It strongly depends on the concentration of impurities. Semiconductors whose electrophysical properties depend on impurities of other chemical elements are called impurity semiconductors. There are two types of impurities, donor and acceptor.

Donor an impurity is called, the atoms of which give the semiconductor free electrons, and the electrical conductivity obtained in this case, associated with the movement of free electrons, is electronic. A semiconductor with electronic conductivity is called an electronic semiconductor and is conventionally denoted by the Latin letter n - the first letter of the word "negative".

Let us consider the process of formation of electronic conductivity in a semiconductor. We 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 electrophysical properties (that is, 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 the 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 easily separates from the atom and gets the opportunity to move in the substance.

acceptor An impurity is called an impurity whose atoms accept electrons from the atoms of the main semiconductor. The resulting electrical conductivity, associated with the movement of positive charges - holes, is 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 bond with three electrons of silicon, but this bond turns out to be incomplete: one more electron is missing to bond with the fourth electron of silicon. The impurity atom attaches the missing electron from one of the nearby atoms of the main semiconductor, after which it becomes associated with all four neighboring atoms. Due 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 left for the impurity atom, turns out to be connected with neighboring atoms by only three electrons. thus, there is an excess of positive charge and an unfilled bond appears, that is hole.

One of the important properties of a semiconductor is that in the presence of holes, a current can pass through it, even if there are no free electrons in it. This is due to the ability of holes to move from one semiconductor atom to another.

Moving "holes" in a semiconductor

By introducing a donor impurity into a part of a semiconductor and an acceptor impurity into another part, it is possible to obtain regions with electron and hole conductivity in it. A so-called electron-hole transition is formed at the boundary between the regions of electronic and hole conduction.

P-N junction

Consider the processes that occur when current passes through electron-hole transition. The left layer, labeled n, is electronically conductive. 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, denoted by the letter p, has hole conductivity. The current in this layer is associated with the movement of holes, which are indicated by circles with a “plus” in the figure.

Motion of electrons and holes in the direct conduction mode

Movement of electrons and holes in the reverse conduction regime.

When semiconductors with different types of conductivity come into contact, electrons due to diffusion will begin to move to the p-region, and holes - to the n-region, as a result of which the boundary layer of 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 is, as it were, barriers 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 the p-n junction is called a potential barrier, and the p-n junction is called a blocking layer. If the direction of the external electric field is opposite to the direction of the field of the p-n junction ("+" in the p-region, "-" in the n-region), then the potential barrier decreases, the concentration of charges in the p-n junction increases, the width and, therefore, the transition resistance decreases. When the polarity of the source is changed, the external electric field coincides with the direction of the field of the p-n junction, the width and resistance of the junction increases. Therefore, the p-n junction has valve properties.

semiconductor diode

diode called an electrically converting semiconductor device with one or more p-n junctions and two leads. Depending on the main purpose and the phenomenon used in the p-n junction, there are several main functional types of 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 I–V characteristic has a different form, but they are all based on the I–V characteristic of a junction 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 is the threshold voltage; rdyn is dynamic resistance; Uprob - breakdown voltage

The scale along the y-axis for negative values ​​of currents is chosen many times larger than for positive ones.

The current-voltage characteristics of the diodes pass through zero, but a sufficiently noticeable current appears only when threshold voltage(U then), which for germanium diodes is 0.1 - 0.2 V, and for silicon diodes it is 0.5 - 0.6 V. In the region of negative voltage values ​​​​on the diode, at already relatively low voltages (U arr. ) occurs 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. With an increase in the reverse voltage, an increase in current does not occur, since the number of minority carriers that appear at the transition boundary per unit time does not depend on the voltage applied from outside, if it is not very large. The reverse current for silicon diodes is several orders of magnitude less than for germanium ones. Further increase in reverse voltage to breakdown voltage(U samples) leads to the fact that electrons from the valence band pass into the conduction band, there is 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 definition of the main electrical parameters of diodes

Semiconductor diode designation

As mentioned earlier, the diode conducts current in one direction (i.e., ideally, it is just a conductor with low resistance), in the other direction it does not (i.e., it turns into a conductor with very high resistance), in a word, it has unilateral conduction. Accordingly, he has only two conclusions. They, as has been customary since the time of lamp technology, are called anode(positive conclusion) and cathode(negative).

