Parting 

Which graph corresponds to the current-voltage characteristic of a semiconductor. Features of the current-voltage characteristics of rectifier diodes. Semiconductor diodes and their characteristics

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 average rectified current values ​​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 the 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.

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 the same 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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To control the direction of the electric current, it is necessary to use different radio and electrical parts. In particular, modern electronics uses a semiconductor diode for this purpose, its use ensures a smooth current.

Device

A semiconductor electrical diode or diode valve is a device that is made of semiconductor materials (usually silicon) and operates only with a one-way flow of charged particles. The main component is a crystalline part, with a p-n junction, which is connected to two electrical contacts. Vacuum diode tubes have two electrodes: a plate (anode) and a heated cathode.

Photo - semiconductor diode

To create semiconductor diodes, germanium and selenium are used, as they were more than 100 years ago. Their structure allows the use of parts to improve electronic circuits, convert AC and DC to unidirectional pulsating, and to improve various devices. On the diagram, it looks like this:


 Photo - diode designation

There are different types of semiconductor diodes, their classification depends on the material, principle of operation and field of use: zener diodes, pulsed, alloy, point, varicaps, laser and other types. Quite often, analogues of bridges are used - these are planar and polycrystalline rectifiers. Their message is also made with the help of two contacts.

The main advantages of a semiconductor diode:

  1. Complete interchangeability;
  2. Excellent throughput parameters;
  3. Availability. They can be bought at any electrical goods store or removed for free from old circuits. The price starts from 50 rubles. In our stores, both domestic brands (KD102, KD103, etc.) and foreign ones are presented.

Marking

The marking of a semiconductor diode is an abbreviation for the main parameters of the device. For example, KD196V is a silicon diode with a breakdown voltage of up to 0.3 V, a voltage of 9.6, a model of the third development.

Based on this:

  1. The first letter identifies the material from which the device is made;
  2. Device name;
  3. The number that determines the purpose;
  4. Device voltage;
  5. A number that defines other parameters (depends on the type of part).

Video: the use of diodes

Principle of operation

Semiconductor or rectifier diodes have a fairly simple principle of operation. As we have already said, the diode is made of silicon in such a way that one of its ends is p-type and the other end is n-type. This means that both contacts have different characteristics. One has an excess of electrons, while the other has an excess of holes. Naturally, there is a region in the device in which all the electrons fill certain gaps. This means that there are no external charges. Due to the fact that this region is depleted of charge carriers and is known as the unifying region.


 Photo - the principle of operation

Despite the fact that the connecting section is very small (often its size is a few thousandths of a millimeter), the current cannot flow in it in the usual way. If a voltage is applied such that the p-type area becomes positive and the n-type area, respectively, negative, the holes go to the negative pole and help the electrons pass through the pooling area. In the same way, electrons move to the positive contact and, as it were, bypass the unifying one. Despite the fact that all particles move with different charges in different directions, in the end they form a unidirectional current, which helps to rectify the signal and prevent voltage surges at the diode contacts.

If voltage is applied to a semiconductor diode in the opposite direction, no current will flow through it. The reason is that the holes are attracted by the negative potential, which is in the p-type region. Similarly, electrons are attracted to a positive potential that is applied to the n-type region. This causes the merging area to increase in size, making the flow of directional particles impossible.


 Photo - characteristics of semiconductors

IV-characteristics

The current-voltage characteristic of a semiconductor diode depends on the material from which it is made and some parameters. For example, an ideal semiconductor rectifier or diode has the following parameters:

  1. Direct connection resistance - 0 ohm;
  2. Thermal potential - VG \u003d + -0.1 V .;
  3. In the straight section, RD > rD, i.e., the direct resistance is greater than the differential one.

If all parameters match, then the following graph is obtained:


 Photo - CVC of an ideal diode

Such a diode is used in digital electrical engineering, the laser industry, and it is also used in the development of medical equipment. It is necessary for high demands on logic functions. Examples are laser diode, photodiode.

