What is the volt-ampere characteristic of the diode. Features of the current-voltage characteristics of the diode. Tutorial: Semiconductor diodes. Types of diodes by purpose

Low power diodes do not require additional heat dissipation (heat is removed using the diode body)

For diodes of medium and high power, which do not efficiently dissipate heat with their bodies, additional heat sink is required (a radiator is a cube of metal, in which spikes are made using casting or milling, as a result of which the surface of the heat sink increases. Material - copper, bronze, aluminum, silumin )

2. Constant forward voltage (U pr.)

Direct forward voltage is the voltage drop between the anode and cathode when the maximum allowable direct direct current flows.

It manifests itself especially at low supply voltage.

The constant forward voltage depends on the diode material (germanium - Ge, silicon - Si)

U pr. Ge ≈ 0.3 ÷ 0.5 V (Germanium) U pr. Si ≈ 0.5 ÷ 1 V (Silicon)

Germanium diodes mean - GD (1D) Silicon diodes mean - KD (2D)

3. Repetitive pulse reverse maximum voltage (U arr. Max)

Electrical breakdown occurs according to the amplitude value (pulse) U arr. max ≈ 0.7U El. breakdown (10 ÷ 100 V)

For powerful diodes U arr. max \u003d 1200 V.

This parameter is sometimes called the class of the diode (12 class -U arr. Max \u003d 1200 V)

4. Maximum diode reverse current (I max ..rev.)

Corresponds to the maximum reverse voltage (in mA units).

For silicon diodes, the maximum reverse current is half that for germanium diodes.

5. Differential (dynamic) resistance.

1.I pr max ≤30 A

2.U pr max ↓ ≤1.2 V

3.U obr max ≤1600

4.I arr max<100мА

The voltage drop across an individual diode depends on the magnitude of the forward current and temperature and is applied in the range for germanium diodes and for silicon ones.

The reverse current flowing through the diode strongly depends on the temperature, and at a certain value approaches a certain constant value (with increasing temperature, the reverse current increases).

The temperature limit for germanium diodes is; silicon diodes.

In electrical circuits, diodes are included in the circuit in the forward direction. E - power supply voltage. In practical circuits, some kind of load, such as a resistor, is always included in the diode circuit. This mode of operation of the diode is called workers ... It is calculated using the known values \u200b\u200band the I - V characteristic of the diode. The calculation is made according to the formula.

There are two unknowns in the formula. The decision is made graphically. A direct load is imposed on the I - V characteristic of the diode, which is built along 2 points on the coordinate axes at:

T. A in the picture.

Which corresponds to T. B.

We draw a straight line through these points, which is the load line. Coordinates t. T determine the operating mode of the diode.

The operating mode is characterized by the following parameters: - the maximum allowable power dissipated by the diode; temperature parameters.

Consider a group of semiconductor diodes, the peculiarity of which is associated with the use of nonlinear properties p-n-transition.

Rectifier diodes are designed to convert low frequency AC voltage () to DC. They are classified into diodes

• small,
• average
• high power.

Main parametersthat characterize rectifier diodes are:

• Reverse current at a certain value of reverse voltage;
• Maximum current in the forward direction;
• The voltage drop across the diode at a certain value of the forward current through the diode;
• Barrier capacitance of a diode when a reverse voltage of a certain value is applied to it;
• The frequency range in which the diode can work without a significant reduction in the rectified current;
• Working temperature range.

In the operating mode, a current flows through the diode, and power is released in its electrical junction, as a result of which the junction temperature rises. In the steady state, the power supplied to the junction and the power removed from it must be equal and not exceed the maximum allowable power dissipated by the diode, i.e. ... Otherwise, thermal breakdown of the diode occurs.

Today, diodes can be found in almost any household appliance. Many even assemble some devices in their home lab. But in order to use these elements of the electrical circuit correctly, you need to know what the VAC of the diode is. It is this characteristic that this article will be devoted to.