All semiconductor diodes can be divided into two groups: rectifier and special. Rectifier Diodes, as the name implies, are designed to rectify alternating current. Depending on the frequency and shape of the alternating 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, and according to manufacturing technology, into alloy, diffusion and epitaxial. Planar diodes, due to the large area of ​​the p-n junction, are used to rectify high currents. Point diodes have a small junction area and, accordingly, are designed for rectification small currents. To increase the avalanche breakdown voltage, rectifier poles are used, consisting of a series of diodes connected in series.

High power rectifier diodes are called power. 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 CR on the diode at a certain value of forward current;
• reverse current I arr at a certain value of the reverse voltage;
• mean direct current I pr.cf. ;
• impulsive reverse voltage U arr. ;

To dynamic parameters diode are its time and frequency characteristics. These options include:

• recovery time t reverse voltage;
• rise time direct current I out. ;
• limit frequency without reducing the modes of the diode f max .

Static parameters can be set according to the current-voltage characteristic of the diode.

The reverse recovery time of the diode tvos is the main parameter of rectifier diodes, which characterizes their inertial properties. It is determined by switching the diode from a given forward current I CR to a given reverse voltage U arr. During switching, the voltage across the diode acquires the opposite value. Due to the inertia of the diffusion process, the current in the diode does not stop instantly, but over time t nar. In essence, there is a resorption of charges at the boundary of the p-n junction (ie, a discharge of equivalent capacity). It follows from this that the power losses in the diode increase sharply when it is turned on, especially when it is turned off. Consequently, losses in the diode 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 (voltage temperature coefficient) Upr \u003d -2 mV / K, and the reverse current of the diode has a positive coefficient. So with an increase in temperature for every 10 ° C, the reverse current of germanium diodes increases by 2 times, and silicon - 2.5 times.

Diodes with a Schottky barrier

For rectification of small voltages of high frequency are widely used schottky barrier diodes. In these diodes, instead of a p-n junction, a metal surface contact with is used. At the point of contact, semiconductor layers depleted in charge carriers, which are called shut-off layers, appear. Diodes with a Schottky barrier differ from diodes with a p-n junction in the following ways:

• 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 fast reverse voltage recovery time. In addition, the absence of minor media requiring recovery time means physical no loss 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 less than the forward voltage of diodes with a p-n junction by 0.2 ... 0.3 V.

The advantages of the 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 the rectifier makes it possible to reduce losses by about 10 ... 15%. The maximum operating frequency of Schottky diodes exceeds 200 kHz.

Theory is good, but without practical application it's just words.

Currently, three main groups of methods are used to fabricate junctions in gallium arsenide: diffusion, vapor phase epitaxy, and liquid phase epitaxy. The fusing method, which was previously used in semiconductor technology, is no longer used in PCD technology, since it does not produce a carved and flat electron-hole transition and, therefore, is unsuitable for the manufacture of laser diodes. Therefore, now the main methods for manufacturing PCG diodes are diffusion and epitaxy.

8.3.1. Diffusion method

The theory of diffusion is based on the assumption that impurity atoms do not interact with each other during diffusion, and the diffusion rate does not depend on their concentration. On the basis of this assumption, the fundamental equations of diffusion - Fick's laws - are derived. Fick's first law defines the diffusion flux as a quantity proportional to the concentration gradient (under isothermal conditions with one-dimensional diffusion)

where is the concentration of diffusing atoms; x - distance coordinate; diffusion coefficient.

Fick's second law determines the rate of diffusion

Based on these laws, one can find the distribution of impurity concentration in a semi-limited sample. For the case when the initial concentration in the bulk of the crystal is close to zero, while the surface concentration is and remains constant, the impurity concentration after time x at depth x is

If diffusion occurs from a thin layer with a thickness of impurity concentration per unit

surface, then the impurity distribution is expressed by the equation

Determination of the concentration profiles of the distribution of impurities in the sample is carried out either by the method of radioactive tracers, or by the probe method of measuring the "spreading of resistance" along the oblique cut of the sample.

