What is the maximum forward voltage of a diode. Reverse voltage

Semiconductor diode device.

Direct and reverse connection of the diode, characterize the forward and reverse voltage, forward and reverse currents of the diode.

The direct connection graph is drawn in the first quadrant. This shows that the greater the voltage, the greater the current. Moreover, up to a certain point, the voltage increases faster than the current. But then a turning point occurs, and the voltage remains almost unchanged, but the current begins to increase. For most diodes, this turning point occurs in the range of 0.5...1 V. It is this voltage that is said to “drop” across the diode. That is, if you connect a light bulb according to the first diagram in Fig. 3, and your battery voltage is 9 V, then the light bulb will no longer receive 9 V, but 8.5 or even 8 (depending on the type of diode). This 0.5...1 V is the voltage drop across the diode. A slow increase in current to a voltage of 0.5...1V means that in this section there is practically no current flowing through the diode, even in the forward direction.

The reverse switching graph is drawn in the third quadrant. From this it can be seen that over a significant area the current remains almost unchanged, and then increases like an avalanche. What does it mean? If you turn on the light bulb according to the second diagram in Fig. 3, then it will not light up, because the diode does not pass current in the opposite direction (more precisely, it does, as can be seen in the graph, but this current is so small that the lamp will not light up). But the diode cannot withstand the voltage indefinitely. If you increase the voltage, for example, to several hundred volts, then this high voltage the diode will “break through” (see the inflection point on the reverse branch of the graph) and current will flow through the diode. But “breakdown” is an irreversible process (for diodes). That is, such a “breakdown” will lead to burnout of the diode and it will either completely stop passing current in any direction, or vice versa – it will pass current in all directions.

Fig.3. Direct connection of p-n junction

Let electrons 1, 2, 3 diffuse into the p-layer, which momentarily loses electrical neutrality, acquiring an excess negative charge. An electric field arises between the p-layer and its output, which ejects electrons 4, 5, 6 from the nearest orbits of pair-electronic bonds of the p-type semiconductor into the external circuit. Next, electrons 1, 2, 3 begin to diffusely move through the holes to the right to the right contact.

During electron diffusion, the 1, 2, 3 n-layer also loses electrical neutrality, acquiring an excess positive charge. An electric field arises between the n-layer and its output, which draws in electrons 7, 8, 9 from the external circuit. As a result, direct current flows at the left and right contacts, as well as through the structure. The magnitude of the forward current is determined by the area of ​​the pn junction and depends on the applied forward voltage and limiting resistance.

Fig.4. Reverse switching of p-n junction

The circuit diagram for the reverse connection of the pn junction is shown in Fig. 4. Under the influence of reverse voltage, the majority carriers 1 and 2 flow away from the junction boundaries, so the p-n junction expands. A strong retarding field is created for majority carriers, so carrier diffusion is impossible. The field acting at the transition is accelerating for minority carriers, so carrier drift occurs. The drift current has three components: thermal generation current, thermal current, and leakage current.

The thermal generation current is created by minority carriers 5 and 6, which are generated in the transition region, and depends on the temperature Itr(T) = Itr(T0)eT, where T0 - initial value temperature (250C); T - current temperature value; T - temperature change; - temperature coefficient. Thermal generation current predominates in silicon diodes, which have a larger p-n junction width compared to germanium diodes.

The thermal current is created by minority carriers 3 and 4, which are generated in the semiconductor layers adjacent to the junction. Thermal current predominates at germanium p-n junctions. It depends on the temperature It(T) = It(T0)eT. There is a rule of thumb for estimating temperature-dependent currents: with an increase in temperature of 100C, the reverse current increases by a factor of 2.

The leakage current is created by minority carriers that are generated on the surface of the layers. This current does not depend on temperature, because determined by the state of the surface of the semiconductor crystal. The main feature of leakage current is instability over time, which is called creep.