What it is

CVC stands for the current-voltage characteristic of a diode semiconductor. It reflects the dependence of the current that passes through the pn junction of the diode. The I - V characteristic determines the dependence of the current on the magnitude, as well as on the polarity of the applied voltage. The current-voltage characteristic has the form of a graph (diagram). This graph looks like this:

CVC for diode

For each type of diode, the I - V characteristic graph will have its own specific form. As you can see, the graph contains a curve. The values \u200b\u200bof the forward current (direct connection) are marked here vertically at the top, and in the opposite direction at the bottom. But the horizontal lines of the diagram and graph show the voltage, similarly in the forward and reverse directions. Thus, the current-voltage characteristic circuit will consist of two parts:

• top and right side - the element is functioning in the forward direction. It reflects the forward current. The line in this part goes up sharply. It characterizes a significant increase in forward voltage;
• bottom left - the element acts in the opposite direction. It corresponds to a closed (reverse) current through the junction. Here the line runs almost parallel to the horizontal axis. It reflects the slow rise of the reverse current.

Note! The steeper the vertical upper part of the graph, and the closer to the horizontal axis the lower line, the better the rectifier properties of the semiconductor will be.

It should be noted that the I – V characteristic is highly dependent on the ambient temperature. For example, an increase in air temperature can lead to a sharp increase in reverse current.
You can build a CVC with your own hands as follows:

• we take the power supply;
• we connect it to any diode (minus to the cathode, and plus to the anode);
• using a multimeter we take measurements.

From the data obtained, the current-voltage characteristic for a specific element is built. Its scheme or graph can be as follows.

Nonlinear CVC

The graph shows the I - V characteristic, which in this design is called nonlinear.
Let's look at examples of different types of semiconductors. For each individual case, this characteristic will have its own schedule, although they will all be the same with only minor changes.

VAC for Schottky

One of the most common diodes today is Schottky. This semiconductor was named after the German physicist Walter Schottky. For Schottky, the current-voltage characteristic will be as follows.

I - V characteristic for schottky

As you can see, Schottky is characterized by a low voltage drop in a direct connection situation. The graph itself is clearly asymmetrical. An exponential increase in current and voltage is observed in the forward bias zone. With reverse and forward bias for a given element, the current in the barrier is due to electrons. As a result, such elements are characterized by fast action, since they do not have diffuse and recombination processes. In this case, the asymmetry of the I – V characteristic will be typical for barrier-type structures. Here, the dependence of the current on the voltage is determined by the change in the number of carriers that take part in the charge-transfer processes.

Silicon diode and its CVC

In addition to Schottky, silicon semiconductors are currently very popular. For a silicon type diode, the current-voltage characteristic looks as follows.

CVC of silicon and germanium diode

For such semiconductors, this characteristic starts at about 0.5-0.7 volts. Silicon semiconductors are often compared to germanium semiconductors. If the ambient temperatures are equal, then both devices will exhibit a band gap. In this case, the silicon element will have a lower forward current than from germanium. The same rule applies to reverse current. Therefore, in germanium semiconductors, thermal breakdown usually occurs immediately if there is a large reverse voltage.
As a result, in the presence of the same temperature and forward voltage, the potential barrier for silicon semiconductors will be higher, and the injection current will be lower.

CVC and rectifier diode

In conclusion, I would like to consider this characteristic for a rectifier diode. A rectifier diode is a type of semiconductor that is used to convert alternating to direct current.

CVC for rectifier diode

The diagram shows the experimental I - V characteristic and theoretical (dashed line). As you can see, they do not match. The reason for this lies in the fact that some factors were not taken into account for theoretical calculations:

• the presence of ohmic resistance of the base and emitter regions of the crystal;
• his findings and contacts;
• the presence of the possibility of leakage currents on the crystal surface;
• the course of the processes of recombination and generation in the transition for carriers;
• various types of breakdowns, etc.