In practice, these parameters are very different from the real ones. Many devices are simply not capable of working with such high accuracy, or such requirements are not needed. The equivalent circuit characteristic of a real semiconductor demonstrates that it has serious drawbacks:


 Photo - CVC in a real semiconductor diode

This IV characteristic of a semiconductor diode indicates that during direct switching, the contacts must reach the maximum voltage. Then the semiconductor will open to the passage of electronic charged particles. These properties also demonstrate that the current will flow normally and without interruption. But until all parameters are matched, the diode does not conduct current. At the same time, for a silicon rectifier, the voltage varies within 0.7, and for a germanium one - 0.3 Volts.

The operation of the device is very dependent on the level of maximum forward current that can pass through the diode. On the diagram, it is defined by ID_MAX. The device is designed in such a way that when switched on in a direct way, it can only withstand an electric current of limited strength. Otherwise, the rectifier will overheat and burn out like a normal LED. Various types of devices are used to control temperature. Naturally, some of them affect the conductivity, but they prolong the performance of the diode.

Another disadvantage is that when passing AC current, the diode is not an ideal isolating device. It only works in one direction, but leakage current must always be taken into account. Its formula depends on the remaining parameters of the diode used. Most often, schemes designate it as I OP. A study by independent experts found that germanium passes up to 200 µA, and silicon up to 30 µA. At the same time, many imported models are limited to a leakage of 0.5 µA.


 Photo - domestic diodes

All types of diodes are susceptible to voltage breakdown. This is a property of the network, which is characterized by limited voltage. Any stabilizing device must withstand it (zener diode, transistor, thyristor, diode bridge and capacitor). When the external potential difference of the contacts of a rectifying semiconductor diode is significantly higher than the limited voltage, then the diode becomes a conductor, reducing the resistance to a minimum in one second. The purpose of the device does not allow it to make such sharp jumps, otherwise it will distort the CVC.

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 the voltage is applied directly (plus to the anode, minus to the cathode), and an absolute insulator with infinite resistance when the voltage is reversed.

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.


semiconductor diode — This is a semiconductor device with one p-n junction and two electrodes. The principle of operation of a semiconductor diode is based on the phenomenon of p-n junction, therefore, for further study of any semiconductor devices, you need to know how it works.

rectifier diode (also called a valve) is a type of semiconductor diode that is used to convert alternating current to direct current.

A diode has two leads (electrodes) an anode and a cathode. The anode is attached to the p layer, the cathode to the n layer. When a plus is applied to the anode, and a minus to the anode (direct connection of the diode), the diode passes current. If a minus is applied to the anode, and a plus (reverse switching on of the diode) is applied to the cathode, the current through the diode will not be visible from the current-voltage characteristic of the diode. Therefore, when an alternating voltage is supplied to the input of the rectifier diode, only one half-wave passes through it.

Volt-ampere characteristic (VAC) of the diode.

The current-voltage characteristic of the diode is shown in fig. I. 2. The first quadrant shows the direct branch of the characteristic, which describes the state of high conductivity of the diode with a forward voltage applied to it, which is linearized by a piecewise linear function

u \u003d U 0 + R D i

where: u is the voltage across the valve during the passage of current i; U 0 - threshold voltage; R d - dynamic resistance.

In the third quadrant is the reverse branch of the current-voltage characteristic, which describes the state of low conductivity when the reverse voltage is applied to the diode. In the state of low conductivity, the current through the semiconductor structure practically does not flow. However, this is only true up to a certain value of the reverse voltage. With a reverse voltage, when the electric field strength in the p-n junction reaches about 10 s V / cm, this field can impart to mobile charge carriers - electrons and holes that constantly arise throughout the entire volume of the semiconductor structure as a result of thermal generation - a kinetic energy sufficient for ionization neutral silicon atoms. The resulting holes and conduction electrons, in turn, are accelerated by the electric field of the p-n junction and also ionize neutral silicon atoms. In this case, an avalanche-like increase in the reverse current occurs, .t. e. avalanche breakdowns.

The voltage at which there is a sharp increase in reverse current, called the breakdown voltage U 3 .