Semiconductor materials. Structure, connection of atoms in a crystal lattice. Formation of charge carriers in intrinsic and impurity semiconductors.

Semiconductors are, as a rule, solids with a regular crystalline structure (single crystals). Their crystal lattice consists of many repeating and adjacent unit cells.

Types of cubic lattice:

Simple cubic lattice

Cubic body-centered lattice

-cubic face-centered lattice

- diamond type grating

Contact phenomena. Classification. Electron-hole transition. Education, operating principle r-n transition in equilibrium and nonequilibrium states. Current-voltage characteristics. Electric field effect.

Transitions between two regions of a semiconductor with different types electrical conductivity is called electron-hole or R- n-junctions . .

Equilibrium analysis p-n- transition

The height of the equilibrium potential barrier is determined by the difference in electrostatic potentials in R- And n- Dj o = j Ep – j En .

Dj o = j T ln ( n n o r r o / n i 2)

the equilibrium height of the potential barrier is determined by the ratio of the concentrations of carriers of the same type (electrons or holes) on both sides of the transition, at its boundaries:

Dj o = j T ln ( n n o / n r o);Dj o = j T ln (p p o / p no) potential barrier width in an asymmetric transition:

l o= Ö(2e o eDj o) / ( qN) ,

the width of the equilibrium smooth transition in the following form:l o = 3 Ö(9e o eDj o) / (qN"), where N"- effective concentration gradient. Since the gradient is the same in both parts of the transition, the width l o divided equally between n- And R-layers, i.e. the smooth transition is symmetrical.

Analysis of nonequilibrium p-n- transition

If you connect an EMF source U between R- And n- layers, then the equilibrium of the transition will be disrupted and current will flow in the circuit. The resistivity of the depletion layer is much higher than resistivities neutral layers, therefore the external voltage almost completely drops at the transition, which means that the change in the height of the potential barrier is equal to the value of the applied emf.

When EMF U attached plus to R- layer, the height of the barrier decreases

Dj = Dj o – U.

Voltage of this polarity is direct. With a negative potential at p- layer, the height of the barrier increases and the minus sign should be changed to plus.

the width of the nonequilibrium barrier in the form

l = Ö(2e o e(Dj o – U)) / (qN).

Semiconductor diodes. Classification. Rectifier semiconductor diode, zener diode, LED, photodiode, Schottky diode. Operating principles, characteristics, parameters, areas of application of diodes.

Semiconductor diode called a semiconductor device with one electrical p-n- transition and two outputs. Semiconductor diodes use the property of one-way conductivity p-n transition - contact between semiconductors with different types of impurity conductivity, or between a semiconductor and a metal. Depending on the technological processes, used in their manufacture, are distinguished point diodes, alloy And microalloy, with diffusion base, epitaxial etc. By functional purpose diodes are divided into rectifier, universal, pulse, switching, multiplying, zener diodes (reference), tunnel, parametric, photodiodes, LEDs, magnetodiodes, etc.

Most semiconductor diodes are based on asymmetrical p-n- transitions. The low-resistance region of diodes is called emitter, and high-resistance - base. To create transitions with gate properties, use p-n-, р-i, n-i- transitions, as well as metal-semiconductor transitions.

Rectifier diodes designed to convert alternating current to permanent.

Pulse diodes have a short duration of transient processes and are designed for operation in pulse circuits. They differ from rectifier diodes in their small p-n junction capacitances (fractions of picofarads) and a number of parameters that determine the transient characteristics of the diode.

Schottky diode - semiconductor diode with a low voltage drop when connected directly.

Light-emitting diode- a semiconductor device with an electron-hole junction that creates optical radiation when passed through it electric current. The emitted light lies in a narrow range of the spectrum. Its spectral characteristics depend largely on the chemical composition of the semiconductors used in it.

Photodiode- optical radiation receiver, which converts light incident on its photosensitive area into electric charge due to processes in the p-n junction.

Principle of operation:

When exposed to radiation quanta in the base, free carriers are generated, which rush to the boundary of the p-n junction. The width of the base (n-region) is made such that holes do not have time to recombine before moving to the p-region. The photodiode current is determined by the minority carrier current - drift current. The speed of the photodiode is determined by the rate of carrier separation by the field of the p-n junction and the capacitance of the p-n junction C p-n

The photodiode can operate in two modes:

photovoltaic - no external voltage

photodiode - with external reverse voltage

Zener diode– a semiconductor diode designed to stabilize voltage.

Varicap is a nonlinear controlled capacitor. In semiconductor diodes, the dependence of the barrier capacitance on voltage is nonlinear, therefore any semiconductor device with p-n- The junction can, in principle, be used as a capacitor with a voltage-controlled capacitance.

In tunnel diodes, charge carriers pass through a potential barrier due to the tunneling effect.

Pulse properties of diodes. Explain the characteristics and explain what physical phenomena these properties are conditioned.

Pulse diodes have a short duration of transient processes and are designed for operation in pulse circuits. They differ from rectifier diodes in their small capacitances p-n- transition and a number of parameters that determine the transient characteristics of the diode. Reducing capacities is achieved by reducing area p-n- transition, therefore their permissible dissipation powers are low (30–40 mW).