The temperature dependence of the diffusion coefficient has the form

However, this dependence is not always maintained in binary semiconductors due to deviations from Fick's law, since the impurity interacts with one of the components of the compound or with vacancies formed due to the evaporation of a volatile component during the dissociation of the compound. Sometimes, as a result of the interaction of an impurity with the components of a compound, new compounds are formed that are more stable than the original binary semiconductor. In compounds of the diffusion type, diffusion occurs through the movement of atoms along the sites of the sublattice of elements of groups III and V. The activation energy of diffusion in this case depends on the type of sublattice, through the nodes of which diffusion occurs. However, this mechanism is not the only one; possible, for example, is the diffusion of an impurity along interstices. Diffusion of various impurities into binary semiconductors is considered in reviews. Data on the diffusion of impurities in gallium arsenide are given in Table. 8.3.

Fabrication of junctions by diffusion can be done by diffusing both donors into the -type gallium arsenide and acceptors into the -type material. Since the diffusion of donors is very slow, the diffusion of acceptors is usually carried out. The most common dopants used for the manufacture of injection are the acceptor - zinc and the donor - tellurium. The industry produces single crystals of gallium arsenide, intended for the production of PKG, doped with tellurium to concentrations These

(click to view scan)

concentrations, as shown above, and are optimal. An electron-hole transition in plates cut from these single crystals is produced by zinc diffusion, which allows, at not too high temperatures, to quickly produce a transition at any desired depth.

Gallium arsenide plates supplied for diffusion must be specially prepared. First of all, a plane with index (100) is revealed in the crystal by X-ray method. Then the crystal is cut into plates parallel to this crystallographic plane. The choice of the plane is determined by the following considerations. The crystals of the compounds are easily cleaved along the (110) plane. In the cubic structure of sphalerite, which is characteristic of these compounds, there are three (110) planes perpendicular to the (111) plane and two perpendicular (100) ones. If the (111) plane is chosen, triangular PKG diodes can be fabricated.

Diodes with typical Fabry-Perot resonators are easily fabricated from plates cut parallel to the (100) plane by a simple double cleavage along (110). These resonator planes must be strictly perpendicular to the future transition, since the thickness of the active layer of the diode is only 1-2 microns. Consequently, insignificant deviations of the resonator plane can lead to the emission of radiation from the active region. In order to fulfill this requirement, one side of the wafer is ground with 5 µm powder perpendicular to the cleaved planes prior to diffusion. The ground surface of the plate is manually polished on glass with polishing powder (grain size first 1 µm and then 0.3 µm). Sometimes chemical polishing is also used.

The process of zinc diffusion into a polished gallium arsenide plate is carried out either in a closed volume (in a sealed ampoule) or in a flow system. More often, however, a closed system is used. To do this, the ampoule is preliminarily pumped out to a residual pressure of about mm Hg. Art. As a source of zinc, either elemental zinc or its compounds are taken. The latter compound is a mixture of solid phases, the ratio

which are chosen depending on the temperature conditions of diffusion. If elemental zinc is used as an impurity source, then elemental arsenic is also placed in the ampoule in the ratio or As will be shown below, the pressure of arsenic in the ampoule is of great importance in this process.

There are three variants of diffusion processes used in technology to form junctions.

1. Single-stage zinc diffusion in an atmosphere of arsenic in a plate (100) or (111) is carried out at a temperature of Zinc and arsenic is loaded into the ampoule in the ratio of their total concentration in the gas phase should be After the process is completed, the ampoule is rapidly cooled with water. The duration of the process is chosen depending on the desired depth of the transition.

As a result of three-hour diffusion under these conditions, the transition is formed at a depth of about 20 μm.

2. Zinc diffusion followed by annealing in an arsenic atmosphere. The diffusion process is similar to that described above, but at the end of the diffusion process, the plate is placed in another ampoule, where arsenic is also placed in an amount. The ampoule with the load is pumped out to mm Hg. Art. and kept in a furnace at a temperature of 900 °C for Annealing contributes to the expansion of the compensated area, the alignment of the active transition layer, and the creation of a smooth, unsharp transition. The optimal conditions are as follows: stage I (diffusion) - temperature zinc concentration ratio duration stage I stage II (annealing) - temperature 900 or - arsenic concentration duration stage II The diffusion depth under these conditions is about 8 microns.

