What is the current-voltage characteristic of a diode. Features of the current-voltage characteristic of the diode. Tutorial: Semiconductor Diodes. Types of diodes by purpose

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

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

2. Constant forward voltage(U ex.)

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

It manifests itself especially at low supply voltage.

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

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

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

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

Electrical breakdown goes on the amplitude value (impulse) U arr. max ≈ 0.7U breakdown (10÷100 V)

For powerful diodes U arr. max = 1200 V.

This parameter is sometimes called the class of the diode (class 12 - U arr. max = 1200 V)

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

Corresponds to the maximum reverse voltage (is units of mA).

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

5. Differential (dynamic) resistance.

1. I pr max ≤30 A

2. U pr max ↓ ≤1.2 V

3. U arr max ≤1600

4. I arr max<100мА

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

The reverse current flowing through the diode is highly dependent on 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 connected in the circuit in the forward direction. E is the power supply voltage. In practical circuits, a load, such as a resistor, is always included in the diode circuit. This mode of operation of the diode is called workers . Its calculation is made according to the known values ​​\u200b\u200band the CVC of the diode. The calculation is made according to the formula.

The formula has two unknowns. The solution is made graphically. A direct load is imposed on the CVC of the diode, which is built on 2 points on the coordinate axes at:

T. And in the figure.

What corresponds to t. B.

Through these points we draw a straight line, which is the load line. T. T coordinates 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 settings.

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

Rectifier Diodes designed to convert low-frequency alternating voltage () to direct. They are classified as diodes.

• small,
• middle
• high power.

Main parameters characterizing rectifier diodes are:

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

During operation, current flows through the diode, and power is released in its electrical junction, as a result of which the temperature of the junction rises. In steady state, the power supplied to the junction and 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 properly use these elements of the electrical circuit, you need to know what the CVC of the diode is. It is this characteristic that this article will focus on.

What it is

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

CVC for diode

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

• top and right side - the element functions in the forward direction. It reflects the throughput. The line in this part goes sharply upwards. It characterizes a significant increase in forward voltage;
• lower left part - the element acts in the opposite direction. It corresponds to 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 top of the graph, and the closer the bottom line is to the horizontal axis, the better the rectifier properties of the semiconductor will be.

It should be noted that the CVC strongly depends on the ambient temperature. For example, an increase in air temperature can lead to a sharp increase in reverse current.
You can build a VAC with your own hands as follows:

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

From the data obtained, the current-voltage characteristic for a particular element is built. Its scheme or graph may look like this.

Nonlinear IV

The graph shows the CVC, which in this design is called non-linear.
Consider the examples of various types of semiconductors. For each individual case, this characteristic will have its own schedule, although they will all be of the same nature with only minor changes.

VAC for Schottky

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

VAC for Schottky

As you can see, Schottky is characterized by a small voltage drop in a direct connection situation. The graph itself is clearly asymmetric. In the zone of forward biases, an exponential increase in current and voltage is observed. 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 CVC will be typical for structures of the barrier type. Here, the dependence of current on voltage is determined by changing the number of carriers that take part in 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 like this.

CVC of silicon and germanium diode

For such semiconductors, this characteristic starts at about 0.5-0.7 Volts. Silicon semiconductors are often compared with germanium semiconductors. If the ambient temperatures are equal, then both devices will exhibit a bandgap. 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 of silicon semiconductors will be higher, and the injection current will be lower.

VAC 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 AC to DC.

CVC for rectifier diode

The diagram shows the experimental CVC and the theoretical one (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 conclusions and contacts;
• the possibility of leakage currents along the crystal surface;
• the course of recombination and generation processes in the transition for carriers;
• various types of breakdowns, etc.