Main parameters of pulse diodes:

● total diode capacity WITH d, (fractions of pF – several pF);

● maximum pulse forward voltage U pr and mach;

● maximum permissible pulse current I pr and mach;

● diode forward voltage setting time t mouth

● recovery time of diode reverse resistance t sun

The presence of recovery time is due to the charge accumulated in the base of the diode during injection. To turn off the diode, this charge must be “liquidated.” This occurs due to recombination and reverse transition of non-majority charge carriers to the emitter.

In high-speed pulse circuits, Schottky diodes (DS) are widely used, in which the transition is made on the basis of a metal-semiconductor contact. The structure of the DS is shown in Fig. 3.2 e. These diodes do not waste time accumulating and dissolving charges in the base. In DS, the current flow is carried out by the majority charge carriers and does not lead to the occurrence of processes of injection of non-majority carriers with their subsequent resorption when switching the voltage from direct to reverse.

In addition, their performance depends only on the speed of the barrier capacitance recharging process. The current-voltage characteristic of the DS resembles the characteristic of diodes based on p-n- transitions. The difference is that the forward branch within 8–10 decades (a decade is a 10-fold change in value) of the applied voltage represents an almost ideal exponential curve, and the reverse currents are small (fractions - tens of nA).

distinctive features of DS are: high speed, low voltage drop at forward bias (0.3–0.4 V), high efficiency straightening and wide possibilities of use as additional elements in the designs of various transistors and other semiconductor devices for the purpose of expanding functionality. Schottky diodes are also used in high current rectifiers and logarithmic devices.

Bipolar transistors. Structure, operating principle, operating modes of the transistor, transistor switching circuits. Integrated multi-emitter bipolar transistor. Structure, operating principle, application. Bipolar transistors in key and analog circuits.

BT are semiconductor devices with two or more interacting electrical p-n transitions and three or more outputs. Their amplifying properties are due to the phenomena of injection and extraction of non-majority charge carriers: injection from E to B, extraction from B to C.

Figure page 133

Principle of operation bipolar transistor is based on changing the resistance of a reverse-biased p-n junction due to the injection of charge carriers.

Operating modes

Regardless of the switching circuit, transistors can operate in one of four, differing in the polarity of the voltage at the EB and BC junction:

1)Normal active mode- E-transition is included in the forward direction, K-transition in the reverse direction

2) Saturation mode - E- and K-transitions are included in the forward direction

3) Cut-off mode - E- and K-transitions are turned on in the opposite direction

4) Inverse active mode - the E-transition is switched on in the reverse direction, the K-transition is switched on in the forward direction.

The main parameter of a bipolar transistor is the emitter current transfer coefficient:

Close to 1. Determined by 2 parameters, where is the injection coefficient, B is the transfer coefficient.

A huge number of modern electronic devices use electrical impulses in their work. These can be low-current signals or current pulses (which is much more serious technically) in the circuits of power supplies and other pulse converters, inverters, etc.

And the action of impulses in converters is always critical to the duration of forts and recessions, which have time boundaries of approximately the same order as transient processes in electronic components, in particular - in the same diodes. Therefore, when using diodes in pulse circuits, it is imperative to take into account the transient processes in the diodes themselves - during their switching on and off (during the opening and closing of the p-n junction).

In principle, in order to reduce the switching time of a diode from a non-conducting state to a conducting state and back, in some low-voltage circuits it is advisable to resort.

Diodes of this technology differ from conventional rectifier diodes by the presence of a metal-semiconductor junction, which, although it has a pronounced rectifying effect, at the same time has a relatively small throughput capacitance of the junction, the charge in which accumulates in such non-critical quantities and is absorbed so quickly that the circuit with Schottky diodes it can operate at a fairly high frequency when the switching time is on the order of a few nanoseconds.

Another advantage of Schottky diodes is that the voltage drop across their junction is only about 0.3 volts. So, the main advantage of Schottky diodes is that they do not waste time on the accumulation and dissolution of charges; the performance here depends only on the recharging rate of a small barrier capacitance.

As for, the original purpose of these components does not involve operation in pulse modes at all. The pulse mode for a rectifier diode is an atypical, non-standard mode, and therefore the developers do not place particularly high demands on the speed of rectifier diodes.

Rectifier diodes are used mainly to convert low-frequency alternating current into direct or pulsating current, where low throughput capacitance of the p-n junction and speed are not required at all; more often, simply high conductivity and, accordingly, high resistance to relatively long-term continuous current are required.

Rectifier diodes are therefore distinguished by low on-state resistance, a larger p-n junction area, and the ability to transmit high currents. But due to the large junction area, the diode capacitance is greater - on the order of hundreds of picofarads. This is a lot for a pulse diode. For comparison, Schottky diodes have a throughput capacitance of the order of tens of picofarads.

So, pulsed diodes are specially designed diodes for operation specifically in pulsed modes in high-frequency circuits. Their principled distinctive feature from rectifier diodes is the short duration of transient processes due to the very small capacitance of the pn junction, which can reach several picofarads and be even smaller.

Reducing the pn junction capacitance in pulsed diodes is achieved by reducing the junction area. As a result, the power dissipated on the diode body should not be very large, the average current through a small-area junction should not exceed the maximum permissible value indicated in the documentation for the diode.