3. Three-stage diffusion. To the two-stage diffusion process described above, a third stage is added - a shallow diffusion of zinc to form a layer

At the end of the diffusion process and cooling of the ampoule, the gallium arsenide plate is removed and its edge is cut off to identify the transition, determine the depth of its occurrence and visually observe its characteristics: evenness, width, etc. In order to

to make the transition clearly visible, the chip is etched in a solution or a drop of the solution is applied to the chipped surface and held for 15–30 s, after which the plate is rinsed with distilled water. Two lines can be seen on the etched surface: the lower line defines the transition boundary, and the upper one is the place where the degeneration of the β-type material begins.

Mechanism of diffusion of zinc into gallium arsenide. The distribution of zinc concentration in gallium arsenide as a result of diffusion is anomalous. For zinc diffusion at temperatures below, it can be described by the Gaussian error function, i.e., equations (8.4) and (8.5); in this case, the values ​​of the diffusion coefficients can be calculated taking into account the parameters given in Table. 8.3. For diffusion temperatures above 800°C, the distribution of zinc in gallium arsenide does not follow this classical pattern. Typical examples of anomalous distribution of zinc are shown in fig.

8.13 for diffusion at temperature during

Anomalous phenomena during the diffusion of zinc into gallium arsenide are the subject of numerous studies. The following facts have been noted.

Rice. 8.13. Zinc concentration distribution profiles in a gallium arseiide plate for various surface concentrations at a diffusion temperature and a duration of about

At diffusion temperatures above, the diffusion coefficient of zinc strongly depends on the concentration of arsenic, and the solubility of zinc in gallium arsenide increases even by three orders of magnitude (from 1017 to , i.e., in the absence of a zinc concentration gradient on the sample.

Zinc atoms can be located in gallium arsenide either in places of gallium or in interstices. Therefore, zinc diffusion can occur along gallium vacancies and along interstices. Fick's law for such a double diffusion mechanism can be expressed by the equation

where and are the diffusion coefficients of zinc over interstices and over the gallium substitution mechanism.

This equation can be simplified by introducing the effective diffusion coefficient:

The results of isoconcentration diffusion show that, at high zinc concentrations, diffusion along interstices predominates, i.e.

Consequently, isoconcentration diffusion can also be described by equation (8.4). The isoconcentration diffusion coefficient can be calculated based on an analysis of the concentration of interstitial zinc atoms and gallium vacancies. Its strong dependence on zinc concentration is shown in fig. 8.14.

Rice. 8.14, Dependence of the diffusion coefficient of zinc in gallium arsenide on the concentration of zinc.

However, under real technological conditions at high temperatures, the surface concentration of zinc on gallium arsenide reached slightly exceeded the zinc vapor density in the ampoule. In the absence of arsenic pressure in the ampoule, the distribution of zinc in the sample was irreproducibly distorted, and

The transition was uneven, especially at low zinc concentrations. The introduction of arsenic into the ampoule substantially corrected the situation. The dependence of the diffusion coefficient on the zinc concentration significantly decreased, diffusion proceeded more regularly, and the transition turned out to be smooth.

Attention should be paid to the fact that anomalous phenomena in the diffusion of zinc occur at temperatures above the temperature of the onset of decomposition of gallium arsenide. Therefore, an arsenic pressure must be created in the ampoule, at least equal to the dissociation pressure of gallium arsenide at a given temperature. In addition, since zinc forms two congruently melting compounds with arsenic, one can expect their formation both on the zinc source and on the surface of gallium arsenide. These processes, as well as the dissociation of gallium arsenide, can lead to the release of liquid gallium and the formation of gallium solutions of zinc and gallium arsenide, as a result of which local surface disturbances arise, further distorting the diffusion profile and transition. To eliminate these surface disturbances and bring diffusion closer to the isoconcentration regime, zinc is sometimes diffused through a film deposited on gallium arsenide, or from a film doped with zinc.

The conditions for achieving reproducible diffusion of zinc into gallium arsenide can be determined by n? based on the consideration of phase equilibrium diagrams of gallium-arsenic-zinc (Fig. 8.15).

If only elemental zinc is used as a diffusant, then arsenic will be transferred from gallium arsenide to the zinc source until equilibrium phases of zinc arsenides are formed on both surfaces. Naturally, this will lead to the release of liquid gallium, damage to the wafer surface, and distortion of the diffusion front.

