Static volt-ampere characteristic of the diode. Characteristics and principle of operation of rectifier diodes. Main characteristics and parameters of diodes

In fig. 2.9 shows the current-voltage characteristic of a silicon rectifier diode at different ambient temperatures.

Maximum allowable forward currents of silicon junction diodes different types are 0.1 ... 1600 A. The voltage drop across the diodes at these currents usually does not exceed 1.5 V. With an increase in temperature, the forward voltage drop decreases, which is associated with a decrease in the height of the potential barrier

p – n-transition and with the redistribution of charge carriers over energy levels.

The reverse branch of the current-voltage characteristic of silicon diodes does not have a section of saturation of the reverse current, since reverse current in silicon diodes is caused by the process of generation of charge carriers in p – n-transition. Breakdown of silicon diodes is avalanche. Therefore, the breakdown voltage increases with increasing temperature. For some types of silicon diodes at room temperature, the breakdown voltage can be 1500 ... 2000 V.

The operating temperature range for silicon rectifier diodes is limited to values \u200b\u200b- 60 ... + 125 C. The lower limit of operating temperatures is due to the difference in temperature coefficients of linear expansion of various elements of the diode structure: at low temperatures, mechanical stresses arise, which can lead to cracking of the crystal. With a decrease in temperature, it is also necessary to take into account an increase in the forward voltage drop across the diode, which occurs due to an increase in the height of the potential barrier by p – n-transition.

The upper limit of the operating temperature range of rectifier diodes is determined by a sharp deterioration in rectification due to an increase in reverse current - the thermal generation of charge carriers as a result of ionization of semiconductor atoms affects. Based on this, the upper limit of the operating temperature range of silicon rectifier diodes, like most other semiconductor devices, is associated with the band gap of the original semiconductor material.

In fig. 2.10 shows the current-voltage characteristic of a germanium rectifier diode at different ambient temperatures.

The forward voltage across a germanium diode at the maximum allowable forward current is almost half that on a silicon diode. This is due to the lower height of the potential barrier to the germanium transition, which is an advantage, but, unfortunately, the only one.

For germanium diodes, the existence of a reverse saturation current is characteristic, which is associated with the formation mechanism of the reverse current - the process of extraction of minority charge carriers.

The reverse current density in germanium diodes is much higher, because all other things being equal, the concentration of minority charge carriers in germanium is several orders of magnitude higher than in silicon. This leads to the fact that for germanium diodes the breakdown has a thermal character. Therefore, the breakdown voltage decreases with increasing temperature, and the values \u200b\u200bof this voltage are less than the breakdown voltage of silicon diodes.

The upper limit of the operating temperature range for germanium diodes is about 75 ° C.

An essential feature of germanium diodes and their disadvantage is that they poorly withstand even very short-term impulse overloads with reverse bias p – n-transition. This is determined by the breakdown mechanism - a thermal breakdown that occurs when the current is laced with the release of high specific power at the breakdown site.

The listed features of silicon and germanium rectifier diodes are associated with the difference in the band gap of the original semiconductors. From this comparison, it can be seen that rectifier diodes with a wider band gap have significant advantages in properties and parameters. One of these representatives is gallium arsenide.

At present, commercially available gallium arsenide rectifier diodes are still far from optimal. For example, a diode of type AD112A has a maximum permissible forward current of 300 mA at a forward voltage of 3 V. A large amount of forward voltage is a disadvantage of all rectifier diodes, p – n-the transitions of which are formed in a material with a wide band gap. The maximum allowable reverse voltage for this diode is -50 V. This is most likely due to the fact that in the region p – n-transition there is a large concentration of defects due to imperfect technology.

The advantages of gallium arsenide rectifier diodes are a large operating temperature range and better frequency properties. The upper limit of operating temperatures for AD112A diodes is 250 C. Gallium arsenide diodes AD110A can operate in low-power rectifiers up to a frequency of 1 MHz, which is ensured by the short lifetime of charge carriers in this material.

Findings:

1. With an increase in temperature, the reverse current of germanium rectifier diodes increases sharply due to an increase in the thermal current.

2. Silicon diodes have a very small thermal current, and therefore they can operate at more high temperatures and with less reverse current than germanium diodes.

3. Silicon diodes can operate at significantly higher reverse voltages than germanium diodes. The maximum permissible constant reverse voltage for silicon diodes increases with increasing temperature to a maximum value, while for germanium diodes it drops sharply.

4. Due to these advantages, rectifier diodes are currently mainly manufactured on the basis of silicon.

Wah-wah-wah ... Usually these words are used when telling anecdotes about Caucasians))) Caucasians, please do not be offended - I respect the Caucasus. But, as they say, you cannot erase words from a song. And in our case, this word has a different meaning. And not even a word, but an abbreviation.

