgap width. Indeed, the current through I-transition is defined as I As the value increases, the current
0 the current decreases
also decreases. For the most common semiconductor materials Ge, Si and GaAs, the current-voltage characteristics are related as follows (Fig. 3.13). p–n. CVC during thermal breakdown
Rice. 3.13
. VAC dependence p–n-transition
from material p– n 3.11. Capacity
In the depletion layer 
,
Where -transition there are space charges that are formed by the charges of ionized donor and acceptor impurities. These charges are equal in magnitude and opposite in sign. Therefore, the depletion layer is like a capacitor. Since charges determine the potential barrier, the capacitance is called barrier capacitance. Its value is equal Where p–n-transition, U S n- square n– external voltage applied to the junction,
= 0.5 for a sharp transition,
The larger the band gap, the lower the rate of thermal generation and the lower the concentration of minority carriers creating the reverse current p–n-transition occurs the process of injection of minority charge carriers. Excess concentrations of minority carriers appear in each region and, in accordance with the condition of electrical neutrality, excess concentrations of majority carriers are equal to them. Thus, in n-regions (as in a capacitor) end up with an equal amount of positive charge of excess holes (minority carriers) and negative charge of excess electrons (majority carriers). Likewise p-region behaves like a capacitor with a negative charge of excess electrons (minority carriers) and an equal positive charge of excess holes (majority carriers).
The process of accumulation of excess charges is usually characterized by diffusion capacitance, which takes into account the change in excess carriers (holes and electrons) in both regions when the voltage changes.
Diffusion capacitance is determined by direct diffusion currents of holes Ip and electrons I n(hence the name of the capacity) and the lifetime of minority carriers and:
.
Diffusion currents Ip 1. Areas adjacent to the transition I n increase with increasing forward voltage p-n-transition and quickly vanish during the reverse transition. Therefore the dependence WITH voltage differential approximately repeats the course of the direct branch of the current-voltage characteristic p–n-transition.
Equivalent circuit p–n-junction, taking into account its capacitive properties, is shown in Fig. 3.15.
3.12. Metal-semiconductor contact
Contacts between semiconductor and metal are widely used to form external terminals from the semiconductor device fields and the creation of high-speed diodes. The type of metal-semiconductor contact is determined by the work function of electrons from the metal and semiconductor, the conduction current of the semiconductor, and the impurity concentration in it.
The electron work function is the energy required to transfer electrons from the Fermi level to the top of the upper free band.
With ideal contact between the metal and the semiconductor and without taking into account surface states, electrons diffuse predominantly from the material with a lower work function. As a result of diffusion and redistribution of charges, the electrical neutrality of the areas adjacent to the interface is disrupted, a contact electric field and a contact potential difference arise
Where A m, A n is the work function of electrons leaving the metal and semiconductor.
The transition layer in which a contact (or diffusion) electric field exists and which is formed as a result of contact between a metal and a semiconductor is called a Schottky junction.
Depending on the type of electrical conductivity of the semiconductor and on the ratio of work functions, a depleted or enriched layer may appear in the semiconductor. If the work function in a metal is less than the work function in a semiconductor A m< A n, then electrons are more likely to move from the metal to the semiconductor. This leads to the formation of a depletion layer in the semiconductor if the semiconductor p-type, or even an inverse layer, if A m<< A n. If the semiconductor n-type, then an enriched layer is formed.
In depleted layers, the space charge is formed as a result of a violation of the compensation of the charge of ionized impurities by the main carriers, and in enriched layers - due to the accumulation of the main charge carriers. The enriched layer causes a low resistance of the near-contact region of the semiconductor compared to the resistance of the bulk of the semiconductor. Therefore, such a transition does not have rectifying properties and forms an ohmic contact. In the presence of a depletion or inverse layer, the Schottky junction has rectifying properties, since the external voltage, falling mainly at the high-resistance junction, will change the height of its potential barrier, changing the conditions for the passage of charge carriers through the junction.
A characteristic feature of the rectifying Schottky junction, in contrast to p–n-transition is a different height of potential barriers for electrons and holes. As a result, the injection of minority charge carriers into the semiconductor may not occur through the Schottky junction. Therefore, they do not accumulate and there is no need for their resorption. Hence the high performance of the Schottky junction.