If the source is zinc and arsenic or zinc arsenides, then everything depends on the amount of diffusant, its composition and temperature. With small amounts of diffusant (several ampoules), no condensed phase is formed - all zinc and arsenic are in the vapor phase. The surface disturbances of the transition from the duration of diffusion and temperature is expressed by

The contact of two semiconductors of n- and p-types is called p-n-junction or n-p-junction. Diffusion begins as a result of contact between semiconductors. Some of the electrons go to the holes, and some of the holes go to the side of the electrons.

As a result, the semiconductors are charged: n is positive, and p is negative. After the electric field that will arise in the transition zone begins to impede the movement of electrons and holes, diffusion will stop.

When connecting a pn junction in the forward direction, it will pass current through itself. If you connect the pn-junction in the opposite direction, then it will practically not pass current.

The following graph shows the current-voltage characteristics of the forward and reverse connection of a pn junction.

Fabrication of a semiconductor diode

The solid line shows the current-voltage characteristic of the direct connection of the pn-junction, and the dotted line shows the reverse connection.
It can be seen from the graph that the pn-junction is asymmetric with respect to the current, since in the forward direction the junction resistance is much less than in the reverse direction.

The properties of the pn junction are widely used to rectify electric current. To do this, a semiconductor diode is made on the basis of a pn junction.

Typically, germanium, silicon, selenium and a number of other substances are used to make semiconductor diodes. Let us consider in more detail the process of creating a pn junction using germanium with n-type semiconductor.

Such a transition cannot be obtained by mechanically connecting two semiconductors with different types of conductivity. This is not possible because the gap between the semiconductors is too large.

And we need the thickness of the pn-junction to be no more than the interatomic distances. To avoid this, indium is melted into one of the sample surfaces.

To create a semiconductor diode, a p-type doped semiconductor containing indium atoms is heated to a high temperature. Pairs of n-type impurities are deposited on the surface of the crystal. Further, due to diffusion, they are introduced into the crystal itself.

On the surface of the crystal, which has p-type conductivity, a region with n-type conductivity is formed. The following figure shows schematically what this looks like.

In order to exclude the effect of air and light on the crystal, it is placed in a sealed metal case. On circuit diagrams, a diode is designated with the following special icon.

Solid-state rectifiers have very high reliability and a long service life. Their main disadvantage is that they can only work in a small temperature range: from -70 to 125 degrees.

Semiconductor diodes

A semiconductor diode is an element of an electrical circuit that has two terminals and has one-sided electrical conductivity. All semiconductor diodes can be divided into two groups: rectifier and special. Rectifier diodes, as the name suggests, are designed to rectify alternating current. Depending on the frequency and shape of the alternating voltage, they are divided into high-frequency, low-frequency and pulse. Special types of semiconductor diodes use different properties pn transitions: breakdown phenomenon, barrier capacitance, the presence of sections with negative resistance, etc.

Structurally, rectifier diodes are divided into planar and point, and according to manufacturing technology, into alloy, diffusion and epitaxial. Planar diodes due to large area pn-junctions are used to rectify high currents. Point diodes have a small junction area and, accordingly, are designed to rectify small currents. To increase the avalanche breakdown voltage, rectifier poles are used, consisting of a series of series-connected diodes.

High power rectifier diodes are called power diodes. The material for such diodes is usually silicon or gallium arsenide. Germanium is practically not used due to the strong temperature dependence of the reverse current. Silicon alloy diodes are used to rectify alternating current up to 5 kHz. Silicon diffusion diodes can operate at elevated 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.

The operation of diodes is based on the use of an electron-hole transition - a thin layer of material between two areas of different types of electrical conductivity - n and p. The main property of this transition is asymmetric electrical conductivity, in which the crystal passes current in one direction and does not pass in the other. The device of the electron-hole transition is shown in Fig. 1.1, a. One part of it is doped with a donor impurity and has electronic conductivity ( n-region); the other, doped with an acceptor impurity, has hole conductivity ( p-region). The carrier concentrations in the regions differ sharply. In addition, both parts contain a small concentration of minority carriers.

Fig.1.1. pn transition:

a - device, b - space charges

Electrons in n- areas tend to penetrate into p- region where the electron concentration is much lower. Likewise, holes in p-areas are moved to n-region. As a result of the oncoming movement of opposite charges, a so-called diffusion current arises. Electrons and holes, having passed through the interface, leave behind opposite charges, which prevent the further passage of the diffusion current. As a result, dynamic equilibrium is established at the boundary, and when closing p- and n- areas no current flows in the circuit. The distribution of the space charge density in the transition is shown in Fig. 1.1, b. In this case, inside the crystal at the interface there is an own electric field E oct. , the direction of which is shown in Fig. 1.1, a. Its intensity is maximum at the interface, where there is an abrupt change in the sign of the space charge. And then the semiconductor is neutral.

