What is the difference between a pulse diode and a rectifier diode? Pulse diode

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 V electronic components, in particular - in the same diodes. Therefore, when used in pulse circuits diodes, it is necessary to take into account 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 imply 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 characterized by low on-state resistance, a larger p-n junction area, and the ability to pass large currents. But due to the large junction area, the diode capacitance is larger - 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, pulse diodes are specially designed diodes for operation specifically in pulse 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 specified in the documentation for the diode.

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

Pulse diode – This is a diode with short transient duration, designed for use in pulsed operating modes. They are used as switching elements (for example, in computers), for detecting high-frequency signals and for other purposes.

At rapid changes voltage across the diode in the - junction, transient processes occur due to two main processes. The first is the accumulation of minority carriers in the base of the diode when it is directly turned on, i.e. diffusion capacitance charge. And when the voltage changes to the opposite (or when it decreases), this charge dissolves. The second phenomenon is the recharging of the barrier capacitance, which also does not occur instantly, but is characterized by a time constant, where is the differential resistance of the diode (resistance across alternating current), a - barrier capacitance - transition.

The first phenomenon plays a major role at high densities of forward current through the diode; the charge of the barrier capacitance in this case plays a secondary role. At low current densities, transient processes in the diode are determined by the second phenomenon, and the accumulation of minority charge carriers in the base plays a secondary role.

Let's consider the process of switching a diode from a state of high conductivity (diode open) to a state of low conductivity (diode closed) (Figure 1.11). When direct voltage is applied, a significant forward current arises, which leads to the accumulation of minority charge carriers in the base area (this is a high-resistance n- region).

When a diode switches from the forward direction to the reverse direction, at the initial moment a large reverse current flows through the diode, limited mainly by the volumetric resistance of the base. Over time, the minority carriers accumulated in the base recombine or leave through the junction, and the reverse current decreases to its stationary value. This whole process takes reverse resistance recovery time– the time interval from the moment the current passes through zero after switching the diode until the reverse current reaches a specified low value. This is one of the main parameters of pulsed diodes, and according to its value they are divided into six groups: >500 ns; =150…500 ns;=30…150 ns, =5…30 ns; =1…5 ns and<1 нс.

Figure 1.11 - The process of switching a diode from open to closed state

When a current pulse is passed in the forward direction, a voltage surge is observed at the first moment after switching on (Figure 1.12), which is associated with an increase in voltage until the accumulation of minority carriers in the diode base ends. After this, the base resistance decreases and the voltage decreases.

Figure 1.12 -. The process of switching a diode from a closed state to an open state

This process is characterized by the second parameter of the pulse diode - forward voltage establishment time, equal to the time interval from the beginning of the current pulse until the specified value of the forward voltage is reached.

The values of these parameters depend on the structure of the diode and on the lifetime of minority charge carriers in the diode base. To reduce the lifetime of minority carriers, a small amount of gold impurity is introduced into the base. Gold atoms serve as additional recombination centers; as a result of their introduction, the lifetime of charge carriers decreases, and, consequently, the diffusion capacity of the transition decreases. Reducing the barrier capacity is achieved by technological and constructive methods. Pulse diodes are manufactured based on planar technology, epitaxial growth, and ion beam technology. The main semiconductor material in this case is silicon.

In high-speed pulse circuits Schottky diodes (Figure 1.13) are widely used, in which the transition is made based on a metal-semiconductor contact. The symbol is shown in Fig. 16.

Figure 1.13 - Schottky diode symbol

These diodes do not spend time accumulating and dissolving charges in the base; their performance depends only on the speed of the barrier capacitance recharging process. Current-voltage characteristic Schottky diodes resemble the characteristics of junction-based diodes. The difference is that the forward branch within 8 - 10 decades of applied voltage represents an almost ideal exponential curve, and the reverse currents are small (fractions to tens of nanoamperes).

Structurally, Schottky diodes are made in the form of a low-resistance silicon wafer, on which a high-resistance epitaxial film with electrical conductivity of the same type is applied. A layer of metal is applied to the surface of the film by vacuum deposition.

Schottky diodes are also used in high current rectifiers and logarithmic devices.

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Ministry of Education and Science of Ukraine

Dnepropetrovsk National University named after Oles Gonchar

Faculty of Physics, Electronics

and computer systems

Department of Radio Electronics

Test on “Solid-state electronics”

On the topic: “Features of a diode”

Completed

student of group KM-11-1

Mironenkov R.D.

