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

On fig. 2.9 presents volt- ampere characteristic silicon rectifier diode at different ambient temperature.

Maximum allowable forward currents of silicon planar diodes various types are 0.1 ... 1600 A. The voltage drop across the diodes at these currents usually does not exceed 1.5 V. With increasing 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 reverse current saturation section, since reverse current in silicon diodes is caused by the process of generation of charge carriers in p–n-transition. The breakdown of silicon diodes has an avalanche character. 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 -60 ... +125 C. The lower limit of operating temperatures is due to the difference in the temperature coefficients of linear expansion of various elements of the diode design: at low temperatures, mechanical stresses arise that can lead to crystal cracking. With a decrease in temperature, it is also necessary to take into account the increase in the direct 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 the 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.

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

The forward voltage on a germanium diode at the maximum allowable forward current is almost two times less than on a silicon diode. This is due to the lower height of the potential barrier of 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 mechanism of formation of a reverse current - the process of extraction of minor charge carriers.

The reverse current density in germanium diodes is much higher, because 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 ​​of this voltage are less than the breakdown voltages of silicon diodes.

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

An essential feature of germanium diodes and their disadvantage is that they do not withstand even very short-term impulse overloads with reverse bias. p–n-transition. This is determined by the breakdown mechanism - thermal breakdown that occurs when the current is laced with the release of a large 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 such a comparison, it can be seen that rectifier diodes with a larger band gap have significant advantages in properties and parameters. One such representative is gallium arsenide.

At present, commercially produced gallium arsenide rectifier diodes are still far from optimally possible. For example, an AD112A diode has a maximum allowable forward current of 300 mA at a forward voltage of 3 V. A large forward voltage is a disadvantage of all rectifier diodes, p–n- transitions of which are formed in a material with a wide bandgap. 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 has a large concentration of defects due to the imperfection of the technology.

The advantages of gallium arsenide rectifier diodes are a large operating temperature range and better frequency properties. The upper operating temperature limit for AD112A diodes is 250 C. AD110A gallium arsenide diodes 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.

Conclusions:

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

2. Silicon diodes have very low 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 much higher reverse voltages than germanium diodes. The maximum allowable direct 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, at present, rectifier diodes are mainly made on the basis of silicon.

Wah-wah-wah ... Usually these words are used when telling jokes about Caucasians))) Caucasians, please do not be offended - I respect the Caucasus. But, as they say, you can’t throw words out of a song. And in our case, this word has a different meaning. And it's not even a word, but an abbreviation.

VAC is the volt-ampere characteristic. Well, in this section we are interested in current-voltage characteristic of a semiconductor diode.

The I–V curve of the diode is shown in fig. 6.

Rice. 6. CVC 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 direct 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 (Iobr and Uobr) displays the characteristics of the diode when it is turned back on (that is, when a "minus" is applied to the anode).

On 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 for the current and voltage axes, since they depend on the specific brand of the diode.

What do we see on the chart? Well, for starters, let's define, as for any flat coordinate system, four coordinate angles (quadrants). Let me remind you that the first quadrant is considered, which is located at the top right (that is, where we have the letters Ge and Si). Next, the quadrants are counted counterclockwise.

So, the II and IV 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 switched on in the forward direction, or, in other words, it is impossible to apply both "plus" and "minus" to one output at the same time. More precisely, it is possible, but then it will be a short circuit))). It remains to consider only two cases - direct connection of the diode And reverse diode switching.

The direct connection graph is drawn in the first quadrant. This shows that the higher the voltage, the greater the current. Moreover, up to a certain point, the voltage grows faster than the current. But then a fracture occurs, and the voltage almost does not change, and the current begins to grow. For most diodes, this break occurs in the range of 0.5 ... 1 V. It is this voltage that is said to "drop" on the diode. That is, if you connect the light bulb according to the first circuit in fig. 3, and you will have a battery voltage of 9 V, then not 9 V will fall on the bulb, 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 flow even in the forward direction.

The reversal graph is drawn in the third quadrant. From this it can be seen that in a significant area the current almost does not change, and then increases like an avalanche. What does it mean? If you turn on the light bulb according to the second circuit in fig. 3, then it will not glow, because the diode does not pass current in the opposite direction (more precisely, it passes, as can be seen on the graph, but this current is so small that the lamp will not glow). But a diode cannot hold 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 on the reverse branch of the graph) and the current will flow through the diode. That's just a "breakdown" - this is an irreversible process (for diodes). That is, such a “breakdown” will lead to the 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.

The characteristics of specific diodes always indicate the maximum reverse voltage - 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 designing 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 (at the same voltage values ​​​​at the terminals). This is due to the fact that silicon has a larger band gap and for the transition of electrons from the valence band to the conduction band, they need to impart a large additional energy.

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

• Principle of operation
• Basic device parameters
• Rectifier circuits
• Pulse 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 barrier layer. His resistance is great.