Potential barrier height at pn transition is determined by the contact potential difference n- and p-areas, which, in turn, depends on the concentration of impurities in them:

, (1.1)

where is the thermal potential, N n and Pp are the concentrations of electrons and holes in n- and p-areas, n i is the concentration of charge carriers in the undoped semiconductor.

The contact potential difference for germanium is 0.6 ... 0.7 V, and for silicon - 0.9 ... 1.2 V. The height of the potential barrier can be changed by applying an external voltage to pn transition. If the field of the external voltage coincides with the internal one, then the height of the potential barrier increases; when the applied voltage is reversed, the barrier height decreases. If the applied voltage is equal to the contact potential difference, then the potential barrier disappears completely.

Hence, if an external voltage lowers the potential barrier, it is called direct, and if it increases it, it is called reverse.

The symbol and current-voltage characteristic (CVC) of an ideal diode are shown in Fig. 1.2.

The output to which a positive potential must be applied is called the anode, the output with a negative potential is called the cathode (Fig. 1.2, a). An ideal diode in the conductive direction has zero resistance. In the non-conductive direction - an infinitely large resistance (Fig. 1.2, b).

Fig. 1.2. Symbol (a) and CVC

characteristic of an ideal diode (b)

in semiconductors R-type, holes are the main carriers. Hole electrical conductivity was created by introducing atoms of an acceptor impurity. Their valency is one less than that of semiconductor atoms. In this case, impurity atoms capture semiconductor electrons and create holes - mobile charge carriers.

in semiconductors n-type the main carriers are electrons. Electronic electrical conductivity is created by introducing donor impurity atoms. Their valency is one more than that of semiconductor atoms. Forming covalent bonds with semiconductor atoms, impurity atoms do not use 1 electron, which becomes free. The atoms themselves become immobile positive ions.

If a voltage source is connected to the external terminals of the diode in the forward direction, then this voltage source will create in district transition electric field directed towards the internal. The resulting field will decrease. This will start the diffusion process. A direct current will flow in the diode circuit. The greater the value of the external voltage, the smaller the value of the internal field, the narrower the blocking layer, the greater the value of the direct current. With an increase in external voltage, the direct current increases exponentially (Fig. 1.3). When a certain value of the external stress is reached, the width of the barrier layer will decrease to zero. The forward current will be limited only by the volume resistance and will increase linearly as the voltage increases.

Fig.1.3. IV characteristic of a real diode

In this case, the voltage drop across the diode is a forward voltage drop. Its value is small and depends on the material:

germanium Ge: U pr= (0.3 - 0.4) V;

silicon Si: U pr\u003d (0.6 - 1) V.

If you change the polarity of the external voltage, then the electric field of this source will coincide with the internal one. The resulting field will increase, the width of the barrier layer will increase, and the current will ideally not flow in the opposite direction; but since semiconductors are not ideal and in addition to the main mobile carriers there are a small number of minor ones, as a result, a reverse current arises. Its value depends on the concentration of minority carriers and is usually a few to tens of microamperes.

The concentration of minority carriers is less than the concentration of major ones, so the reverse current is small. The magnitude of this current does not depend on the magnitude of the reverse voltage. Silicon reverse current is several orders of magnitude less than that of germanium, but silicon diodes have a higher forward voltage drop. The concentration of minority carriers depends on the temperature, and as it increases, the reverse current increases, so it is called the thermal current I o:

I o (T) \u003d I o (T o)e a D T,

DT=T-T o ; and Ge =0.09k -1; and Si \u003d 0.13k -1; I oGe >>I oSi . .

There is an approximate formula

I o (T)=I o (T o)2 T * ,

where T *- temperature increment, which corresponds to a doubling of the thermal current,

T*Ge=8...10 o C; T*Si=6°C.

Analytical expression for VAC r-p transition looks like:

, (1.2)

where U is the applied external voltage.

For a temperature of 20 ° C φ t = 0.025V.