Checked

Ph.D. physics and mathematics Sciences, Associate Professor of the Department of Radio Electronics.

Makarov V.A.

Dnepropetrovsk 2013

Abstract

Key words: pulse diode, high-frequency diode, Gunn diode, current-voltage characteristic of the diode.

Purpose of the work: study of the characteristics and principles of operation of pulsed and high-frequency diodes

Introduction

1. Pulse diode. Operating principle

2. High frequency diode. Operating principle

2.1 Gunn diode

3. Manufacturing of diodes

Conclusion

References

Introduction

Semiconductors became a real goldmine of technology when they learned to make structures like layered cakes from them.

By growing a layer of an n-semiconductor on a wafer of a p-semiconductor, we obtain a two-layer semiconductor. The transition layer between them is called a pn junction. If you solder a connecting wire to each half, you will get a semiconductor diode that acts on the current like a valve: it passes current well in one direction, and almost not in the other direction.

How does the straightening barrier layer occur? The formation of a layer begins with the p-half having more holes and the n-half having more electrons. The difference in charge carrier density begins to balance through the transition: holes penetrate into the n-half, electrons into the p-half.

Using an external current source, you can increase or decrease the external potential barrier. If a direct voltage is applied to the diode, that is, the positive pole is connected to the p-half, then an external electrical force will begin to act against the double layer, and the diode passes a current that quickly increases with increasing voltage. If you change the polarity of the conductors, the voltage drops almost to zero. If a diode is connected to an alternating voltage circuit, it will serve as a rectifier, that is, the output will have a constant pulsating voltage in one direction (from plus to minus). In order to smooth out the amplitude, or as it is also called the “peak value,” of current ripple, it is effective to add a capacitor in parallel with the diode. Rectifier devices are quite often required in industry. For example, rectifiers are needed for the proper operation of household appliances (since almost all electrical appliances consume constant voltage. These are TVs, radios, VCRs, etc.). Also, semiconductor diodes are needed to decipher video, radio, photos and other signals into frequency-electric signals. Using this property of semiconductors, we watch TV or listen to the radio.

There are also unusual semiconductor diodes - LEDs and photodiodes. Photodiodes pass current only when light hits their body. And LEDs, when current passes through them, begin to glow. The color of the LEDs depends on what type it belongs to.

Semiconductor diodes are divided into groups depending on their power, operating frequency range, voltage and operating frequency range. Both diodes and transistors have one unique property. When the temperature changes, their internal resistance changes and, consequently, the value of the rectified current voltage also changes up or down. Light and photodiodes are used as sensors and indicators.

1. Pulse diode. Operating principle

These are ordinary diodes, with a normal current-voltage characteristic, but operating in switching mode. Their field of application is digital circuits, the elements of which are either in the open state “0” or in the closed state “1”. Therefore, in this application, the timing parameters of the diode are of interest: how quickly it goes from off to on and vice versa. Figure 1 shows a pulse diode based on an asymmetrical contact. Let us accept the condition that the emitter has n - conductivity. This gives grounds to consider the behavior and current of only electrons. With reverse asymmetry, everything said will apply to holes.

Fig.1. Pulse diode

Let's consider the processes during switching. Let's apply a direct voltage to it - an ideal stage (Fig. 2.a). Initially, the electrons with the highest energy, located directly near the p-n junction, will begin to move, then those located inside the n region will join them. Thus, due to the difference in carrier energies, their number gradually increases, and the forward current gradually increases. This process over time is shown in Fig. 2.b, and for evaluation, the parameter t mouth is introduced - the time to establish the open state. For a long time, the current does not change and a large number of minority carriers, electrons, accumulate in the “p” junction region. A nonequilibrium concentration of carriers arises in the p region of the crystal.

Let us apply an equally sharply changing reverse voltage polarity to the junction. Nonequilibrium electrons accumulated in the “p” region will begin to be removed under the influence of an electric field into the “n” region. Their concentration is high, so the reverse current will be large for some time. This stage of the process is shown in Fig. 2.b as t 1. Eventually, the output process will end, the transition becomes a closed state. Now there are two semiconducting regions p and n b and a dielectric layer between them. This is a capacitor that begins to charge when exposed to reverse voltage. The charge current will decrease according to the exponential law, in Fig. 2.b this is time t 2. In general, the recovery time of the closed state is equal to t 1 +t 2 =t recovery.