Schottky diodes are often used as high-speed diodes, but they rarely have high reverse voltage, so pulsed diodes are distinguished as a separate type of diode.

Universal and pulse diodes

Universal (high frequency) diodes are used to convert high-frequency signals. Pulse semiconductor diodes designed primarily for operation in pulsed modes (conversion pulse signals). These diodes are characterized by minimal values ​​of reactive parameters, which is achieved thanks to special design and technological measures.

One of the main reasons for the inertia of semiconductor diodes is related to diffusion capacitance (see § 3.7, 3.8). To reduce the lifetime, doping of the material (for example, with gold) is used, which creates many trap levels in the band gap, increasing the recombination rate.

A type of universal diode is short base diode. In such a diode, the length of the base is less than the diffusion length of minority carriers. Consequently, the diffusion capacity will not be determined by the lifetime of minority carriers in the base, but by the actual shorter residence time (time of flight). However, to reduce the thickness of the base at large area pn junctions are technologically very difficult. Therefore, manufactured diodes with a short base and a small area are low-power.

Currently widely used diodes with p-i-n structure, in which two heavily doped p- and n-type regions are separated by a fairly wide region with conductivity close to its own (i-region). The charges of donor and acceptor ions are located near the boundaries i-regions The distribution of the electric field in it, in the ideal case, can be considered uniform (unlike a conventional p-n junction). Thus, i-a region with a low concentration of charge carriers, but with a dielectric constant, can be taken as a capacitor, the “plates” of which are narrow (due to the high concentration of carriers in R- And n-regions) layers of charges of donors and acceptors. Barrier capacity p-i-n-diode is determined by size i-layer and with a sufficiently wide i-region from the applied DC voltage practically does not depend.

Features of work p-i-n-diode is that under forward voltage, holes are simultaneously injected from the p-region and electrons from the n-region into the i-region. At the same time, its direct resistance drops sharply. With reverse voltage, carriers are extracted from i-regions to neighboring regions. A decrease in concentration leads to an additional increase in the resistance of the i-region compared to the equilibrium state. Therefore for p-i-n-diodes are characterized by a very large ratio of forward and reverse resistances, which is important when using them in switching modes.

Structures with Schottky and Mott barriers are used as high-frequency universal diodes. In these devices, direct conduction processes are determined only by the majority charge carriers. Thus, the diodes under consideration do not have a diffusion capacitance associated with the accumulation and resorption of charge carriers in the base, which determines their good high-frequency properties.

The difference between the Mott barrier and the Schottky barrier is that a thin i-layer is created between the metal M and a heavily doped semiconductor, so that the structure is M-i-n. IN high resistance i In the -layer, all the voltage applied to the diode drops, so the thickness of the depletion layer in the -region is very small and does not depend on voltage. And therefore, the barrier capacitance is practically independent of the voltage and base resistance.

Diodes with a Mott and Schottky barrier have the highest operating frequency, which, unlike a pn junction, almost do not accumulate minority charge carriers in the diode base during the passage of forward current and therefore have a short recovery time (about 100 ps).

A type of pulse diodes are charge storage diodes (CSDs) or diodes with a sharp recovery of reverse current (resistance). The reverse current pulse in these diodes has an almost rectangular shape (Fig. 4.2). In this case, the value can be significant, but must be extremely small for the use of DNC in high-speed pulsed devices.

Obtaining a short duration is associated with the creation of an internal field in the base near the depletion layer of the pn junction through the uneven distribution of the impurity. This field is inhibitory for carriers arriving through the depletion layer under forward voltage, and therefore prevents the injected carriers from leaving the boundary of the depletion layer, forcing them to concentrate more compactly near the boundary. When a reverse voltage is applied to the diode (as in a conventional diode), the charge accumulated in the base is dissolved, but the internal electric field will already contribute to the drift of minority carriers to the depletion layer of the junction. At the moment when the concentration of excess carriers at the transition boundaries drops to zero, the remaining excess charge of minority carriers in the base becomes very small, and, consequently, the time for the reverse current to drop to the value is short.

Varicaps

Varicap is a semiconductor diode used as an electrically controlled capacitance with a sufficiently high quality factor in the operating frequency range. It uses the property p-n- transition to change the barrier capacitance under the influence of external voltage.

The value of the varicap quality factor at low frequencies ;

on high frequencies ah –

Capacitance temperature coefficient , Where DT And D– changes in temperature and capacity of the varicap.

To increase the quality factor of a varicap, a Schottky barrier is used; these varicaps have low loss resistance, since metal is used as one of the diode layers.

Quality factor of the oscillatory system a characteristic of the resonant properties of a system, showing how many times the amplitude of forced oscillations during resonance exceeds the amplitude in its absence. The higher the quality factor of the oscillatory system, the less energy loss in it over a period.

The main application of varicaps is electrical frequency tuning oscillatory circuits. The dependence of its capacitance on voltage is reflected by the capacitance-voltage characteristic, similar to the dependence of the barrier capacitance p-n-transition from reverse voltage applied to it. Currently, there are several types of varicaps used in various devices continuous action. These are parametric diodes, designed to amplify and generate microwave signals, and multiplying diodes, designed to multiply the frequency over a wide frequency range. Sometimes diffusion capacitance is also used in multiplier diodes.