The total value of the minority carrier current at temperatures up to 400C is much less than the diffusion current: Ipr/Ibr = 104 - 105. From this relationship it follows that the asymmetric stepped p-n junction has valve properties.

To rectify low-frequency alternating currents, that is, to transform alternating current into direct or pulsating, they are used, the operating principle of which is based on the one-sided electron-hole conductivity of the p-n junction. Diodes of this type used in multipliers, rectifiers, detectors, etc.

Rectifier diodes are produced with a planar or point junction, and the area of ​​the junction itself can range from tenths of a square millimeter to a few square centimeters, depending on the rated current rectified over a half-cycle for a given diode.

The current-voltage characteristic (CVC) of a semiconductor diode has forward and reverse branches. The direct branch of the current-voltage characteristic practically shows the connection between the current through the diode and the forward voltage drop across it, their interdependence.

The reverse branch of the current-voltage characteristic reflects the behavior of the diode when a voltage of reverse polarity is applied to it, where the current through the junction is very small and practically does not depend on the magnitude of the voltage applied to the diode, until the limit is reached at which an electrical breakdown of the junction occurs and the diode fails.

Maximum diode reverse voltage - Vr

The first and main characteristic of a rectifier diode is the maximum permissible reverse voltage. This is the voltage that, if applied to the diode in the opposite direction, one can confidently say that the diode will withstand it, and that this fact will not negatively affect the further performance of the diode. But if this voltage is exceeded, then there is no guarantee that the diode will not be broken.

This parameter differs for different diodes; it ranges from tens of volts to several thousand volts. For example, for the popular rectifier diode 1n4007, the maximum constant reverse voltage is 1000V, and for 1n4001 it is only 50V.

Average diode current - If

The diode rectifies the current, so the next the most important characteristic rectifier diode will be the average current of the diode - the average value of the rectified value over the period direct current, flowing through the p-n junction. For rectifier diodes, this parameter can range from hundreds of milliamps to hundreds of amperes.

For example, for a 2D204A rectifier diode the maximum forward current is only 0.4A, and for 80EBU04 it is as much as 80A. If the average current turns out to be greater in value for a long time than the value given in the documentation, then there is no guarantee that the diode will survive.

Maximum diode pulse current - Ifsm (single pulse) and Ifrm (repeated pulses)

The maximum pulse current of a diode is the peak value of current that a given rectifier diode can only withstand certain time, which is indicated in the documentation along with this parameter. For example, a 10A10 diode is capable of withstanding a single current pulse of 600A with a duration of 8.3ms.

As for repeated pulses, their current must be such that the average current falls within the permissible range. For example, the 80EBU04 diode will withstand repeated rectangular pulses with a frequency of 20 kHz even if their maximum current is 160 A, but the average current should remain no more than 80 A.

Average diode reverse current - Ir (leakage current)

The average reverse current of a diode shows the average current through the junction in the reverse direction over a period. Typically this value is less than a microamp, the maximum is a few milliamps. For 1n4007, for example, the average reverse current does not exceed 5 μA at a junction temperature of +25°C, and does not exceed 50 μA at a junction temperature of +100°C.

Average forward voltage of the diode - Vf (junction voltage drop)

The average forward voltage of the diode at a specified average current value. This is the voltage that is applied directly to the p-n junction of the diode when a direct current of the value specified in the documentation passes through it. Usually no more than fractions, maximum - units of volts.

For example, the documentation for the EM516 diode gives a forward voltage of 1.2V for a current of 10A, and 1.0V for a current of 2A. As you can see, the diode resistance is nonlinear.

Diode differential resistance

The differential resistance of the diode expresses the ratio of the voltage increment at the p-n junction of the diode to the small increment of current through the junction that caused this increment. Typically from fractions of an ohm to tens of ohms. It can be calculated from graphs of voltage drop versus forward current.