CVC Is the volt-ampere characteristic. Well, in this section we are interested in volt-ampere characteristic of a semiconductor diode.

The I - V characteristic of the diode is shown in Fig. 6.

Figure: 6. I - V characteristic of a semiconductor diode.

The graph shows the I - V characteristics for forward and reverse switching on of the diode. They also say that the forward and reverse branch of the current-voltage characteristic. The direct branch (Ipr and Upr) displays the characteristics of the diode during direct connection (that is, when a "plus" is applied to the anode). The reverse branch (Irev and Urev) displays the characteristics of the diode during reverse connection (that is, when a minus is applied to the anode).

In fig. 6, the blue thick line is the characteristic of the germanium diode (Ge), and the black thin line is the characteristic of the silicon (Si) diode. The figure does not indicate the units of measurement for the current and voltage axes, since they depend on the specific brand of diode.

What do we see on the chart? Well, to begin with, let's define, as for any plane coordinate system, four coordinate angles (quadrants). Let me remind you that the first is the quadrant, which is located in the upper right (that is, where we have the letters Ge and Si). Further, the quadrants are counted counterclockwise.

So, the II-nd and IV-th quadrants are empty. This is because we can only turn on the diode in two ways - forward or reverse. A situation is impossible when, for example, a reverse current flows through the diode and at the same time it is turned on in the forward direction, or, in other words, it is impossible to simultaneously apply both "plus" and "minus" to one terminal. More precisely, it is possible, but then it will be a short circuit))). It remains to consider only two cases - direct diode and reverse diode.

The direct inclusion graph is drawn in the first quadrant. From this it can be seen that the greater the voltage, the greater the current. Moreover, until a certain moment, the voltage grows faster than the current. But then a break occurs, and the voltage remains almost unchanged, and the current begins to rise. For most diodes, this break occurs in the range of 0.5 ... 1 V. It is this voltage, as they say, "drops" across the diode. That is, if you connect a light bulb according to the first circuit in fig. 3, and the voltage of the battery will be 9 V, then it will not be 9 V, but 8.5 or even 8 (depending on the type of diode). These 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 the current through the diode practically does not go even in the forward direction.

The reverse engagement graph is drawn in the third quadrant. From this it can be seen that the current remains almost unchanged over a significant section, and then increases like an avalanche. What does it mean? If you turn on the light bulb according to the second scheme in fig. 3, then it will not glow, because the diode does not transmit 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 glow). But a diode cannot hold back the voltage indefinitely. If you increase the voltage, for example, up to several hundred volts, then this high voltage The diode will "break through" (see the inflection on the reverse branch of the graph) and the current will flow through the diode. Here are just "breakdown" - this is an irreversible process (for diodes). That is, such a "breakdown" will lead to burnout of the diode and it will either stop passing current in any direction altogether, or vice versa - it will pass current in all directions.

In the characteristics of specific diodes, the maximum reverse voltage is always indicated - that is, the voltage that the diode can withstand without "breakdown" when turned on in the opposite direction. This must be taken into account when developing devices where diodes are used.

Comparing the characteristics of silicon and germanium diodes, we can conclude that in the p-n junctions of a silicon diode, the forward and reverse currents are less than in a germanium diode (with the same voltage values \u200b\u200bat the terminals). This is due to the fact that silicon has a wider bandgap and for the transition of electrons from the valence band to the conduction band, they need to be imparted a large additional energy.

A rectifier diode is a device that conducts current in one direction only. Its design is based on one pn junction and two outputs. Such a diode changes the alternating current to direct current. In addition, they are widely practiced in electrical circuits for voltage multiplication, circuits where there are no strict requirements for signal parameters in time and frequency.

• Principle of operation
• Basic device parameters
• Rectifier circuits
• Impulse devices
• Imported appliances

Principle of operation

The principle of operation of this device is based on features p-n transition. Near the junctions of two semiconductors, there is a layer in which there are no charge carriers. This is the locking layer. His resistance is great.

When a certain external alternating voltage is applied to the layer, its thickness becomes smaller, and subsequently disappears altogether. The current increasing in this case is called direct. It runs from the anode to the cathode. If the external alternating voltage has a different polarity, then the blocking layer will be larger, the resistance will increase.

Varieties of devices, their designation

By design, there are two types of devices: point and plane. In industry, the most common silicon (designation - Si) and germanium (designation - Ge). The former have a higher operating temperature. The advantage of the latter is a low voltage drop with forward current.