HIGH FREQUENCY, PULSE DIODES, VARICAPS

High frequency diodes

High-frequency diodes are universal-purpose devices. They can be used for rectification, detection and other nonlinear transformations of electrical signals in the frequency range up to 600 MHz. High-frequency diodes are usually made of germanium or silicon and have a point structure. The design of a point germanium diode is shown in Fig. 6.8. The diode consists of a germanium crystal soldered to a crystal holder, a contact electrode in the form of a thin tungsten wire and a glass balloon. The dimensions of the crystal are 1x1x0.2 mm. The radius of the area of ​​contact between the wire and germanium usually does not exceed 5 – 7 µm.

For getting r-p The junction diode is subjected to current molding during the manufacturing process. For this purpose, a short-term current pulse of up to 400 mA is passed through it in the forward direction. As a result of molding, a thin layer of semiconductor adjacent to the tip acquires hole conductivity, and at the boundary between this layer and the main mass of the plate a r-p transition. This diode design provides a small capacitance r-p transition (no more than 1 pF), which allows the diode to be used effectively at high frequencies. However, the small contact area between parts of a semiconductor with conductivity type P And R does not allow dispersion in the area r-p transition of significant power. Therefore, point diodes are less powerful than planar diodes and are not used in rectifiers designed for high voltages and currents. They are used mainly in circuits of radio receiving and measuring equipment operating at high frequencies, as well as in rectifiers for voltages not exceeding several tens of volts at a current of the order of tens of milliamps.

The inclusion of high-frequency point diodes in a circuit is not fundamentally different from the inclusion of planar rectifier diodes. The operating principle of a point diode is similar, based on the property of one-way conductivity r-p transition.

A typical current-voltage characteristic of a point diode is shown in Fig. 6.9, A. The reverse branch of the characteristics of a point diode differs significantly from the corresponding branch of the characteristics of a planar diode.

Due to the small area p- n transition reverse current diode is small, the saturation region is small and not so pronounced. As the reverse voltage increases, the reverse current increases almost uniformly. The effect of temperature on the magnitude of the reverse current is weaker than in planar diodes - the reverse current doubles with a temperature increase of 15 – 20°C (Fig. 6.9, b). Let us recall (section 6.1) that in planar r-p transitions, the reverse current increases approximately 2 – 2.5 times with an increase in temperature for every 10°C.

The properties of high-frequency diodes are characterized by parameters similar to those specified in paragraph 6.1. The following are essential for assessing the properties of high-frequency diodes:

Total capacity diode WITH D is the capacitance measured between the diode terminals at a given bias voltage and frequency.

Differential resistance r diff - the ratio of the voltage increment on the diode to the small current increment that caused it.

Frequency range ∆f- the difference in the limiting frequency values ​​at which the average rectified current of the diode is not less than a given fraction of its value at the lowest frequency.

High-frequency point diodes can be used in detection circuits, as limiters, nonlinear resistances, switching elements, etc.

IN last years Diodes based on the rectifying action of the metal - semiconductor contact - the so-called Schottky diodes. Unlike conventional point diodes, in which contact is made by pressing a metal needle, in Schottky diodes the contact is a thin film of metal (gold, nickel, aluminum, platinum, tungsten, molybdenum, vanadium, etc.). As was shown above (section 3.8), devices using a metal-semiconductor contact operate on majority charge carriers, which can significantly reduce their inertia and, therefore, increase performance. The switching time of Schottky diodes from a locked state to an open state and vice versa is determined by the small value of the barrier capacitance, which usually does not exceed 0.01 pF.

The main advantage of Schottky diodes compared to diodes based on r-p transitions - the possibility of obtaining lower values ​​of direct contact resistance, since the metal layer in these properties is superior to any, even a heavily doped semiconductor layer.

The low forward resistance and small capacitance of the Schottky barrier allows the diodes to operate at ultra-high frequencies. Typical operating frequency range is 5-250 GHz and switching time is less than 0.1 ns. The reverse currents of Schottky diodes are small and amount to several microamps. Reverse voltages lie in the range of 10...1000 V.

It should be noted that Schottky diodes became widespread relatively recently (in the early 70s), although their theory dates back more than 50 years. This is explained by the fact that only in recent years, thanks to improvements in the production technology of semiconductor devices and integrated circuits, has it been possible to obtain Schottky barriers with characteristics and parameters close to ideal.

Pulse diodes

Pulse diodes are designed to operate in high-speed pulse circuits with switching times of 1 μs or less. With such short operating pulses, it is necessary to take into account the inertia of the processes of turning on and off the diodes and take design and technological measures aimed at reducing the barrier capacitance and reducing the lifetime of nonequilibrium charge carriers in the region r-p transition.

By manufacturing method r-p transition pulse diodes are divided into point, alloy, welded And diffusion(mesa and planar). The design of diodes of these groups is shown in Fig. 6.10.