All of these factors can have a different effect, leading to a deviating from the theoretical real current-voltage characteristic. Moreover, the ambient temperature has a significant impact on the appearance of the graph in this situation.
The I - V characteristic for a rectifier diode demonstrates a high conductivity of the device at the moment a forward voltage is applied to it. In the opposite direction, low conductivity is observed. In such a situation, the current through the element practically does not flow in the opposite direction. But this only happens with certain parameters of the reverse voltage. If it is exceeded, then the graph shows an avalanche-like increase in current in the opposite direction.

Conclusion

The current-voltage characteristic for diode elements is considered an important parameter, reflecting the specifics of conducting current in the reverse and forward directions. It is determined depending on the voltage and ambient temperature.

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What is an ideal diode?

The main task of a conventional rectifier diode is conduct electric current in one direction, and do not pass it in the opposite direction... Therefore, an ideal diode should be a very good conductor with zero resistance when directly connected to voltage (plus to the anode, minus to the cathode), and an absolute insulator with infinite resistance when the opposite is true.

This is how it looks on the chart:

This diode model is used in cases where only the logical function of the device is important. For example, in digital electronics.

I - V characteristic of a real semiconductor diode

However, in practice, due to its semiconductor structure, this diode has a number of disadvantages and limitations compared to an ideal diode. This can be seen in the graph below.

V ϒ (gamma) - conduction threshold voltage

With direct connection, the voltage across the diode must reach a certain threshold value - V ϒ. This is the voltage at which the PN junction in the semiconductor opens up enough for the diode to conduct well. Before the voltage between the anode and cathode reaches this value, the diode is a very poor conductor. V ϒ for silicon devices is about 0.7V, for germanium devices - about 0.3V.

I D_MAX - maximum current through the diode during direct connection

When directly connected, the semiconductor diode is capable of withstanding the limited current I D_MAX. When the current through the device exceeds this limit, the diode overheats. As a result, the crystal structure of the semiconductor is destroyed, and the device becomes unusable. The magnitude of a given current strength varies greatly depending on the different types of diodes and their manufacturers.

I OP - reverse leakage current

When turned back on, the diode is not an absolute insulator and has a finite resistance, albeit very high. This creates a leakage or reverse current I OP. The leakage current for germanium devices reaches up to 200 µA, for silicon devices up to several tens of nA. The latest high quality silicon diodes with extremely low reverse current have this figure of about 0.5 nA.

PIV (Peak Inverse Voltage) - Breakdown voltage

When turned back on, the diode is able to withstand a limited voltage - the breakdown voltage PIV. If the external potential difference exceeds this value, the diode sharply lowers its resistance and turns into a conductor. This effect is undesirable, since the diode should be a good conductor only when directly connected. The breakdown voltage varies with different types of diodes and their manufacturers.

In most cases, for calculations in electronic circuits, an exact model of a diode with all its characteristics is not used. The non-linearity of this function makes the task too difficult. They prefer to use the so-called approximate models.

An approximate model of a diode "ideal diode + V ϒ"

The simplest and most frequently used is the first level approximation model. It consists of an ideal diode and, added to it, a conduction threshold voltage V.

An approximate model of a diode "ideal diode + V ϒ + r D"

Sometimes a slightly more complex and accurate approximate second level model is used. In this case, the internal resistance of the diode is added to the model of the first level, transforming its function from exponential to linear.

Rectifier diodes They are used in control circuits, switching, in limiting and decoupling circuits, in power supplies for converting (rectifying) alternating voltage to direct voltage, in voltage multiplication circuits and converters of direct voltage, where high requirements are not imposed on the frequency and time parameters of signals. Depending on the value of the maximum rectified current, a distinction is made between low power rectifier diodes (\\ (I_ (pr max) \\ le (0.3 A) \\)), average power (\\ ((0.3 A)< I_{пр max} \le {10 А}\)) и high power (\\ (I_ (pr max)\u003e (10 A) \\)). Low-power diodes can dissipate the heat generated on them by their body, medium and high-power diodes should be located on special heat sinks, which is provided for, incl. and the corresponding design of their bodies.