Fig.2. Processes in a pulse diode

Usually trest >> trest. To improve the parameters of the diode, materials with high carrier mobility (Ge) are used for manufacturing, the junction area is made small, and p-i-n structures. An example of using a pulse diode is shown in Fig. The voltage shape across the load resistance repeats the current shape in Fig. 3.

Fig. 3. Operation of a pulse diode

2. High frequency diodes. Operating principle

In ultrahigh frequency technology (for operation in the centimeter and millimeter wave ranges), special germanium and silicon ultrahigh frequency diodes (microwave diodes) are used. According to their purpose, microwave diodes are divided into video detector, intended for detecting microwave oscillations, switching, intended for use in devices for controlling the level of microwave power, parametric, intended for use in parametric amplifiers Microwave oscillations, and converters. In turn, converter diodes that use the nonlinearity of the current-voltage characteristic of the transition are divided into:

· mixing devices used to convert a microwave signal and a local oscillator signal into an intermediate frequency signal;

· multipliers, used to multiply the frequency of the microwave signal;

· modulator, used to modulate the amplitude of the microwave signal.

Microwave diodes typically use a point contact. The junction in such diodes is not formed. The rectifying contact is made by simply pressing the tip of a metal contact spring against the polished surface of the semiconductor. These diodes are made of very low-resistance material (charge carrier lifetime is short) and have a very small point contact radius (2-3 µm), which provides good high-frequency properties. However, the breakdown voltage of microwave diodes is very low (only 3-5 V), and the forward voltage is relatively high.

Their reverse current, although small, begins to increase almost from zero due to the tunneling effect of carriers through the junction (Fig. 4).

Rice. 4. I-V characteristics of a high-frequency diode

The design of microwave diodes is usually adapted for coupling with elements of a coaxial or waveguide path, with measuring heads and other parts of the microwave system. In the long-wave section of the microwave range (3-10 cm), the main types of housing are metal-ceramic or metal-glass cartridge type. In the wavelength range of 1-3 cm, the dimensions and capacitance of these cases become unacceptably large, and therefore the rectifying contact is mounted in a coaxial type case. In the millimeter wave range, a waveguide design is used.

In addition to the wavelength at which microwave diodes have parameters guaranteed by standards terms of reference and maximum permissible data, microwave diodes are also characterized by electrical parameters that reflect the main value. Thus, mixing microwave diodes are characterized by conversion losses (the ratio of the microwave power at the input to the intermediate frequency power at the diode output), noise ratio (the ratio of the noise power at the diode output in operating mode to the thermal noise power active resistance diode), normalized noise figure, characterizing the general sensitivity of the receiving device, and differential output impedance. In some cases electrical parameter determines not only the properties of the microwave diode itself, but also the properties of the specific microwave device in which this diode is installed.

It should be borne in mind that the power at which the diode “burns out,” accompanied by irreversible deterioration of the current-voltage characteristic or breakdown, is very small. Therefore, it is necessary to exclude any unintended influences and take the necessary protective measures both during operation and during storage of the microwave diode (for example, discharging static electricity accumulated on the operator’s body through the diode is unacceptable; storing the diode in a metal cartridge, etc.).

In millimeter-wave devices (especially integrated ones), avalanche diodes are widely used to build powerful microwave amplifiers, and Gunn diodes are widely used to build microwave generators. These diodes use the phenomenon of limiting electron mobility in electric fields with strengths above critical, and their current-voltage characteristics have a section with negative differential resistance. Avalanche diodes operate in the mode of avalanche multiplication of charge carriers with reverse bias of the electrical transition. Gunn diodes (there is no rectifying junction in the structure of these devices) use the effect of the occurrence of electrical oscillations in a gallium arsenide plate when a constant voltage is applied to it, creating an electric field with a strength of more than 105 V/m.

Industrially produced avalanche diodes and Gunn generators are designed for a continuous microwave output power of several tens of milliwatts. In pulsed mode, this power can be increased by several orders of magnitude. To increase the output power, avalanche diodes and Gunn generators with a larger area of the electron-hole junction and a larger area of the thin semiconductor film are needed. Moreover, they must be uniform not only in thickness, but also in area.