Design of point pulse diodes (Fig. 6.10, A) is practically no different from the design of conventional high-frequency diodes. In some cases, to improve the characteristics of the diode, an impurity (usually indium or aluminum) is applied to the tip of the contact needle, forming acceptor centers in germanium and silicon n-type. During the electroforming process, the near-contact region of the semiconductor heats up greatly and a small-sized material is formed directly under the tip of the needle. R-region.

In alloy diodes (Fig. 6.10, b) р−п The transition is obtained by fusing a piece of an alloy containing atoms of an acceptor impurity into a crystal of an electronically conductive semiconductor. The boundary between the original single crystal and the heavily doped one R-layer represents р−п transition. Typically this method is used in the manufacture of silicon pulse diodes. When creating similar germanium diodes, instead of the fusion method, the pulse welding method is used (Fig. 6.10, V). In this case, a thin gold needle (with a gallium additive) is brought to the germanium crystal and a high-amplitude current pulse is passed through the resulting contact, as a result of which the end of the gold needle is welded to germanium.

The fastest pulsed diodes are produced by the diffusion of donor or acceptor impurities into a solid semiconductor.

Penetrating to a certain depth of the semiconductor, diffusing atoms change the type of conductivity of this part of the crystal, as a result of which a R− P transition. After obtaining the diffusion structure, chemical etching of the semiconductor surface is carried out, after which R− P the transition is preserved only within a small area that rises above the rest of the surface in the form of a table (mesa). This type of crystal is called mesastructure (Fig. 6.10, G). Capacity R− P The transitions of mesadiodes are lower, and the breakdown voltage is higher than that of alloy or welded diodes. The switching time of the interdiodes does not exceed 10 ps.

Diodes obtained using planar epitaxial technology are very promising (Fig. 6.10, d). During their manufacture, an impurity is introduced locally into the semiconductor (usually silicon) − through “windows” in the protective oxide film of SiO 2. The resulting R− P The transitions are characterized by high parameter stability and reliability.

The simplest circuit for connecting a pulse diode is shown in Fig. 6.11, A. Under the influence of an input pulse of positive polarity (Fig. 6.11, b) a direct current flows through the diode, the magnitude of which is determined by the pulse amplitude, the load resistance and the resistance of the open diode. If a reverse voltage is applied to a diode through which direct current flows so as to block it, then the diode does not close instantly (Fig. 6.11, V).

Rice. 6.11. Connection diagram (a) and oscillograms

input voltage (b) and current (c) of a pulse diode

At the first moment there is a sharp increase in reverse current I 1 through a diode and only gradually over time it decreases and reaches a steady-state value I arr. This phenomenon is associated with the specifics of the work R− P transition and is a manifestation of the so-called accumulation effect. The essence of this effect is as follows. When direct current flows through R− P The transition is carried out by injection of carriers. As a result of injection, a concentration of minority nonequilibrium carriers is created in the immediate vicinity of the transition, which is many times higher than the concentration of equilibrium minority carriers in the region R− P transition: the higher the concentration of minority carriers, the greater the reverse current. The lifetime of nonequilibrium carriers is limited − gradually their concentration decreases both due to recombination and due to escape through R− P transition. Therefore, after some time (τ in Fig. 6.11, V) nonequilibrium minority carriers will disappear; reverse current will be restored to normal value I arr.

The main characteristic of pulsed diodes is their transient response. It reflects the process of restoration of the reverse current and reverse resistance of the diode when exposed to a pulse voltage of reverse polarity (see Fig. 6.11, V).

Main parameters of pulse diodes:

Reverse resistance recovery timeτ in − the time interval from the moment the current passes through zero after switching the diode from a given forward current to the state of a given reverse voltage until the reverse current reaches a given low value.

Charge switchingQPC− part of the accumulated charge flowing into the external circuit when the direction of the current changes from forward to reverse.

Total capacity CD− capacitance measured between the leads of a diode at a given bias voltage and frequency.

Pulse forward voltageU at − the peak value of the forward voltage across the diode for a given forward current pulse.

Pulse forward current I at − peak value of a forward current pulse at a given duration, duty cycle and shape.

For pulsed diodes, the value of direct forward voltage is also indicated U when leaking direct current I and the magnitude of the reverse current I arr. at a given reverse voltage value U arr. Limit modes are determined by the value of the maximum permissible constant reverse voltage U arr. max, the maximum permissible value of the pulse reverse voltage U Aubrey. max , as well as the values ​​of the maximum permissible direct direct current I ex. max and maximum permissible pulsed forward current I at. max.

Pulse diodes are widely used in pulse circuits for a wide variety of purposes, for example in logic circuits electronic digital computers.

Varicaps

Varicaps are semiconductor diodes that use a gated barrier capacitance r-p transition, depending on the magnitude of the reverse voltage applied to the diode. The design of the varicap is shown in Fig. 6.12. An aluminum column 4 is fused into the silicon crystal 5 on one side in a vacuum to obtain r-p transition, and on the other hand - a gold - antimony alloy to obtain an ohmic contact 6. This structure is fused in a vacuum into a gold-plated crystal holder 7. An internal terminal 2 is attached to the aluminum column. The connection of the crystal holder with the cylinder 3 and terminal 1 is carried out by fusion in hydrogen.

To use the properties of a varicap, it is necessary to apply reverse voltage to it (Fig. 6.13).