Usually, the permissible current density passing through the \\ (p \\) - \\ (n \\) - junction does not exceed 2 A / mm2, therefore, to obtain the above values \u200b\u200bof the average rectified current in rectifier diodes, planar \\ (p \\) - \\ Such transitions have significant capacitance, which limits the maximum permissible operating frequency (\\ (f_p \\)) of the rectifier diodes.

The rectifying properties of diodes are the better, the lower the reverse current at a given reverse voltage and the lower the voltage drop at a given forward current. The values \u200b\u200bof the forward and reverse currents differ by several orders of magnitude, and the forward voltage drop does not exceed several volts compared to the reverse voltage, which can be hundreds or more volts. Therefore, diodes have one-sided conductivity, which allows them to be used as rectifier elements. The current-voltage characteristics (VAC) of germanium and silicon diodes are different. In fig. Figure 2.3-1 shows typical CVC for germanium and silicon rectifier diodes for comparison at various ambient temperatures.

Figure: 2.3-1. Current-voltage characteristics of rectifier diodes at various ambient temperatures

It can be seen from the given VAC that the reverse current of silicon diodes is much less than the reverse current of germanium diodes. In addition, the reverse branch of the current-voltage characteristic of silicon diodes does not have a pronounced saturation region, which is due to the generation of charge carriers in the \\ (p \\) - \\ (n \\) junction and leakage currents along the crystal surface. When a reverse voltage is applied exceeding a certain threshold level, a sharp increase in the reverse current occurs, which can lead to a breakdown of the \\ (p \\) - \\ (n \\) - junction. In germanium diodes, due to the large amount of reverse current, the breakdown has a thermal character. In silicon diodes, the probability of thermal breakdown is small, and electrical breakdown prevails in them. The breakdown of silicon diodes has an avalanche nature, therefore, in contrast to germanium diodes, the breakdown voltage increases with increasing temperature. The permissible reverse voltage of silicon diodes (up to 1600 V) is significantly higher than that of germanium diodes.

Reverse currents are highly dependent on the junction temperature. It can be seen from the figure that the reverse current increases with increasing temperature. For an approximate assessment, we can assume that with an increase in temperature by 10 ° C, the reverse current of germanium diodes increases by 2, and silicon diodes - by 2.5 times. The upper limit of the operating temperature range for germanium diodes is 75 ... 80 ° С, and for silicon diodes - 125 ° С. A significant disadvantage of germanium diodes is their high sensitivity to short-term pulse overloads.

Due to the lower reverse current of the silicon diode, its forward current, equal to the current of the germanium diode, is achieved at a higher forward voltage. Therefore, the power dissipated at the same currents in germanium diodes is less than in silicon ones. The forward voltage at low forward currents, when the voltage drop across the junction prevails, decreases with increasing temperature. At high currents, when the voltage drop across the resistance of the neutral regions of the semiconductor prevails, the dependence of the forward voltage on temperature becomes positive. The point at which there is no dependence of the forward voltage on temperature (i.e., this dependence changes sign) is called inversion point... For most low and medium power diodes, the permissible forward current, as a rule, does not exceed the inversion point, and for high-power diodes, the permissible current can be higher than this point.

Semiconductor elements are widely used in the field of electronics, one of which is a diode. They are used in almost all devices, but more often in various power supplies and to ensure electrical safety. Each of them has its own specific purpose and technical characteristics. To identify various kinds of malfunctions and obtain technical information, you need to know the CVC of the diode.

General information

Diode (D) - semiconductor element, which serves to pass current through the pn junction in only one direction. With the help of D, you can straighten the variable U, getting a constant pulsating from it. To smooth out pulsations, filters of a capacitor or inductive type are used, and sometimes they are combined.

