I - V characteristic of a diode: application of characteristics for finding complex faults of semiconductor elements

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

General information

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

D consists only of a p-n-junction with leads, which are 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 (-), a negative space charge is generated, which interferes with the free movement of E. E, which have reached the anode, form an anode current, and those that have not reached - a cathodic one. If the anode and cathodic currents are equal to zero, D is in the closed state.

Semiconductor device

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

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

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

Applications

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

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

Volt-ampere characteristics

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

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

I - V characteristics differ from each other and it depends on the type of semiconductor device. The I - V curve is a curve with the values ​​of the straight line I marked along the vertical (top). Below are the values ​​of I when connected back. Horizontally, the U readings are indicated for direct and reverse switching on. The scheme consists of 2 parts:

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

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

Plotting a graph

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

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

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

The I – V 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 - I - V characteristic of Schottky.

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

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

Figure 4 - I - V characteristic of silicon D.

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

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

Figure 5 - I - V characteristic of rectifier D.

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

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

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

There are also additional characteristics. They are needed for the application of D in a certain scheme with U and I. If you use a low-power D in devices with U exceeding the maximum allowable Urev, then there will be breakdown and failure of the element, and this can also lead to a chain of exit of other parts from building.

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

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

Possible malfunctions

According to statistics, D or other semiconductor elements fail more often than other elements of the circuit. The faulty item can be calculated and replaced, but sometimes this leads to 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 the failure of other elements and ending with fire or electric shock current. Major faults include:

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

During breakdown, D does not pass current in one direction. There may be several reasons and they arise with sharp increases in I and U, which are unacceptable values ​​for a certain D. The main types of pn junction breakdowns:

1. Thermal.
2. Electric.

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

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

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

If the leg has disappeared, then it 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 a device.