The operating frequencies of modern silicon microwave diodes are already approaching the theoretical limit. Therefore, in order to further improve the frequency properties, it is necessary to use a different material, as well as develop semiconductor devices with a different operating principle.

2.1 Gunn diode

Gunn diode (invented by John Gunn in 1963) -- type semiconductor diodes, used for generating and converting oscillations in the microwave range at frequencies from 0.1 to 100 GHz. Unlike other types of diodes, the operating principle of a Gunn diode is not based on the properties of p-n junctions, i.e. all its properties are determined not by the effects that arise at the junction of two different semiconductors, but by the intrinsic properties of the semiconductor material used.

In the Russian literature, Gunn diodes were called devices with volumetric instability or with intervalley electron transfer, since the active properties of diodes are determined by the transition of electrons from the “central” energy valley to the “side”, where they can already be characterized by low mobility and large effective mass. In foreign literature, the Gunn diode corresponds to the term TED (Transferred Electron Device). high frequency pulse gunn diode

Based on the Gunn effect, generator and amplification diodes have been created, used as pump generators in parametric amplifiers, local oscillators in superheterodyne receivers, generators in low-power transmitters and in measuring technology.

When creating low-resistance ohmic contacts necessary for the operation of Gunn diodes, there are two approaches:

· The first of them is to search for an acceptable technology for depositing such contacts directly onto high-resistivity gallium arsenide.

· The second approach is to manufacture a multilayer generator structure. In diodes of this structure, epitaxial layers of relatively low-resistance gallium arsenide with n-type electrical conductivity are grown on both sides of a layer of relatively high-resistivity gallium arsenide, which serves as the working part of the generator. These highly alloyed layers serve as transition layers from the working part of the device to the metal electrodes.

A Gunn diode traditionally consists of a gallium arsenide layer with ohmic contacts on both sides. The active part of a Gunn diode usually has a length of the order of l = 1-100 μm and a concentration of donor dopant impurities n = 1014? 1016 cm?3. In this material in the conduction band there are two energy minima, which correspond to two states of electrons - “heavy” and “light”. In this regard, with increasing electric field strength, the average drift velocity of electrons increases until the field reaches a certain critical value, and then decreases, tending to saturation speed.

Thus, if a voltage is applied to the diode that exceeds the product of the critical field strength and the thickness of the gallium arsenide layer in the diode, the uniform distribution of voltage across the thickness of the layer becomes unstable. Then, when even in thin area a slight increase in the field strength, electrons located closer to the anode will “retreat” from this region towards it, and electrons located at the cathode will try to “catch up” with the resulting double layer of charges moving towards the anode. During movement, the field strength in this layer will continuously increase, and outside it will decrease until it reaches an equilibrium value. Such a moving double layer of charges with a high electric field inside is called a strong field domain, and the voltage at which it occurs is called a threshold.

At the moment of domain initiation, the current in the diode is maximum. As the domain is formed, it decreases and reaches its minimum at the end of formation. Reaching the anode, the domain is destroyed and the current increases again. But as soon as it reaches its maximum, a new domain. The frequency with which this process repeats is inversely proportional to the thickness of the semiconductor layer and is called the transit frequency.

On the current-voltage characteristic of a semiconductor device, the presence of a falling section is not a sufficient condition for the occurrence of microwave oscillations in it, but it is necessary. The presence of oscillations means that instability of wave disturbances occurs in the space of the semiconductor crystal. But such instability depends on the parameters of the semiconductor (doping profile, size, carrier concentration, etc.).

Fig.5. I-V characteristic of a Gunn diode

When placing a Gunn diode in a resonator, other generation modes are possible, in which the oscillation frequency can be made both lower and higher than the flight frequency. The efficiency of such a generator is relatively high, but maximum power does not exceed 200--300mW.

A Gunn diode can be used to create an oscillator in the 10 GHz and higher (THz) frequency range. And a resonator, which can take the form of a waveguide, is added to control the frequency. The frequency of generators on a Gunn diode is determined mainly by the resonant frequency of the oscillatory system, taking into account the capacitive conductivity of the diode and can be tuned within a wide range by mechanical and electrical methods. However, the service life of Gunn generators is relatively short, which is due to the simultaneous impact on the semiconductor crystal of such factors as a strong electric field and overheating of the crystal due to the power released in it.