For example, for an 80EBU04 diode, a 1A increment in current (from 1 to 2A) gives an increment in the junction voltage drop of 0.08V. Therefore, the differential resistance of the diode in this current range is 0.08/1 = 0.08 Ohm.

Average power dissipation of Pd diode

The average power dissipation of a diode is the average power over a period dissipated by the diode body when current flows through it in the forward and reverse directions. This value depends on the design of the diode housing, and can vary from hundreds of milliwatts to tens of watts.

For example, for the KD203A diode, the average power dissipated by the case is 20 W; this diode can even be installed, if necessary, on a radiator for heat removal.

What is forward and reverse voltage? I'm trying to understand the principle of operation of a field-effect transistor. and got the best answer

Answer from Vovik[active]
Direct - a plus is applied to a plus, a minus is applied to a minus. The opposite is true - to a plus - a minus, to a minus - a plus.
In relation to a field-effect transistor - between the source and the gate.
A bipolar transistor has a base and an emitter, not a field effect transistor.
A bipolar transistor consists of two back-to-back r-n transition and with one common output - emitter - base (common type) - collector, like two diodes, only the common “layer” is thin and conducts current if you apply a direct voltage, which is called opening, between the emitter and the base.
The greater the forward voltage between the base and emitter, the more open the transistor is and the lower its emitter-collector resistance, i.e., there is an inverse relationship between the emitter-base voltage and the resistance of the bipolar transistor.
If a reverse voltage is applied between the base and emitter, the transistor will turn off completely and will not conduct current.
If you apply voltage only to the base and emitter or base and collector, you get a regular diode.
The field-effect transistor is designed somewhat differently. There are also three terminals, but they are called drain, source and gate. There is only one pn junction, gate -> drain-source or gate<- сток-исток в зависимости от полярности транзистора. Затвор находится между истоком и стоком и к нему (измеряется относительно истока) всегда прикладывается только обратное напряжение, которое создаёт поле в промежутке между истоком и стоком, в зависимости от напряжённости больше или меньше препятствующее движению электронов (следовательно, изменяя сопротивление транзистора) , и, таким образом, создающую обратную зависимость между напряжением исток-затвор и сопротивлением полевого транзистора.

Answer from ALEX R[guru]
On the 1st question, direct and reverse direc- tion occurs in a semiconductor (diode), i.e., the diode passes current in the direct direction, but if the current flows in the opposite direction, everything is closed. For clarity, the nipple of a bicycle tire goes there, there is no way back. Field tr-r, just for the sake of understanding, there is no electronic connection between the gate and the drain-source, but the current passes due to the evil field created at the gate. Something like that.

Answer from Alexander Egorov[guru]
direct - minus to the region with n-conductivity, plus to the region k with p-conductivity
the opposite is the opposite
by supplying only the emitter and collector, no current will pass, since the ionized atoms of the base will repel the free charges of the emitter from the pn junction (which are already difficult to jump over the pn junction, since it is a dielectric). And if you apply voltage to the base, it will “suck” free charges from the base and they will no longer repel the emitter charges, preventing them from crossing the pn junction. The transistor will open.
By the way, the emitter, collector and base are not a field-effect transistor, but a bipolar transistor.
If you apply voltage only to the base and emitter or base and collector, then it will be a simple diode (each pn junction is a diode).

Answer from User user[guru]
The field-effect transistor has a p or n type field-controlled channel. transistor terminals gate drain source

D iodine- the simplest in design in the glorious family of semiconductor devices. If you take a semiconductor plate, for example germanium, and introduce an acceptor impurity into its left half and a donor impurity into the right half, then on one side you will get a semiconductor of type P, respectively, on the other, type N. In the middle of the crystal you will get the so-called P-N junction, as shown in Figure 1.

The same figure shows the conventional graphic designation of a diode in the diagrams: the cathode terminal (negative electrode) is very similar to the “-” sign. It's easier to remember that way.