As is known, in the absence of external voltage between p And n− areas there is a contact potential difference (potential barrier) and an internal electric field. If a reverse voltage is applied to the diode U arr (Fig. 6.14, A), then the height of the potential barrier between p And n− areas will increase by the amount of applied voltage (Fig. 6.14, b), the electric field strength in r-p transition. External reverse voltage pushes electrons deeper inward n- area, and the holes go inward R- areas. As a result, the area expands r-p transition and the more, the higher the voltage U arr (in Fig. 6.14, b And V).

Thus, the change in reverse voltage applied to r-p transition, leads to a change in the barrier capacitance between p And n− regions. The value of the diode barrier capacitance C can be determined from the formula

Where e− relative dielectric constant of the semiconductor;

S − area r-p transition; d− width r-p transition.

Formula (6.3) is similar to the formula for the capacitance of a flat-plate capacitor. However, despite the similarity of these formulas, there is a fundamental difference between the barrier capacitance and the capacitance of the capacitor. In a conventional capacitor, the distance between its plates, and therefore its capacitance, does not depend on the voltage applied to the capacitor. The width of the pn junction depends on the magnitude of the voltage applied to it, therefore, the barrier capacitance depends on the voltage: as the blocking voltage increases, the width of the pn junction increases, and its barrier capacitance decreases.

The main characteristic of a varicap is the dependence of its capacitance on the reverse voltage (capacitance-voltage characteristic). Typical characteristic C = f (U arr) is shown in Fig. 6.15. Depending on the purpose, the nominal capacity of varicaps can range from several picofarads to hundreds of picofarads. The dependence of the varicap capacitance on the applied voltage is determined by the manufacturing technology r-p transition.

Varicap parameters:

Nominal capacity WITH rated - capacitance between the varicap terminals at rated bias voltage (usually U CM = 4 V).

Maximum capacity C max is the capacitance of the varicap at a given bias voltage.

Minimum capacity C min is the capacitance of the varicap at a given maximum bias voltage.

body contours

Overlap coefficient TO o is the ratio of the maximum diode capacitance to the minimum.

Quality factorQ- the ratio of the varicap reactance to the total loss resistance, measured at the nominal frequency at a temperature of 20 O C.

Maximum permissible voltage U max is the maximum instantaneous value of alternating voltage, ensuring a given reliability during long-term operation.

Capacitance temperature coefficient(TKE) – the ratio of the relative change in capacitance at a given voltage to the absolute change in ambient temperature that caused it.

Maximum permissible power P max is the maximum value of power dissipated by the varicap, at which the specified reliability during long-term operation is ensured.

The main application of a varicap is electronic tuning of oscillatory circuits. In Fig. 6.16, A A diagram of the inclusion of a varicap in an oscillatory circuit is shown. The circuit is formed by inductance L and varicap capacity WITH B. Coupling capacitor WITH p serves to ensure that the inductance L I did not short-circuit the DC varicap. Capacitor capacity WITH p should be several tens of times greater than the varicap capacity.

Control DC voltage U supplied to the varicap from potentiometer R2 through high-resistance resistor R1. The restructuring of the circuit is carried out by moving the slider of potentiometer R2.

This circuit has a significant drawback - high frequency voltage affects the varicap, changing its capacitance. This leads to circuit detuning. Switching on varicaps according to the diagram shown in Fig. 6.16, b, allows you to significantly reduce circuit detuning under the influence of alternating voltage. Here the varicaps are switched on at high frequency in series towards each other. Therefore, with any change in voltage on the circuit, the capacitance of one varicap increases and the other decreases. For constant voltage, the varicaps are connected in parallel.

A diode is one of the types of devices designed on a semiconductor basis. It has one p-n junction, as well as anode and cathode terminals. In most cases, it is designed for modulation, rectification, conversion and other actions with incoming electrical signals.

Principle of operation:

1. Electricity acts on the cathode, the heater begins to glow, and the electrode begins to emit electrons.
2. Between two electrodes an electric field is generated.
3. If the anode has a positive potential, then it begins to attract electrons to itself, and the resulting field is a catalyst this process. In this case, an emission current is generated.
4. Between electrodes a negative spatial charge is formed that can interfere with the movement of electrons. This happens if the anode potential is too weak. In this case, some of the electrons fail to overcome the influence of the negative charge, and they begin to move in the opposite direction, returning to the cathode again.
5. All electrons, which reached the anode and did not return to the cathode, determine the parameters of the cathode current. Therefore, this indicator directly depends on the positive anode potential.
6. Flow of all electrons, which were able to get to the anode, is called the anode current, the indicators of which in the diode always correspond to the parameters of the cathode current. Sometimes both indicators can be zero; this happens in situations where the anode has a negative charge. In this case, the field that arises between the electrodes does not accelerate the particles, but, on the contrary, slows them down and returns them to the cathode. The diode in this case remains in a locked state, which leads to an open circuit.