The diode designation principle is an alphanumeric code:

• The first element is the designation of the material from which it is made;
• The second defines the subclass;
• The third denotes work opportunities;
• The fourth is the serial number of the development;
• Fifth - designation of sorting by parameters.

The current-voltage characteristic (VAC) of the rectifier diode can be represented graphically. The graph shows that the I - V characteristic of the device is nonlinear.

In the initial quadrant Volt-ampere characteristics its straight branch reflects the highest conductivity of the device when a direct potential difference is applied to it. The reverse branch (third quadrant) of the I – V characteristic reflects the situation of low conductivity. This occurs with a reverse potential difference.

Actual current-voltage characteristics are subject to temperature. With increasing temperature, the forward potential difference decreases.

From the graph of the current-voltage characteristic, it follows that with low conductivity, no current passes through the device. However, at a certain magnitude of the reverse voltage, an avalanche breakdown occurs.

The CVC of silicon devices differs from germanium ones. I - V characteristics are given as a function of various ambient temperatures. The reverse current of silicon devices is much less than that of germanium devices. It follows from the I – V characteristics that it increases with increasing temperature.

The most important property is the sharp asymmetry of the I – V characteristic. Forward bias - high conductivity, reverse - low. It is this property that is used in rectifiers.

Analyzing the instrument characteristics, it should be noted: such quantities as the rectification coefficient, resistance, and device capacitance are taken into account. These are differential parameters.

It reflects the quality of the rectifier.

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It can be calculated: it will be equal to the ratio of the forward current of the device to the reverse one. This calculation is acceptable for an ideal device. The value of the rectification factor can reach several hundred thousand. The larger it is, the better the rectifier does its job.

Basic device parameters

What parameters characterize the devices? Basic parameters of rectifier diodes:

• The highest value of the average forward current;
• Highest allowable reverse voltage value;
• Maximum allowable potential difference frequency at a given forward current.

Based on the maximum value of the forward current, rectifier diodes are divided into:

• Low power devices. They have a forward current value of up to 300 mA;
• Medium power rectifier diodes. Direct current range from 300 mA to 10 A;
• Power (high power). The value is more than 10 A.

There are power devices depending on the form, material, type of installation. The most common ones are:

• Medium power devices. Their technical specifications allow you to work with voltages up to 1.3 kilovolts;
• Power, high power, capable of passing currents up to 400 A. These are high-voltage devices. There are different housings for power diodes. The most common are pin and tablet types.

Rectifier circuits

Connection diagrams power devices are different. To rectify the mains voltage, they are divided into single-phase and multi-phase, half-wave and full-wave. Most of them are single phase. Below is the design of such a half-wave rectifier and two voltage graphs in a timing diagram.

AC voltage U1 is applied to the input (Fig. A). It is represented by a sinusoid on the right side of the graph. The diode is open. A current flows through the load Rн. With a negative half-cycle, the diode is closed. Therefore, only a positive potential difference is applied to the load. In fig. its time dependence is reflected. This potential difference is valid for one half period. Hence the name of the circuit.

The simplest full-wave circuit consists of two half-wave. For such a rectification design, two diodes and one resistor are sufficient.

Diodes only pass the positive AC waveform. The disadvantage of the design is that in a half-cycle, the variable potential difference is removed from only half of the secondary winding of the transformer.

If four diodes are used instead of two diodes, the efficiency will increase.

Rectifiers are widely used in various industries. The three-phase device is involved in car generators... And the use of the invented alternator helped to reduce the size of this device. In addition, its reliability has increased.

In high-voltage devices, high-voltage poles are widely used, which are composed of diodes. They are connected in series.

Impulse devices

An impulse device is called a device in which the transition time from one state to another is short. They are used to work in impulse circuits. Such devices differ from their rectifier analogs in small containers p-n transitions.

For devices of this class, in addition to the parameters indicated above, the following should be attributed:

• Maximum pulse forward (reverse) voltages, currents;
• Forward voltage setting period;
• Reverse resistance recovery period of the device.

Schottky diodes are widely used in high-speed pulse circuits.

Imported appliances

The domestic industry produces a sufficient number of devices. However, today imported ones are most in demand. They are considered to be of better quality.

Imported devices are widely used in TV and radio circuits. They are also used to protect various devices in case of incorrect connection (incorrect polarity). The number of types of imported diodes is diverse. A full-fledged alternative replacement for them with domestic ones does not yet exist.

The current-voltage characteristic (VAC) is a graph of the dependence of the current in the external circuit of the p-n-junction on the value and polarity of the voltage applied to it. This dependence can be obtained experimentally or calculated on the basis of the current-voltage characteristic equation . The thermal current of the pn junction depends on the impurity concentration and temperature. An increase in the temperature of the p-n-junction leads to an increase in the thermal current, and, consequently, to an increase in the forward and reverse currents. An increase in the concentration of the dopant leads to a decrease in the thermal current, and, consequently, to a decrease in the forward and reverse currents of the p-n-junction.