In total, in such a crystal there are two zones with different conductivities, from which two outputs come out, so the resulting device was called diode, since the prefix “di” means two.

In this case, the diode turned out to be a semiconductor, but similar devices were known before: for example, in the era of electronic tubes there was a tube diode called a kenotron. Now such diodes are a thing of history, although adherents of “tube” sound believe that in a tube amplifier even the anode voltage rectifier should be tube-based!

Figure 1. Diode structure and diode designation on the diagram

At the junction of semiconductors with P and N conductivity, it turns out P-N junction, which is the basis of all semiconductor devices. But unlike a diode, which has only one transition, they have two P-N junctions, and, for example, they consist of four junctions at once.

P-N junction at rest

Even if the P-N junction, in this case the diode, is not connected anywhere, interesting physical processes still occur inside it, which are shown in Figure 2.

Figure 2. Diode at rest

In the N region there is an excess of electrons, it carries a negative charge, and in the P region the charge is positive. Together these charges form an electric field. Since unlike charges tend to attract each other, electrons from the N zone penetrate into the positively charged P zone, filling some holes. As a result of such movement, a current, albeit very small (several nanoamperes), appears inside the semiconductor.

As a result of this movement, the density of the substance on the P side increases, but up to a certain limit. Particles usually tend to spread evenly throughout the entire volume of a substance, just as the smell of perfume spreads throughout the entire room (diffusion), so sooner or later the electrons return back to the N zone.

If for most consumers of electricity the direction of the current does not matter - the light bulb lights up, the tile heats up, then for a diode the direction of the current plays a huge role. The main function of a diode is to conduct current in one direction. It is this property that is provided by the P-N junction.

Turning the diode in reverse

If a power source is connected to a semiconductor diode, as shown in Figure 3, then no current will pass through the P-N junction.

Figure 3. Diode reverse connection

As can be seen in the figure, the positive pole of the power source is connected to area N, and the negative pole is connected to area P. As a result, electrons from region N rush to the positive pole of the source. In turn, positive charges (holes) in the P region are attracted by the negative pole of the power source. Therefore, in the region of the P-N junction, as can be seen in the figure, a void is formed, there is simply nothing to conduct current, there are no charge carriers.

As the voltage of the power source increases, electrons and holes are increasingly attracted by the electric field of the battery, while in the region of the P-N junction there are less and less charge carriers. Therefore, in reverse switching, no current flows through the diode. In such cases it is customary to say that The semiconductor diode is reverse-voltage locked.

An increase in the density of matter near the battery poles leads to occurrence of diffusion, - the desire for a uniform distribution of matter throughout the entire volume. This is what happens when the battery is disconnected.

Semiconductor Diode Reverse Current

This is where the time has come to remember the non-mainstream media that have been conventionally forgotten. The fact is that even in the closed state, a small current passes through the diode, called reverse. This reverse current and is created by minor carriers, which can move in exactly the same way as the main ones, only in the opposite direction. Naturally, such movement occurs under reverse voltage. The reverse current is usually small, which is due to the small number of minority carriers.

As the temperature of the crystal increases, the number of minority carriers increases, which leads to an increase in the reverse current, which can lead to the destruction of the P-N junction. Therefore, operating temperatures for semiconductor devices - diodes, transistors, microcircuits are limited. To prevent overheating, powerful diodes and transistors are installed on heat sinks - radiators.

Turning on the diode in the forward direction

Shown in Figure 4.

Figure 4. Direct connection of the diode

Now let's change the polarity of the source: connect the minus to area N (cathode), and the plus to area P (anode). With this inclusion in the N region, electrons will be repelled from the negative of the battery and move towards the P-N junction. In region P, positively charged holes will be repelled from the positive terminal of the battery. Electrons and holes rush towards each other.

Charged particles with different polarities gather near the P-N junction, and an electric field arises between them. Therefore, the electrons overcome the P-N junction and continue to move through the P zone. In this case, some of them recombine with holes, but most of them rush to the plus of the battery; current Id flows through the diode.