With an increase in temperature due to an increase in the thermal current and a decrease in the potential barrier, a decrease in the resistance of the semiconductor layers, a shift of the direct branch of the I–V characteristic occurs in the region of high currents. The volume resistance of semiconductors decreases n and R. As a result, the forward voltage drop will be less. As the temperature rises, the potential barrier of the barrier layer decreases due to a decrease in the difference between the concentrations of major and minor carriers, which will also lead to a decrease in U pr, since the barrier layer will disappear at a lower voltage.

The same current will correspond to different forward voltages (Fig. 1.4), forming the difference DU,

where e- temperature coefficient of voltage.

If the current through the diode is constant, then the voltage drop across the diode will decrease. With an increase in temperature by one degree, the forward voltage drop decreases by 2 mV.

Rice. 1.4. VAC r-p transition at Fig. 1.5. CVC of germanium and

different temperatures of silicon diodes

As the temperature rises, the reverse branch of the current-voltage characteristic shifts down (Fig. 1.4). The operating temperature range for germanium diodes is 80 ° C, for silicon diodes 150 ° C.

IV characteristics of germanium and silicon diodes are shown in Fig. 1.5.

Differential resistance r-p transition (Fig. 1.6):

(1.3)

With increasing current r d- decreases.

Fig. 1.6. Definition of differential

diode resistance

DC resistance r-p transition: .

DC resistance is characterized by the coefficient of the angle of inclination of a straight line drawn from the origin to a given point. This resistance also depends on the magnitude of the current: with increasing I, the resistance decreases . R Ge< R Si .

The IV characteristic of a semiconductor diode is somewhat different from the IV characteristic of an ideal diode. So, due to current leakage across the crystal surface, the real reverse current will be greater than the thermal current. Accordingly, the reverse resistance of a real diode is less than that of an ideal one. r-p transition.

Forward voltage drop is greater than ideal r-p transition. This is due to the voltage drop across the semiconductor layers. R and P type. Moreover, in real diodes one of the layers R or P has a higher concentration of major carriers than the other. A layer with a high concentration of majority carriers is called an emitter; it has negligible resistance. A layer with a lower concentration of majority carriers is called a base. It has quite a lot of resistance.

The increase in the forward voltage drop occurs due to the voltage drop across the base resistance.

To calculate electronic circuits containing semiconductor diodes, it becomes necessary to represent them in the form of equivalent circuits. The equivalent circuit of a semiconductor diode with a piecewise linear approximation of its CVC is shown in Fig. 1.7. Figure 1.8 shows equivalent circuits using the I–V characteristics of an ideal diode and the I–V characteristics of an ideal pn transition ( r d is the resistance of the diode, r is the leakage resistance of the diode).

Fig.1.7. Approximation of the current-voltage characteristic of a diode

linear segments

Fig.1.8. Replacing Diodes Using I-V Characteristics

ideal diode (a) and CVC ideal pn transition (b)

The operation of a diode in a circuit with a load. Consider the simplest circuit with a diode and a resistor, and the action of a bipolar voltage at its input (Fig. 1.9). The picture of the distribution of voltages on the circuit elements is determined by the position of the load lines (Fig. 1.10) - on the graph of the CVC of the diode along the voltage axis, two points are plotted in both directions, determined by +U m and –Um supply voltage, which corresponds to the voltage across the diode with a shorted load R n, and currents are deposited on the current axis in both directions U m / R n and - U m / R n, which corresponds to a shorted diode. These two points are connected in pairs by straight lines, which are called load. Load line intersections R n in the first and third quadrants with branches

I–V characteristics of the diode for each phase of the supply voltage correspond to

Rice. 1.9. Circuit with diode and Fig. 1.10. CVC diode with load

direct load

their identical currents (which is necessary when they are connected in series) and determine the position of the operating points.

positive half wave U>0, U=U m.

This polarity is direct for a diode. Current and voltage will always satisfy the current-voltage characteristics:

,

Moreover:

U d \u003d U m - I d R H;

at I d \u003d 0, U d \u003d U m;

at U d \u003d 0, I d \u003d U m / R H;

with direct connection U m >> U pr(Fig. 1.10).

In practical application U pr>0 (U pr- forward voltage) when the diode is open. When the diode operates in the forward direction, the voltage across it is minimal - ( Ge-0.4V; Si-0.7 V), and it can be considered approximately equal to zero. The current will then be maximum.