14. Breakdownp- n –Transition- called a sharp change in the operating mode of the junction, which is under reverse voltage. Accompanying

A sharp increase in the reverse current, with a slightly decreasing and even decreasing reverse voltage:

Three types of breakdown:

1. Tunnel (electrical) - the phenomenon of the passage of electrons through a potential barrier;

2. Avalanche (electrical) - occurs if, when moving before the next collision with the atom, the hole (electron) acquires energy sufficient to ionize the atom;

3. Thermal breakdown (irreversible) - occurs when the semiconductor heats up and a corresponding increase in conductivity.

15. Rectifier diode: purpose, wax, basic parameters, ugo

Rectifier diodes are used to convert alternating current into pulsating current of one direction and are used in power supplies for electronic equipment.

Germanium rectifier diodes

The manufacture of germanium rectifier diodes begins by fusing indium into an original n-type germanium semiconductor wafer. In turn, the original plate is soldered to a steel crystal holder for low-power rectifier diodes or to a copper base for high-power rectifier diodes.

Fig. 24 construction of a low-power alloy diode. 1- crystal holder; 2 - crystal; 3 - int. output; 4 - insidious body; 5 - insulator; 6 - kovar pipe; 7 - external output

Rice25 VAC of germanium diode

From Fig. 25 it can be seen that with increasing temperature, the reverse current of the diode increases significantly, and the value of the breakdown voltage decreases.

Germanium diodes for various purposes have a rectified current from 0.3 to 1000A. The forward voltage drop does not exceed 0.5V, and the allowable reverse voltage is 400V. The disadvantage of germanium diodes is their irreversible breakdown even with short-term impulse overloads.

Silicon rectifier diodes

For getting p-n transition in silicon rectifier diodes, aluminum is fused into an n-type silicon crystal, or an alloy of gold with antimony into p-type silicon. Diffusion methods are also used to obtain transitions. The design of a number of low-power silicon diodes practically does not differ from those of similar germanium diodes.

Semiconductor devices

Diodes.

A semiconductor diode is a device that is two connected semiconductors of different conductivity.

Designation on the diagrams:

V or VD - diode designation

VS - diode assembly designation

V7 Anode The number after V, shows the diode number in the circuit

Anode is a P-type semiconductor Cathode is an N-type semiconductor

When an external voltage is applied to the diode in the forward direction ("+" to the anode, and "-" to the cathode), the potential barrier decreases, diffusion increases - the diode is open (short-circuited).

When a voltage is applied in the opposite direction, the potential barrier increases, diffusion stops - the diode is closed (break).

Current-voltage characteristic (VAC) of a semiconductor diode.

U email test \u003d 10 ÷ 1000 V - electric breakdown voltage.

U us. \u003d 0.3 ÷ 1 V - saturation voltage.

I a and U a - anode current and voltage.

Section I:- working section (direct branch of the I - V characteristic)

Sections II, III, IV, - reverse branch of the I - V characteristic (not the working section)

Section II: If you apply a reverse voltage to the diode, the diode is closed, but still a small reverse current (drift current, thermal current) will flow through it, due to the movement of non-basic carriers.

Section III: Electric breakdown section. If a sufficiently large voltage is applied, minority carriers will accelerate and impact ionization occurs when colliding with the nodes of the crystal lattice, which in turn leads to avalanche breakdown (as a result of which the current sharply increases)

The electrical breakdown is reversible; after the voltage is removed, the P-N junction is restored.

Section IV: Thermal breakdown section. The current increases, therefore, the power increases, which leads to heating of the diode and it burns out.

Following an electrical breakdown, a thermal one follows very quickly, therefore, diodes do not work during an electrical breakdown. Thermal breakdown is irreversible.

Current-voltage characteristic of an ideal diode (valve)

The main parameters of semiconductor devices:

1. Maximum permissible average forward current for the period (I PR. SR.)

This is the current that the diode is capable of passing in the forward direction.

The value of the permissible average over the period of the forward current is equal to 70% of the thermal breakdown current.

For forward current, diodes are divided into three groups:

1) Low power diodes (I PR.SR< 0,3 А)

2) Average power diodes (0.3

3) Diodes of high power (I PR.SR\u003e 10 A)

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.)

DC 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 denote - GD (1D)

Silicon diodes denote - CD (2D)

3. Repetitive impulse 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 reverse current of the diode (I max ..rev.)

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

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

5. Differential (dynamic) resistance.