Device

Below is detailed description diode devices, studying this information is necessary for further understanding of the principles of operation of these elements:

1. Frame is a vacuum cylinder that can be made of glass, metal or durable ceramic varieties of material.
2. Inside the cylinder there are 2 electrodes. The first is a heated cathode, which is designed to ensure the process of electron emission. The simplest cathode in design is a filament with a small diameter, which heats up during operation, but today indirectly heated electrodes are more common. They are cylinders made of metal and have a special active layer capable of emitting electrons.
3. Inside the cathode indirect heat There is a specific element - a wire that glows under the influence of electric current, it is called a heater.
4. Second electrode is the anode, it is necessary to accept the electrons that were released by the cathode. To do this, it must have a potential that is positive relative to the second electrode. In most cases, the anode is also cylindrical.
5. Both electrodes vacuum devices are completely identical to the emitter and base of the semiconductor variety of elements.
6. For making a diode crystal Silicon or germanium is most often used. One of its parts is p-type electrically conductive and has a deficiency of electrons, which is formed by an artificial method. The opposite side of the crystal also has conductivity, but it is n-type and has an excess of electrons. There is a boundary between the two regions, which is called a p-n junction.

Such features internal device diodes are endowed with their main property - the ability to conduct electric current in only one direction.

Purpose

Below are the main areas of application of diodes, from which their main purpose becomes clear:

1. Diode bridges are 4, 6 or 12 diodes connected to each other, their number depends on the type of circuit, which can be single-phase, three-phase half-bridge or three-phase full-bridge. They perform the functions of rectifiers; this option is most often used in automobile generators, since the introduction of such bridges, as well as the use of brush-collector units with them, has made it possible to significantly reduce the size of this device and increase its reliability. If the connection is made in series and in one direction, this increases the minimum voltage required to unlock the entire diode bridge.
2. Diode detectors are obtained with combined use of these devices with capacitors. This is necessary so that it is possible to isolate the modulation from low frequencies from various modulated signals, including amplitude-modulated types of radio signals. Such detectors are part of the design of many household appliances, such as televisions or radios.
3. Ensuring protection of consumers from incorrect polarity when switching on circuit inputs from overloads or breakdown switches electromotive force, which occurs during self-induction, which occurs when an inductive load is turned off. To ensure the safety of circuits from overloads that occur, a chain is used consisting of several diodes connected to the supply buses in the reverse direction. In this case, the input to which protection is provided must be connected to the middle of this chain. During normal operation of the circuit, all diodes are in a closed state, but if they have detected that the input potential has gone beyond the permissible voltage limits, one of the protective elements is activated. Due to this, this permissible potential is limited within the permissible supply voltage in combination with a direct drop in the voltage on the protective device.
4. Switches, created on the basis of diodes, are used to switch signals with high frequencies. Such a system is controlled using direct electric current, high-frequency separation and the supply of a control signal, which occurs due to inductance and capacitors.
5. Creation of diode spark protection. Shunt-diode barriers are used, which provide safety by limiting the voltage in the appropriate electrical circuit. In combination with them, current-limiting resistors are used, which are necessary to limit the electric current passing through the network and increase the degree of protection.

The use of diodes in electronics today is very widespread, since virtually no modern type of electronic equipment can do without these elements.

Direct diode connection

The p-n junction of the diode can be affected by voltage supplied from external sources. Indicators such as magnitude and polarity will affect its behavior and the electrical current conducted through it.

Below we consider in detail the option in which the positive pole is connected to the p-type region, and the negative pole to the n-type region. In this case, direct switching will occur:

1. Under voltage from external source, an electric field will be formed in the p-n junction, and its direction will be opposite to the internal diffusion field.
2. Field voltage will decrease significantly, which will cause a sharp narrowing of the barrier layer.
3. Under the influence of these processes a significant number of electrons will be able to freely move from the p-region to the n-region, as well as in the opposite direction.
4. Drift current indicators during this process remain the same, since they directly depend only on the number of minority charged carriers located in the region of the pn junction.
5. Electrons have an increased level of diffusion, which leads to the injection of minority carriers. In other words, in the n-region there will be an increase in the number of holes, and in the p-region an increased concentration of electrons will be recorded.
6. Lack of equilibrium and increased number of minority carriers causes them to go deep into the semiconductor and mix with its structure, which ultimately leads to the destruction of its electrical neutrality properties.
7. Semiconductor at the same time, it is able to restore its neutral state, this occurs due to the receipt of charges from a connected external source, which contributes to the appearance of direct current in the external electrical circuit.

Diode reverse connection

Now we will consider another method of switching on, during which the polarity of the external source from which the voltage is transmitted changes:

1. The main difference from direct connection is that that the created electric field will have a direction that completely coincides with the direction of the internal diffusion field. Accordingly, the barrier layer will no longer narrow, but, on the contrary, expand.
2. Field located in the pn junction, will have an accelerating effect on a number of minority charge carriers, for this reason, the drift current indicators will remain unchanged. It will determine the parameters of the resulting current that passes through the pn junction.
3. As you grow reverse voltage, the electric current flowing through the junction will tend to reach maximum values. It has a special name - saturation current.
4. According to the exponential law, with a gradual increase in temperature, the saturation current indicators will also increase.

Forward and reverse voltage

The voltage that affects the diode is divided according to two criteria:

1. Forward voltage- this is when the diode opens and direct current begins to pass through it, while the resistance of the device is extremely low.
2. Reverse voltage- this is the one that has reverse polarity and ensures that the diode closes with reverse current passing through it. At the same time, the resistance indicators of the device begin to increase sharply and significantly.