This current is called direct current. It is limited by the technical data of the diode, a certain maximum value. If this value is exceeded, there is a risk of diode failure. It should be noted, however, that the direction of the forward current in the figure coincides with the generally accepted direction, opposite to the movement of electrons.

It can also be said that with the forward direction of switching on, the electrical resistance of the diode is relatively small. When turned on in reverse, this resistance will be many times greater; no current flows through the semiconductor diode (insignificant reverse current is not taken into account here). From all of the above, we can conclude that the diode behaves like an ordinary mechanical valve: turned in one direction - water flows, turned in the other - the flow stops. For this property the diode received the name semiconductor gate.

To understand in detail all the abilities and properties of a semiconductor diode, you should get acquainted with its volt-ampere characteristic. It is also a good idea to learn about the different diode designs and frequency properties, advantages and disadvantages. This will be discussed in the next article.

A diode is one of the types of devices designed on a semiconductor basis. It has one pn 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 for this process. In this case, an emission current is generated.
4. Between electrodes a negative spatial charge is formed, which 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.

Although these errors are rare, you must remember that these are all possibilities, as well as a diode that is interrupted and does not work when a large current is passed through it. You should also remember that the diode is made of materials that break easily. The only thing that holds them together is the diode body.

If the diode body expands, the connection opens. Also: How a capacitor works. On these pages you will find a lot of useful material on “electronics in general.” At this small positive voltage, there is virtually no forward current. With a positive voltage across its terminals, we say that the diode is forward biased. A diode is forward biased when its voltage is anywhere on the positive side of the source voltage.

Device

The following is a description of the diode structure; studying this information is necessary to further understand the principles of operation of these elements:

We say that the diode is reversed. In the reverse direction the current is very close to zero, always slightly negative, below the voltage axis. There is a tiny bit of current that flows when the diode is reverse biased. We call this reverse saturation current. In most situations this is close enough to zero that it can be ignored.

In some cases, reverse saturation current becomes important and you give it a bad-sounding name: leakage current. A reverse biased diode cannot last forever. During breakdown, the current increases sharply and becomes very high in the negative direction.

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 of the internal structure give diodes their main property - the ability to conduct electric current in only one direction.

Thanks to its two electrodes, it is called a diode. The diode is then considered to be forward biased. In this state, the height of the potential barrier at the junction is reduced by an amount equal to the specified forward bias voltage. Assuming that the current flowing through the diode will be very large, the diode can be approximated as a short-circuited switch. In this state, a value equal to the reverse bias voltage increases the height of the potential barrier at the junction. However, the process cannot continue indefinitely, so a small current continues to flow in the diode, called reverse saturation current.

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, 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 by combining these devices with capacitors. This is necessary so that it is possible to isolate low-frequency modulation from various modulated signals, including the amplitude-modulated variety of the radio signal. 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 occurring overloads or switches from breakdown by electromotive force that occurs during self-induction, which occurs when the 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 associated 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.

This current is negligible; a diode can be approximated as an open-closed switch. The current-voltage characteristics of the diode are explained by the following equations. Rice. - State of forward displacement. Rice. - Reverse Bias Condition. Tabulate the various forward currents obtained for different forward voltages.

• To obtain a graph in the inverse domain, replace the voltmeter with a nanoampmeter.
• A voltmeter has a lower load resistance compared to a diode.
• The current operates along a short resistance length.
• Take a graphic sheet and divide it into 4 equal parts.
• Mark the origin at the center of the graph sheet.
• In this case, the experiment does not exceed the readings of the diode.

Results: students can.

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.

I-V characteristics and rectifier diode

What are trivalent and pentavalent impurities? Trivalent impurities that form the p-type: aluminum, gallium, boron and indium. . Reverse the polarity of the voltage and it acts as a short circuit. What is diode current equation? Expression of dynamic resistance?