Fig.1.11. Voltage and current signals in a diode circuit with a load

.

negative half wave U<0, U= -U m .

The characteristic of the diode is the same, but

U d \u003d -U m -I d R H,;

I d \u003d 0, U d \u003d U m;

U d =0, I d =U m /R H ; U H<

Capacities r-p transition. When turned on r-p transition in the opposite direction, as well as at small forward voltages in the region r-p transition there is a double electric layer: in R areas - negative, in P areas - positive.

The accumulation of an uncompensated charge in this layer leads to the appearance of a capacitance r-p transition, which is called the barrier capacitance. It characterizes the change in the accumulated charge with a change in the external voltage according to Fig. 1.12. C b \u003d dQ / dU .

Rice. 1.12. Barrier capacitance dependence

from reverse voltage.

Barrier capacitance depends on geometric dimensions r-p transition. With the increase U arr width r-p transition increases, and the capacitance decreases.

When the diode is turned on in the forward direction, the barrier capacitance practically disappears, and the minority carriers transferred from the emitter accumulate in the base layer of the diode. This accumulation of charge also creates a capacitance effect, which is called diffusion capacitance. C d usually exceeds C b.

Diffusion capacity is determined C d \u003d dQ d / dU.

These capacitances affect the operation of diodes at high frequencies. Capacities r-p the transition is included in the equivalent circuit (Fig. 1.13).

Rice. 1.13. Diode equivalent circuits taking into account capacitances:

a – barrier capacitance; b - diffusion capacity

Transient processes in diodes. When diodes operate with high-frequency signals (1-10 MHz), the process of transition from a non-conductive state to a conductive state and vice versa does not occur instantly due to the presence of capacitance in the transition, due to the accumulation of charges in the diode base.

Figure 1.14 shows the timing diagrams of current changes through the diode and the load with rectangular pulses of the supply voltage. Capacitances in the diode circuit distort the leading and trailing edges of the pulses, causing the absorption time to appear tp.

When choosing a diode for a particular circuit, its frequency properties and speed must be taken into account.

Rice. 1.14. Transient processes at

switching diode:

t f1- the duration of the leading edge of the transition;

t f2- the duration of the trailing edge;

tp- dissolution time.

Breakdown r-p transition. The reverse voltage of the diode cannot increase to an arbitrarily large value. At some reverse voltage, characteristic of each type of diode, there is a sharp increase in the reverse current. This effect is called transition breakdown. There are several types of breakdown (Fig. 1.15):

1 - avalanche breakdown, when an increase in the reverse current occurs due to avalanche multiplication of non-main carriers;

Rice. 1.15. CVC for various types of breakdown

2-tunnel breakdown, when the overcoming of the potential barrier and the blocking layer occurs due to the tunnel effect.

During avalanche and tunnel breakdowns, the reverse current increases at a constant reverse voltage.

These are electrical breakdowns. They are reversible. After removal U arr the diode recovers its properties.

3- thermal breakdown, it occurs when the amount of heat released in r-p junction, more heat is given off by the surface of the diode to the environment. However, with increasing temperature r-p transition, the concentration of minority carriers increases, which leads to an even greater increase in the reverse current, which, in turn, leads to an increase in temperature, etc. Since for diodes made on the basis of germanium, I arr more than for silicon-based diodes, then for the former, the probability of thermal breakdown is higher than for the latter. Therefore, the maximum operating temperature for silicon diodes is higher (150 o ... 200 o C) than for germanium ones (75 o ... 90 o C).

With this breakdown r-p the transition is destroyed.

Test questions.

1. What is a semiconductor diode? Current-voltage characteristic of an ideal and real diode?

2. What materials are used to make semiconductor diodes? How to create regions of one or another type of conductivity in a semiconductor substrate?

3. What is the intrinsic electric field in a crystal at the boundary p-n- transition? How does it change when an external voltage is applied?

4. What explains the effect of one-way conduction p-n- junction in a semiconductor?

5. Current-voltage characteristics pn-transitions for germanium and silicon diodes when the external temperature changes?

6. How is the differential resistance of a diode determined?

7. How are the current-voltage characteristics of a diode with a load straight line constructed?

8. Explain the mechanism of formation of the barrier and diffusion capacitances of the diode? How do they affect the operation of the diode in AC circuits?

Lecture 2 Special types