All these factors can have different influences, leading to 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 curve for a rectifier diode demonstrates the high conductivity of the device at the moment a voltage is applied to it in the forward direction. 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 at certain reverse voltage parameters. 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 current conduction 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 electricity in one direction and not in the opposite direction. Therefore, an ideal diode should be a very good conductor with zero resistance when voltage is applied directly (plus to the anode, minus to the cathode), and an absolute insulator with infinite resistance when applied in reverse.

This is how it looks on the chart:

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

IV characteristic of a real semiconductor diode

However, in practice, due to its semiconductor structure, a real diode has a number of disadvantages and limitations compared to an ideal diode. This can be seen in the chart 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 enough for the diode to begin to conduct current 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 - about 0.3V.

I D_MAX - maximum current through the diode with direct connection

When directly connected, a semiconductor diode is able to withstand a 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 value of this 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 a very high one. This causes a leakage current 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 value around 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 connected directly. The breakdown voltage value varies depending on different types of diodes and their manufacturers.

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

Approximate diode model "ideal diode + V ϒ "

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

Approximate diode model "ideal diode + V ϒ + r D "

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

Rectifier Diodes are used in control circuits, switching circuits, in limiting and decoupling circuits, in power supplies for converting (rectifying) alternating voltage into direct voltage, in voltage multiplication circuits and DC voltage converters, where there are no high requirements for frequency and time parameters of signals. Depending on the value of the maximum rectified current, there are low power rectifier diodes(\ (I_ (pr max) \ le (0.3 A) \)), medium power(\((0.3 A)< I_{пр max} \le {10 А}\)) и high power(\ (I_ (pr max) > (10 A) \)). Low power diodes can dissipate the heat generated on them by their case, medium and high power diodes must be located on special heat sinks, which is provided for incl. and the corresponding design of their hulls.

Usually, the allowable 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 \) - \ (n\)-transitions. Such junctions have significant capacitance, which limits the maximum allowable operating frequency (\(f_p\)) of rectifier diodes.

The rectifying properties of diodes are better, the smaller the reverse current for a given reverse voltage and the smaller the voltage drop for a given forward current. The values ​​of the forward and reverse currents differ by several orders of magnitude, and the forward voltage drop does not exceed a few 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 (CVC) of germanium and silicon diodes are different. On fig. Figure 2.3‑1 shows for comparison the typical I–V characteristics for germanium and silicon rectifier diodes at various ambient temperatures.

Rice. 2.3-1. Volt-ampere characteristics of rectifier diodes at various ambient temperatures

It can be seen from the given I–V characteristics 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 over the crystal surface. When a reverse voltage exceeding a certain threshold level is applied, 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. For silicon diodes, the probability of thermal breakdown is low, and electrical breakdown predominates in them. The breakdown of silicon diodes has an avalanche character, therefore, in them, unlike germanium diodes, the breakdown voltage increases with increasing temperature. The allowable reverse voltage of silicon diodes (up to 1600 V) is much higher than that of germanium diodes.

The 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 estimate, we can assume that with an increase in temperature by 10 ° C, the reverse current of germanium diodes increases by 2, and silicon - by 2.5 times. The upper limit of the operating temperature range of germanium diodes is 75 ... 80 ° C, and silicon - 125 ° C. A significant disadvantage of germanium diodes is their high sensitivity to short-term impulse 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 at the junction predominates, decreases with increasing temperature. At high currents, when the voltage drop across the resistance of the neutral regions of the semiconductor predominates, 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 to medium power diodes, the allowable forward current will generally not exceed the inversion point, while high power diodes may have allowable forward current above this point.

Semiconductor elements, one of which is a diode, have received wide application in the field of electronics. 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 p-n junction in only one direction. With the help of D, you can rectify the variable U, obtaining from it a constant pulsating one. To smooth out ripples, filters of a capacitor or inductive type are used, and sometimes they are combined.