What is meant by internal semiconductor? What is the order of the energy gap in a pure semiconductor? What is an extrinsic semiconductor? What is a doped semiconductor? What are the two different types of impurities? What are the charge carriers in a pure semiconductor? What is the effect of temperature on the conductivity of a semiconductor? What is meant by straight slope? What does reverse bias mean? What is reverse breakdown? What are the semiconductor materials used? How many valence electrons are present in each semiconductor atom?

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 an 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

What is the static resistance of the diode? What is the dynamic resistance of a diode? Write the equation for the diode current. When current flows in only one direction and the voltage drop across the diode is always 7 V, the voltage at the anode should be about 6 V higher than the voltage at the cathode. We say that the diode is in forward bias.

When powered, the diode can be checked by measuring the voltage drop. The voltage at the anode should be 7 V higher than at the cathode. Is the voltage the same as the diode, short. When powered, the diode not only creates a 7V voltage drop, but can also separate two different voltages. The voltage at the cathode does not have to be the voltage coming from the anode. It may also come from another voltage source. In general, the voltage at the cathode is higher than the anode, the voltage comes from somewhere else, and the diode keeps the voltages separate.

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

What is a "forward" diode?

As always in electronics, heat is a big issue. If in doubt, carefully check all solder points on the board and separate them. When a diode is bad, choose a larger type if possible. Diode, an electronic component that allows current to flow in one direction. The diodes most used in modern electronic circuits are diodes made of semiconductor material. The simplest germanium contact point diode was created in the early days of radio. In modern germanium diodes, a cable and a tiny glass plate are mounted inside a small glass tube and connected to two wires that are welded to the ends of the tube.

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.

Junction diodes consist of a junction of two different types of semiconductor material. A Zener diode is a special type of diode that uses silicon in which the voltage across the junction is independent of the current passing through it. Due to this feature, Zener diodes are used as voltage regulators. On the other hand, in light-emitting diodes, voltage applied across the semiconductor junction results in the emission of light energy.

Three approaches are currently used to solve diode problems. The first approximation is an ideal diode, in which the diode is considered to have no voltage drop when wired in the positive direction, so in this first approximation the diode will be considered to be short circuited in the positive direction. In contrast, an ideal diode behaves as an open circuit when its polarization is reversed. To a second approximation, we consider that the diode has a voltage drop under forward polarization. The second approach is most often used.

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

Although there is a wide range of types, only a few features differ from their appearance. This is not about size because that is a function of the power they can dissipate. It is typical to find an aylo in the body that points to the cathode. For those whose specific type is designated by a series of letters and numbers, the cathode is marked by a ring in the body next to that terminal. Colors, and in them the cathode corresponds to the terminal closest to the thicker color track. Hermione tips are usually encased in glass.

The anode of these diodes is longer than the cathode, and usually the surface of the capsule near the cathode is flat. A practical way to determine the cathode is to use a meter in an ohmmeter between its terminals. If we use the diode test mode using the multicaster, we get the device's elbow voltage value.

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 1V), 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 permissible 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.

Once two materials join, the electrons and voids found in or near the "junction" region combine, and this results in a lack of carriers in the region close to the junction. This region of detected negative and positive ions is called the depletion region due to the absence of carriers. There are three possibilities for applying voltage to the diode terminals.

• There is no polarization.
• Direct polarization.
• The polarization is reversed.

In the absence of an applied bias voltage, the net charge flow in any direction is zero for a semiconductor diode. Reverse polarization condition. The number of negative ions found in the P-type material will also increase due to the electrons injected by the negative end, which will occupy the voids. The current under reverse polarization conditions is called reverse saturation current. When polarized in the opposite direction, it can be considered as an open circuit.

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:

When the reverse disjunction voltage is reached, there is a sudden increase in current that can destroy the device. This diode has a wide range of applications: rectifier circuits, limiters, level clamps, short circuit protection, demodulators, mixers, generators, blocking and bypass in photofibers, etc.