The resistance of a pn junction is a constantly changing indicator, primarily influenced by the forward voltage applied directly to the diode. If the voltage increases, then the junction resistance will decrease proportionally.

This leads to an increase in the parameters of the forward current passing through the diode. When this device is closed, virtually the entire voltage is applied to it, for this reason the reverse current passing through the diode is insignificant, and the transition resistance reaches peak parameters.

Diode operation and its current-voltage characteristics

The current-voltage characteristic of these devices is understood as a curved line that shows the dependence of the electric current flowing through the p-n junction on the volume and polarity of the voltage acting on it.

Such a graph can be described as follows:

1. Vertical axis: The upper area corresponds to the forward current values, the lower area to the reverse current parameters.
2. Horizontal axis: The area on the right is for forward voltage values; area on the left for reverse voltage parameters.
3. Direct branch of the current-voltage characteristic reflects the passage of electric current through the diode. It is directed upward and runs in close proximity to the vertical axis, since it represents the increase in forward electric current that occurs when the corresponding voltage increases.
4. Second (reverse) branch corresponds to and displays the closed state of the electrical current that also passes through the device. Its position is such that it runs virtually parallel to the horizontal axis. The steeper this branch approaches the vertical, the higher the rectifying capabilities of a particular diode.
5. According to the schedule you can see that after an increase in the forward voltage flowing through the p-n junction, a slow increase in electric current occurs. However, gradually, the curve reaches an area in which a jump is noticeable, after which an accelerated increase in its indicators occurs. This is due to the diode opening and conducting current at forward voltage. For devices made of germanium, this occurs at a voltage of 0.1V to 0.2V ( maximum value 1B), and for silicon elements a higher value is required from 0.5V to 0.6V (maximum value 1.5V).
6. Current increase shown can lead to overheating of semiconductor molecules. If the heat removal that occurs due to natural processes and the operation of radiators is less than the level of its release, then the structure of the molecules can be destroyed, and this process will be irreversible. For this reason, it is necessary to limit the forward current parameters to prevent overheating of the semiconductor material. To do this, special resistors are added to the circuit, connected in series with the diodes.
7. Exploring the reverse branch you can notice that if the reverse voltage applied to the p-n junction begins to increase, then the increase in current parameters is virtually unnoticeable. However, in cases where the voltage reaches parameters exceeding acceptable standards, a sudden jump in the reverse current may occur, which will overheat the semiconductor and contribute to the subsequent breakdown of the p-n junction.

Basic diode faults

Sometimes devices of this type fail, this may occur due to natural depreciation and aging of these elements or for other reasons.

In total, there are 3 main types of common faults:

1. Transition breakdown leads to the fact that the diode instead semiconductor device becomes in essence the most ordinary conductor. In this state, it loses its basic properties and begins to pass electric current in absolutely any direction. Such a breakdown is easily detected using a standard one, which begins to feed sound signal and show low level resistance in the diode.
2. When broken the reverse process occurs - the device generally stops passing electric current in any direction, that is, it essentially becomes an insulator. To accurately determine a break, it is necessary to use testers with high-quality and serviceable probes, otherwise they can sometimes falsely diagnose this malfunction. In alloy semiconductor varieties, such a breakdown is extremely rare.
3. A leak, during which the tightness of the device body is broken, as a result of which it cannot function properly.

Breakdown of p-n junction

Such breakdowns occur in situations where the reverse electric current begins to suddenly and sharply increase, this happens due to the fact that the voltage of the corresponding type reaches unacceptable high values.

There are usually several types:

1. Thermal breakdowns, which are caused by a sharp increase in temperature and subsequent overheating.
2. Electrical breakdowns, arising under the influence of current on the transition.

The graph of the current-voltage characteristic allows you to visually study these processes and the difference between them.

Electrical breakdown

The consequences caused by electrical breakdowns are not irreversible, since they do not destroy the crystal itself. Therefore, with a gradual decrease in voltage, it is possible to restore all the properties and operating parameters of the diode.

At the same time, breakdowns of this type are divided into two types:

1. Tunnel breakdowns occur when high voltage passes through narrow junctions, which allows individual electrons to slip through it. They usually occur if semiconductor molecules contain a large number of various impurities. During such a breakdown, the reverse current begins to increase sharply and rapidly, and the corresponding voltage is at a low level.
2. Avalanche types of breakdowns are possible due to the influence of strong fields capable of accelerating charge carriers to the maximum level, due to which they knock out a number of valence electrons from the atoms, which then fly into the conductive region. This phenomenon is of an avalanche nature, due to which this type breakdowns and received this name.

Thermal breakdown

The occurrence of such a breakdown can occur for two main reasons: insufficient heat removal and overheating of the p-n junction, which occurs due to the flow of electric current through it at too high rates.

Promotion temperature regime in the transition and neighboring areas causes the following consequences:

1. Growth of atomic vibrations, included in the crystal.
2. Hit electrons into the conduction band.
3. A sharp increase in temperature.
4. Destruction and deformation crystal structure.
5. Complete failure and breakdown of the entire radio component.