Heterojunctions
A heterojunction is a transition layer with a diffusion electric field existing there between two semiconductors with different chemical compositions.
The width of the electrical bands of different semiconductors is different. Therefore, at the interface between two semiconductors (at the metallurgical contact of the heterojunction), a gap occurs in the bottom of the conduction band and the top of the valence band. As a result of discontinuities, the height of the potential barriers for electrons and holes in the heterojunction turns out to be different. This is a feature of heterojunctions, which determines the specific properties of heterojunctions in contrast to p–n-transitions.
Heterojunctions can be formed by semiconductors with different types of conductivity: p–n, p–p, n–n. Depending on the type of conductivity and the band gap of the energy diagrams, the current through the junction can be determined by both electrons and holes. For example, through contact germanium p-type and gallium arsenide n-type mainly electron current flows (Fig. 3.16).
Rice. 3.16. Band diagram of the Ge transition ( p-type) – GaAs ( n-type)
Through the germanium transition p-type, gallium arsenide p-type mainly hole current flows (Fig. 3.17).
Rice. 3.17. Band diagram of the Ge transition ( p-type) – GaAs ( p-type)
To form a high-quality heterojunction, it is necessary to match the type, orientation and period of the crystal lattices of the contacting semiconductors, so that the crystal lattice of one semiconductor passes into the crystal lattice of another semiconductor with a minimum number of violations. The most widely used in semiconductor devices are heterojunctions between semiconductors based on arsenides, phosphides and antimonides of gallium and aluminum. Due to the proximity of the covalent radii of gallium and aluminum, changes in the chemical composition of semiconductors in a heterojunction occur without changing the lattice period. Heterojunctions are also created on the basis of multicomponent solid solutions, in which the lattice period does not change when the composition changes over a wide range.
3.14. Metal–insulator–semiconductor structure
Metal–insulator–semiconductor (MIS) structures form the basis of MIS field-effect transistors, photovoltaic devices, voltage-controlled capacitors, and are also widely used in integrated circuits.
The simplest MIS structure contains a semiconductor crystal - substrate, a dielectric layer, a metal electrode - gate, and an ohmic contact to the substrate (Fig. 3.17).
Rice. 3.17. The simplest TIR structure
The structure has two outputs - a gate and a contact to the substrate and is an MIS capacitor, the capacitance of which depends on the voltage U between the gate and the substrate lead.
The gate voltage creates an electric field that penetrates through a thin (0.03...0.1 μm) dielectric layer into the near-surface layer of the semiconductor, where it changes the carrier concentration. Depending on the voltage value, enrichment, depletion or inversion modes are observed.
The equivalent circuit of a MIS structure can be represented by connecting two capacitors in series C D– dielectric capacity and With g:
where J p is the charge density of uncompensated impurity ions and mobile charge carriers in the semiconductor, j pov is the voltage in the surface layer of the semiconductor, -transition there are space charges that are formed by the charges of ionized donor and acceptor impurities. These charges are equal in magnitude and opposite in sign. Therefore, the depletion layer is like a capacitor. Since charges determine the potential barrier, the capacitance is called barrier capacitance. Its value is equal– gate area.
The most widely used MIS structure is based on silicon, where the dielectric is silicon dioxide and the gate is an aluminum film.
Related information.
Based on their ability to conduct electric current, 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:
- Significant dependence of the conductivity of semiconductors on temperature.
- Even a small amount of impurities has a strong influence on the conductivity of semiconductors.
- 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 the 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 in absolute value to the charge of the electron. 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. Current-voltage characteristic of p-n junction. 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 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 negatively charged elements act as electric charge carriers. electrons. In p-type semiconductor materials, 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 an extremely high degree of 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 is formed between them, that is, a certain space that is depleted of charges. It is also called " barrier layer", Where holes 1. Areas adjacent to the transition 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 between semiconductors of different types, holes from the p-type region partially follow into the n-type region, and electrons, accordingly, move in the opposite 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 electric current will flow 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.