Gunn diodes operating in various modes are used in the frequency range 1-100 GHz. In continuous mode, real generators based on Gunn diodes have an efficiency of about 2-4% and can provide output power from units of mW to units of W. But when switching to pulse mode, the efficiency increases by 2-3 times. Special resonant systems that make it possible to add some higher harmonics to the power of the useful output signal serve to increase efficiency and this mode is called relaxation.

There are several different modes, in one of which a Gunn diode generator can perform work, depending on the supply voltage, temperature, load properties: domain mode, hybrid mode, limited space charge accumulation mode and negative conductivity mode.

The most commonly used mode is the domain mode, which is characterized by the existence of a dipole domain during a significant part of the oscillation period. Domain mode can have three various types: span, with a delay in the formation of domains and with suppression of domains, which are obtained when the load resistance changes.

For Gunn diodes, a mode of limiting and accumulating space charge was also invented and implemented. Its existence occurs at large voltage amplitudes at frequencies several times greater than the flight frequency and at constant voltages on the diode, which are several times higher than the threshold value. However, there are requirements for implementation to this regime: diodes with a very uniform doping profile are needed. The uniform distribution of the electric field and electron concentration along the length of the sample is ensured by the high rate of change in voltage across the diode.

Along with gallium arsenide and indium phosphide InP (up to 170 GHz) using the epitaxial growth method, gallium nitride (GaN) is also used for the manufacture of Gunn diodes, on which the most achieved high frequency oscillations in Gunn diodes - 3 THz. The Gunn diode has low level amplitude noise and low operating supply voltage (from units to tens of V).

The operation of diodes occurs in resonant chambers, which are in the form of microcircuits on dielectric substrates with resonating capacitive and inductive elements, or in the form of a combination of resonators with microcircuits.

3. Manufacturing of diodes

The diode manufacturing technology can be based on any of the methods described above for producing p-hc junctions on silicon and germanium. However, a device with the best amplifying qualities is obtained by diffusion, using mesa technology.

The manufacturing technology of Gunn diodes is relatively simple. Diodes are made either on the basis of single crystals or on the basis of epitaxial GaAs films. The dimensions of the plates for the manufacture of diodes are selected based on the conditions of their operating mode and the required parameters.

For the parameters and manufacturing technology of diodes and thyristors, the following abbreviations are used in the text and tables: Si - silicon, Qe - germanium, GaAs - gallium arsepide, CaP - gallium phosphite, Si(CO 3) 2 - silicon carbide.

Conclusion

In this work, we examined the operating principles of pulsed and high-frequency diodes. Each of the diodes has its own parameters, characteristics, and its purpose in the electrical circuit. A diode is an electronic element that has different conductivity depending on the direction of the electric current. The diode electrode connected to the positive pole of the current source when the diode is open (that is, has low resistance) is called anode, connected to the negative pole - the cathode.

Pulse diodes operate in the mode electronic key. The pulse duration can be very short, so the diode must transition from one state to another very quickly. The main parameter characterizing the performance of pulsed diodes is the recovery time of the reverse resistance. To reduce this, special measures are used that accelerate the process of resorption of minority charge carriers in the base. The requirements for pulsed diodes are well met by diodes based on the Schottky barrier, which have very low inertia due to the absence of injection and accumulation of minority charge carriers in the base.

The high-frequency diode is used for linear or nonlinear conversion of high-frequency signals up to 600 MHz. (Microwave diodes - up to 12 GHz.) It is used in detector circuits - these are rectifiers of high-frequency signals.

Barrier capacitance Sat [µF]

f slave [MHz]

Modern imported diodes use such a characteristic as “Recovery time”. In ultra-fast diodes it reaches values of 100 ns.

References

1. Alferov Zh. I. // Physics and technology of semiconductors. 1998. T.32. No. 1. P.3-18.

2. Berg A., Dean P. LEDs / Transl. from English edited by A.E. Yunovich. M., 1979.

3. Kogan L. M. Semiconductor light-emitting diodes. M., 1983.

4. Losev O. V. At the origins of semiconductor technology: Selected works. L., 1972.

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Basic parameters of pulsed diodes

1. Total capacity diode (several shares).

2. Maximum pulse forward voltage.

3. Maximum permissible impulse current.

4. The time to establish the forward voltage of the diode is thick - the time interval from the moment the direct current pulse is applied to the diode until the specified value of the forward voltage is reached on it - depends on the speed of movement of minority charge carriers injected through the junction into the base, as a result of which a decrease in its resistance (fraction) is observed not - shares).