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

Types of devices, their designation

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

The principle of diode designation is an alphanumeric code:

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

The current-voltage characteristic (CVC) of a rectifier diode can be represented graphically. It can be seen from the graph that the CVC of the device is non-linear.

In the initial quadrant of the current-voltage characteristic, its direct branch reflects the highest conductivity of the device when a direct potential difference is applied to it. The reverse branch (third quadrant) of the CVC reflects the situation of low conductivity. This happens when the potential difference is reversed.

Real volt-ampere characteristics are subject to temperature. As the temperature rises, the direct potential difference decreases.

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

The CVC of silicon devices is different from that of germanium. I–V characteristics are given depending on various ambient temperatures. The reverse current of silicon devices is much less than that of germanium devices. It follows from the I–V curves that it increases with increasing temperature.

The most important property is the sharp asymmetry of the CVC. With forward bias - high conductivity, with reverse - low. It is this property that is used in rectifiers.

Analyzing instrument characteristics, it should be noted: such quantities as rectification factor, resistance, 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 direct current of the device to the reverse. This calculation is acceptable for an ideal device. The value of the rectification factor can reach several hundreds of thousands. The larger it is, the better the rectifier does its job.

Basic device parameters

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

• The highest value of the average forward current;
• The highest allowable value of the reverse voltage;
• The maximum allowable potential difference frequency for a given forward current.

Based on the maximum 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). Value more than 10 A.

There are power devices that depend on the shape, material, type of installation. The most common ones are:

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

Rectifier circuits

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

AC voltage U1 is applied to the input (Fig. a). On the right side of the graph, it is represented by a sinusoid. The state of the diode is open. A current flows through the load Rn. With a negative half-cycle, the diode is closed. Therefore, only a positive potential difference is applied to the load. On fig. its time dependence is reflected in. This potential difference is valid for one half cycle. This is where the scheme's name comes from.

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

Diodes pass only positive wave of alternating current. The disadvantage of the design is that in the 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 in the design, the efficiency will increase.

Rectifiers are widely used in various industries. A three-phase device is involved in automotive alternators. And the use of the invented alternator contributed to the reduction in 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.

Pulse devices

An impulse device is 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 counterparts in small capacities p-n transitions.

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

• Maximum impulse forward (reverse) voltages, currents;
• Forward voltage setting period;
• The recovery period of the device's reverse resistance.

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

Imported appliances

The domestic industry produces a sufficient number of devices. However, today the most demanded are imported ones. They are considered to be of higher quality.

Imported devices are widely used in the circuits of televisions and radios. They are also used to protect various devices in case of incorrect connection (wrong 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 (CVC) is a graph of the dependence of the current in the external circuit of a 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 dopant concentration leads to a decrease in the thermal current, and, consequently, to a decrease in the direct and reverse currents of the p-n junction.

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

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

Three types of breakdown:

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

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

3. Thermal breakdown (irreversible) - occurs when the semiconductor is heated and the corresponding increase in conductivity.

15. Rectifier diode: purpose, wah, basic parameters, angle

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

germanium rectifier diodes

The fabrication of germanium rectifier diodes begins with the fusing of indium into the original n-type germanium 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.

Figure 24 low power alloy diode design. 1- crystal holder; 2 - crystal; 3 - int. conclusion; 4 - insidious case; 5 - insulator; 6 - kovar tube; 7 - external output

Rice25 CVC germanium diode

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

Germanium diodes for various purposes have a rectified current value 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 transitions in silicon rectifier diodes carry out the fusing of aluminum 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 the designs of similar germanium diodes.

Semiconductors

Diodes.

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

Designation on the diagrams:

V or VD - diode designation

VS - designation of the diode assembly

V7 Anode The number after V indicates the number of the diode 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 circuit).

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

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

U el.prob. = 10 ÷1000 V – electrical breakdown voltage.

U us. = 0.3 ÷ 1 V - saturation voltage.

I a and U a - anode current and voltage.

Plot I:- working section (direct branch of the CVC)

Sections II, III, IV, - the reverse branch of the CVC (not a working section)

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

Plot III: Area of ​​electrical breakdown. If a sufficiently large voltage is applied, the minority carriers will accelerate and, upon collision with the lattice sites, impact ionization occurs, which in turn leads to avalanche breakdown (as a result of which the current increases sharply)

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

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

After electrical breakdown, thermal breakdown follows very quickly, so diodes do not work during electrical breakdown. Thermal breakdown is irreversible.

Current-voltage characteristic of an ideal diode (valve)

The main parameters of semiconductor devices:

1. The maximum allowable average forward current for the period (I PR. SR.)

This is the current that the diode is able to pass in the forward direction.

The value of the allowable average for the period of direct current is equal to 70% of the thermal breakdown current.

For forward current, diodes are divided into three groups:

1) Diodes of low power (I PR.SR< 0,3 А)

2) Medium power diodes (0.3

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

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 pr.)

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 denote - 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 ..reverse)

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.