When using a diode in a circuit, the following considerations must be taken into account. The maximum reverse voltage applied to a component, repeating or not exceeding the maximum it will support. The maximum direct current that can pass through a component, repeating or not, must be greater than the maximum it will support.

1. Transition breakdown leads to the fact that the diode, instead of a semiconductor device, becomes essentially the most common 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 starts beeping and shows a low resistance level 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

The maximum power a diode can handle must be greater than the maximum it can handle. In Figure No.01 we can see a graphical representation or symbol for this type of diode. One of the important parameters for a diode is the resistance at the point or area of ​​operation.

Therefore, the diode represents a short circuit for the conduction region. If we consider the area of ​​potential negatively applied. Therefore, the diode is an open circuit in the region of no conduction. The current in the Zener region has the opposite direction to that of a straight polarized diode. A Zener diode is a diode that has been designed to operate in the Zener zone.

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.

By definition, we can say that a Zener diode was designed to operate with negative voltages. It is important to note that the Zener region is controlled or manipulated by changing doping levels. It applies to voltage regulators or supplies.

In the circuit shown in Figure 03, it is desirable to protect the load from overvoltage, the maximum voltage that the load can withstand is 8 volts. According to other considerations, the operation of this diode is approximately as follows. In the disturbance zone, between the elbow voltage and the zener voltage, we can consider an open circuit.

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 different 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 avalanche-like in nature, which is why this type of breakdown received its 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.

An increase in temperature 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 structures.
5. Complete failure and breakdown of the entire radio component.

thermal current, and the proportion of thermal current in the reverse current of a silicon diode is very small. The reverse current of a silicon diode is determined mainly by generation-recombination processes in p- n-transition. For engineering calculations of reverse current versus temperature, you can use the previously given simplified expression (2.4).

The direct branch of the diode’s current-voltage characteristic deviates from the idealized one due to the presence of recombination currents in p- n-transition, voltage drop at the base of the diode, changes (modulation) of the base resistance when minority charge carriers are injected into it, and the presence of an internal field in the base that occurs at a high injection current. Let us write the equation of the current-voltage characteristic of the ideal p- n-transition (2.3) taking into account the voltage drop at the diode base:

Where r b– ohmic resistance of the diode base.

The solution to this transcendental equation can be obtained by taking logarithms of the right and left sides of the equation:

. (3.2)

For low currents this expression can be simplified:

. (3.3)

A analysis of equation (3.3) allows us to draw some interesting conclusions. The voltage drop across the diode depends on the current through it and is of great importance for diodes with small I T. Since the thermal current of silicon diodes is small, the initial section of the direct branch of the current-voltage characteristic is much flatter than that of germanium diodes. This can also be explained by the fact that a noticeable current appears in the diode when the external voltage exceeds the contact potential difference  To, A  To(in accordance with (2.1)) for silicon p- n-transition is higher than that of germanium. The initial sections of the forward branch of the current-voltage characteristics of germanium and silicon diodes are shown in Fig. 3.2. The figure shows that the voltage on an open silicon diode is usually 0.60.8 V, the voltage on an open germanium diode is 0.20.3 V.

Due to the huge variety of diodes used for domestic semiconductor devices, a special notation system is used. The notation system is based on an alphanumeric code.

First element code indicates the source semiconductor material on which the device is manufactured. The following symbols are used:

Г or 1 – for germanium and its compounds;

K or 2 – for silicon and its compounds;

A or 3 – for gallium compounds (for example, gallium arsenide);

And or 4 – for indium compounds (for example, indium phosphide).

Second element designations - a letter defining a subclass (or group) of devices. Here are just some of the notations:

D – rectifier and pulse diodes;

C – rectifying posts and blocks;

B – varicaps;

I – tunnel diodes;

A – ultra-high frequency diodes;

C – zener diodes;

O – optocouplers;

N – dinistors;

U - triode thyristors...