D consists only of a pn junction with leads, which are called the anode (+) and cathode (-). The current passing through the conductor has a thermal effect on it. When heated, the cathode emits negatively charged particles - electrons (E). The anode attracts electrons because it has a positive charge. In the process, an emission field is formed, at which a current (emission) arises. Between (+) and (-), a space negative charge is generated, which interferes with the free movement of electrons. E, which have reached the anode, form an anode current, and not reached - a cathodic one. If the anode and cathodic currents are equal to zero, D is in the closed state.

D consists of a housing made of durable dielectric material. The housing contains a vacuum space with 2 electrodes (anode and cathode). Electrodes, which are metal with an active layer, are indirectly heated. The active layer emits electrons when heated. The cathode is designed in such a way that there is a wire inside it, which heats up and emits electrons, and the anode serves to receive them.

In some sources, the anode and cathode are called a crystal, which is made from silicon (Si) or germanium (Ge). One of its constituent parts has an artificial lack of electrons, and the other has an excess (Fig. 1). There is a boundary between these crystals, which is called a p-n-junction.

Figure 1 - Schematic representation of a p-n-type semiconductor.

Applications

D is widely used as a variable U rectifier in the construction of power supplies (PSU), diode bridges, as well as in the form of a single element of a specific circuit. D is able to protect the circuit from reversing the polarity of the power supply. A breakdown of any semiconductor part (for example, a transistor) may occur in the circuit and lead to the process of failure of the chain of radioelements. In this case, a chain of several Ds connected in the opposite direction is used. Semiconductors are used to create switches for switching high-frequency signals.

D are used in the coal and metallurgical industries, especially when creating intrinsically safe switching circuits in the form of diode barriers that limit U in the required electrical circuit. Diode barriers are used together with current limiters (resistors) to reduce the values \u200b\u200bof I and increase the degree of protection, and, consequently, the electrical safety and fire safety of the enterprise.

Volt-ampere characteristics

The I – V characteristic is a characteristic of a semiconductor element showing the dependence of I passing through a p-n junction on the magnitude and polarity of U (Fig. 1).

Figure 1 - An example of the current-voltage characteristic of a semiconductor diode.

The I - V characteristics differ from each other and it depends on the type of semiconductor device. The I – V characteristic graph is a curve, along the vertical of which the values \u200b\u200bof direct I are marked (top). Below are the values \u200b\u200bof I for reverse connection. The U readings are shown horizontally for direct and reverse switching on. The scheme consists of 2 parts:

1. Top and right - D functions in direct connection. Shows throughput I and the line goes up, which indicates an increase in direct U (Upr).
2. The lower part on the left - D is in the closed state. The line runs almost parallel to the axis and indicates a slow increase in Iobr (reverse current).

From the graph, we can conclude: the steeper the vertical part of the graph (1 part), the closer the bottom line is to the horizontal axis. This indicates the high rectifying properties of the semiconductor device. It should be borne in mind that the I - V characteristic depends on the ambient temperature; with a decrease in temperature, a sharp decrease in Iobr occurs. If the temperature rises, then Iobr also rises.

Plotting

It is not difficult to build a CVC for a specific type of semiconductor device. This requires a power supply, a multimeter (voltmeter and ammeter) and a diode (can be built for any semiconductor device). The algorithm for constructing the I - V characteristic is as follows:

1. Connect the PSU to the diode.
2. Measure U and I.
3. Enter data into the table.
4. Based on the tabular data, build a graph of the dependence of I on U (Fig. 2).

Figure 2 - An example of a nonlinear I - V characteristic of a diode.

The CVC will be different for each semiconductor. For example, one of the most common semiconductors is the Schottky diode, named by the German physicist W. Schottky (Figure 3).

Figure 3 - VAC Schottky.