D consists only of a p-n junction with leads called the anode (+) and cathode (-). The current, when 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 (-) there is a generation of a spatial negative charge that interferes with the free movement of E. The E that have reached the anode form the anode current, and those that have not reached the cathode current. If the anode and cathode currents are 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 representing a metal with an active layer have an indirect glow. The active layer emits electrons when heated. The cathode is designed in such a way that inside it there is a wire that 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 of silicon (Si) or germanium (Ge). One of its components 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 (PSUs), diode bridges, and also as a single element of a specific circuit. D is able to protect the circuit from non-observance of the polarity of the power supply connection. A breakdown of any semiconductor part (for example, a transistor) can occur in the circuit and lead to the process of failure of the chain of radio elements. In this case, a chain of several D connected in the opposite direction is used. On the basis of semiconductors, switches are created 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 ​​of I and increase the degree of protection, and hence the electrical safety and fire safety of the enterprise.

Volt-ampere characteristics

CVC is a characteristic of a semiconductor element, showing the dependence of I passing through a p-n junction on the value and polarity of U (Fig. 1).

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

I–V characteristics differ from each other and it depends on the type of semiconductor device. The VAC graph is a curve, along the vertical of which the values ​​of the direct I are marked (top). The values ​​of I at the reverse connection are marked below. The horizontal indications U are indicated for direct and reverse switching. The scheme consists of 2 parts:

1. Top and right - D functions in direct connection. It shows throughput I and the line goes up, which indicates the growth of 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 Irev (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 testifies to the high rectifying properties of the semiconductor device. It must be taken into account that the CVC depends on the ambient temperature, with a decrease in temperature, a sharp decrease in Iobr occurs. If the temperature rises, then I rises as well.

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 CVC is as follows:

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

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

The IV characteristic 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 in nature, it can be seen that this type of diode is characterized by a small drop in U when connected directly. There is an exponential increase in I and U. The current in the barrier is due to negatively charged particles at reverse and forward biases. Schottky have high speed, as there are no diffuse and recombination processes. I depends on U due to the change in the number of carriers involved in charge transfer processes.

Silicon semiconductor is widely used in almost all electrical circuits of devices. Figure 4 shows its CVC.

Figure 4 - CVC of silicon D.

In Figure 4, the CVC 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 a smaller Ipr and Iabr, so the thermal irreversible breakdown of germanium D occurs faster (when a high Uabr is applied) than that of its competitor.

Rectifier D is used to convert the variable U into a constant, and Figure 5 shows its current-voltage characteristic.

Figure 5 - CVC rectifier D.

The figure shows the theoretical (dashed curve) and practical (experimental) CVC. They do not coincide due to the fact that some aspects were not taken into account in the theory:

1. The presence of R (resistance) of the emitter region of the crystal, leads and contacts.
2. leakage currents.
3. Processes of generation and recombination.
4. Breakouts of various types.

In addition, the ambient temperature significantly affects the measurements, and the current-voltage characteristics do not match, 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 case.

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

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

VAC helps to identify such complex malfunctions D: breakdown of the transition and depressurization of the case. Complex malfunctions can lead to failure of expensive parts, therefore, before mounting D on the board, it is necessary to check it.

Possible malfunctions

According to statistics, D or other semiconductor elements fail more often than other circuit elements. A failed element can be identified and replaced, but sometimes this results in a 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 failure of other elements to fire or electric shock. The main faults are:

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 ​​for 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 the vibration of atoms, deformation of the crystal lattice, overheating of the junction, and electrons entering the conduction band. The process is irreversible and leads to damage to the radio component.

Electrical breakdowns are temporary (the crystal does not deform) and upon returning 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 unsolder one leg and ring it with a multimeter or ohmmeter for the presence of a transition breakdown (should only ring in one direction). As a result, the R value of the p-n junction will appear in one direction, and in the other direction the device will show infinity. If you call in 2 directions, then the radio component is faulty.

If the leg fell off, then it needs to 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 on a diode or any semiconductor element, but also identifying complex faults that cannot be determined when checking with an instrument.