Third element designation – a number that defines the main functionality of the device. The standard specifies the use of each digit in relation to various subclasses of devices. If necessary, you can find this in special reference literature.

Fourth element – a number indicating the serial number of the development.

Fifth Element – a letter that conditionally defines the classification (sorting according to parameters) of devices manufactured using a single technology.

Thus, knowing the notation system, we can say that GD107B is a germanium rectifier diode with I Wed VP10 A, development number 7, group B, and 2Ts202G - rectifier column made of silicon diodes with 0.3 A I Wed VP10 A, development number 2, group G.

3.2. Rectifier diodes

Diodes designed to convert alternating current to direct current, to speed, capacity p- n-transition and stability parameters of which usually do not have special requirements are called rectifying. Alloy, epitaxial and diffusion diodes made on the basis of asymmetrical p- n-transitions.

It is characteristic of rectifier diodes that they have low resistance in the conducting state and allow large currents to pass. Barrier capacity due to large area p- n-transitions is large and reaches values ​​of tens of picofarads.

The main parameters of diodes given in technical documentation and reference literature include:

1. Maximum permissible diode reverse voltage (U arr. max). This is the amount of voltage applied in the reverse direction that the diode can withstand for a long time without affecting its performance. For different diodes, this voltage can range from tens to thousands of volts.

2. Average rectified diode current (I Wed VP) – the maximum permissible, average over the period, value of the rectified direct current flowing through the diode. For various diodes, this current can range from hundreds of milliamps to tens of amperes.

3. Pulse forward current diode (I at) – permissible peak value of the current pulse at a given maximum duration and duty cycle of the pulses.

4. Diode reverse current (I arr.) – constant reverse current caused by constant reverse voltage.

5. Constant forward voltage (U etc) – constant forward voltage, conditioned by a given value of forward current. The ratio of these quantities determines the direct current resistance of the diode at a given point of the current-voltage characteristic.

3.3. Pulse diodes

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 (fractions of picofarads) 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 small (3050 mW).

Consider the effect on an electrical circuit consisting of a diode VD and resistor R(Fig. 3.3) alternating pulse voltage U input(Fig. 3.4, A). Voltage at the input of the circuit at a time t = 0 jumps to a positive value U m. Due to the inertia of the diffusion process, the current in the diode does not appear instantly, but increases over time t mouth. At a moment in time t = t 1 a stationary mode is established in the circuit, in which the diode current

,

A diode voltage U d =U etc .

At t = t 2 voltage U input changes polarity. However, the charges accumulated at the boundary p- n- transition, the diode is kept open for some time, but the direction of the current in the diode is reversed. Essentially over time t diss Charges dissolve at the boundary p- n- transition (i.e. discharge of equivalent capacity). After the resorption time interval t diss The process of turning off the diode begins, i.e. the process of restoring its locking properties.

By the time t 3 The voltage across the diode becomes zero and subsequently acquires the opposite value. The process of restoring the blocking properties of the diode continues until the point in time t 4 . By this time, the current through the diode becomes zero, and the voltage across it reaches the value - U m . So the time t sun can be counted from the transition U d through zero until the diode current reaches zero value.

Consideration of the processes of turning on and off a rectifying diode shows that the diode is not an ideal valve, but under certain conditions has conductivity in the opposite direction. These effects are especially pronounced at high input voltage frequencies and when working with pulse signals. In connection with this feature of the operation of pulsed diodes, the technical documentation for them, in addition to the parameters characterizing the normal rectification mode, provides additional parameters characterizing the transient process:

maximum pulse forward voltage –U at max ;

maximum permissible pulse forward current –I at max ;

settling time (t mouth) – time interval from the moment the forward voltage pulse is applied to the diode until the specified value of the forward current in it is reached;

recovery time diode reverse resistance – ( t sun).