5. Recovery time of the reverse resistance of the diode - the time interval elapsed from the moment the current passes through zero (after changing the polarity of the applied voltage) until the moment when the reverse current reaches a given small value (of the order of , where is the current at the forward voltage; - fractions of ns - fractions mks).

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 recombinations and reverse transition of minority charge carriers to the emitter. The latter leads to an increase in reverse current. After changing the voltage polarity for some time, the reverse current changes little (Fig. 2.13, a, b) and is limited only by the external resistance of the circuit. In this case, the charge of minority carriers accumulated during injection into the base of the diode (concentration ) is resolved (dotted lines in Fig. 2.13, c).

Rice. 2.13. Change in current through the diode (a) when a reverse voltage is connected (b) and change in the concentration of minority charge carriers in the base of the pulse diode (c); symbol Schottky barrier diode (g); diode equivalent circuit (d): -transition resistance; - transfer capacitance; - ohmic resistance of the base and emitter body; C - interelectrode capacitance of leads

After time, the concentration of minority charge carriers at the transition boundary is equal to the equilibrium one, but deep in the base there is still a nonequilibrium charge. From this moment, the reverse current of the diode decreases to its static value. Its change will stop at the moment of complete resorption of the charge accumulated in the base.

In high-speed pulse circuits, Schottky diodes are widely used, in which the transition is made on the basis of a metal-semiconductor contact. These diodes do not spend time accumulating and dissolving charges in the base; their performance depends only on the speed of the barrier capacitance recharging process. The current-voltage characteristic of Schottky diodes resembles the characteristic of diodes based on -junctions. The difference is that the forward branch within 8-10 decades of applied voltage represents an almost ideal exponential curve, and the reverse currents are small (fractions to tens of nA). Structurally, Schottky diodes are made in the form of a low-resistance silicon wafer, on which a high-resistance epitaxial film with electrical conductivity of the same type is applied. A layer of metal is applied to the surface of the film by vacuum deposition.

Schottky diodes are also used in high current rectifiers and logarithmic devices.

The symbol of a Schottky diode and the equivalent circuit of the diode are shown in Fig. 2.13, d.

With rapid changes in voltage across the diode at the junction, transient processes occur due to two main processes. The first is the accumulation of minority carriers in the base of the diode when it is directly turned on, i.e. diffusion capacitance charge. And when the voltage changes to the opposite (or when it decreases), this charge dissolves. The second phenomenon is the recharging of the barrier capacitance, which also does not occur instantly, but is characterized by a time constant, where is the differential resistance of the diode (alternating current resistance), and is the barrier capacitance of the junction.

When a diode switches from the forward direction to the reverse direction, at the initial moment a large reverse current flows through the diode, limited mainly by the volumetric resistance of the base. Over time, the minority carriers accumulated in the base recombine or leave through the junction, and the reverse current decreases to its stationary value. This whole process takes reverse resistance recovery time– the time interval from the moment the current passes through zero after switching the diode until the reverse current reaches a specified low value. This is one of the main parameters of pulsed diodes, and according to its value they are divided into six groups: >500 ns; =150...500 ns; =30…150 ns, =5…30 ns; =1…5 ns and<1 нс.

Figure 1.11 - The process of switching a diode from open to closed state

Figure 1.12 -. The process of switching a diode from a closed state to an open state

In high-speed pulse circuits, Schottky diodes (Figure 1.13) are widely used, in which the transition is made on the basis of a metal-semiconductor contact. The symbol is shown in Fig. 16.

Figure 1.13 - Schottky diode symbol

These diodes do not spend time accumulating and dissolving charges in the base; their performance depends only on the speed of the barrier capacitance recharging process. The current-voltage characteristic of Schottky diodes resembles the characteristic of diodes based on junctions. The difference is that the forward branch within 8 - 10 decades of applied voltage represents an almost ideal exponential curve, and the reverse currents are small (fractions to tens of nanoamperes).

Schottky diodes are also used in high current rectifiers and logarithmic devices.

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