Based on the graph, which is asymmetric, it can be seen that this type of diode is characterized by a small drop in U with direct connection. There is an exponential increase in I and U. The current in the barrier is due to negatively charged particles in reverse and forward bias. Schottky have a high speed of response, since there are no diffuse and recombination processes. I depends on U due to a change in the number of carriers participating in charge transfer processes.

Silicon semiconductor is widely used in almost all electrical circuits of devices. Figure 4 shows its I - V characteristic.

Figure 4 - I - V characteristic of silicon D.

In Figure 4, the I - V characteristic starts from 0.6-0.8 V. In addition to silicon D, there are also germanium ones, which will work normally at normal temperatures. Silicon has lower Ipr and Irev, therefore, thermal irreversible breakdown in germanium D occurs faster (when high Urev is applied) than in its competitor.

Rectifier D is used to convert AC U to DC and Figure 5 shows its I - V characteristic.

Figure 5 - CVC of rectifier D.

The figure shows the theoretical (dashed curve) and practical (experimental) I – V characteristics. They do not coincide due to the fact that the theory did not take into account some aspects:

1. The presence of R (resistance) of the emitter region of the crystal, leads and contacts.
2. Leakage currents.
3. Generation and recombination processes.
4. Breakdowns of various types.

In addition, the ambient temperature significantly affects the measurements, and the I - V characteristics do not coincide, since the theoretical values \u200b\u200bare obtained at a temperature of +20 degrees. There are other important characteristics of semiconductors that can be understood from the markings on the package.

There are also additional characteristics. They are needed for the use of D in a certain circuit with U and I. If you use a low-power D in devices with U exceeding the maximum allowable Uobre, then a breakdown and failure of the element will occur, and this can also lead to a chain of failure of other parts.

Additional characteristics: maximum values \u200b\u200bof Iobr and Uobr; direct values \u200b\u200bof I and U; overload current; Maximum temperature; working temperature and so on.

The I - V characteristic helps to determine such complex faults D: junction breakdown and case depressurization. Complex malfunctions can lead to the failure of expensive parts, therefore, before installing D on the board, it must be checked.

Possible malfunctions

According to statistics, D or other semiconductor elements fail more often than other circuit elements. The faulty item can be calculated and replaced, but sometimes this leads to loss of functionality. For example, when a p-n-junction breaks down, D turns into an ordinary resistor, and such a transformation can lead to sad consequences, ranging from the failure of other elements and ending with a fire or electric shock. Major faults include:

1. Breakdown. The diode loses its ability to pass current in one direction and becomes an ordinary resistor.
2. Structural damage.
3. A leak.

During breakdown, D does not pass current in one direction. There may be several reasons, and they arise with sharp increases in I and U, which are unacceptable values \u200b\u200bfor a certain D. The main types of breakdowns of the p-n junction:

1. Thermal.
2. Electric.

At the thermal level, at the physical level, there is a significant increase in atomic vibrations, deformation of the crystal lattice, overheating of the transition and the ingress of electrons into the conductive zone. The process is irreversible and leads to damage to the radio component.

Electrical breakdowns are temporary (the crystal is not deformed) and when it returns to normal operation, its semiconductor functions return. Structural damage is physical damage to the legs and body. Current leakage occurs when the case is depressurized.

To check D, it is enough to evaporate one leg and ring it with a multimeter or ohmmeter on the presence of a breakdown of the transition (it should only ring in one direction). As a result, the value of R p-n-transition will appear in one direction, and in the other the device will show infinity. If you call in 2 directions, then the radio component is faulty.

If the leg has disappeared, then it must be soldered. If the case is damaged, the part must be replaced with a serviceable one.

When the case is depressurized, it will be necessary to plot the I - V characteristic and compare it with the theoretical value taken from the reference literature.

Thus, the I - V characteristic allows not only obtaining reference data about a diode or any semiconductor element, but also identifying complex faults that cannot be determined when checking with the device.