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CVC of the diode.
(volt-voltage characteristic) - a graph of the dependence of the current through a two-terminal network on the voltage on this two-terminal network. Most often, the current-voltage characteristic of nonlinear elements is considered (the degree of nonlinearity is determined by the nonlinearity coefficient, since for linear elements the current-voltage characteristic is a straight line and is not of particular interest.
The nonlinearity of the current-voltage characteristic is due to the fact that the resistance of the NE depends on the applied voltage (diodes, zener diodes) or on the current (thermistors). The current-voltage characteristics of nonlinear elements are described by equations whose degrees are higher than the first. Since the resistance of NEs is variable, the instantaneous current value in them is not proportional to the instantaneous voltage values. (p. 117 manual)
Forward and reverse current. Forward and reverse voltage.
When the p-n junction resistance is low, a current called direct current. The larger the area of the p-n junction and the voltage of the power source, the greater this forward current. If the poles of the element are reversed, the diode will be in the closed state. A zone depleted of electrons and holes is formed; it provides very high resistance to the current. However, in this zone, a small exchange of current carriers between regions of the diode will still occur. Therefore, a current will flow through the diode, but many times less than the direct current. This current is called reverse diode current. If a diode is connected to a circuit with alternating current, it will open at positive half-cycles on the anode, freely passing a current in one direction - forward current Ipr., and close at negative half-cycles at the anode, almost without passing a current in the opposite direction - reverse current Irev. The voltage at which the diode opens and direct current flows through it is called direct(Upp.), and the voltage of reverse polarity, at which the diode closes and reverse current flows through it, is called reverse(Urev.) At forward voltage, the diode resistance good quality does not exceed several tens of ohms, but with reverse voltage its resistance will reach tens, hundreds of kilo-ohms and even mega-ohms.
Breakdown voltage.
A dielectric, being in an electric field, loses its electrical insulating properties if the field strength exceeds a certain critical value. This phenomenon is called dielectric breakdown or violation of its electrical strength. The property of a dielectric to resist breakdown is called electrical strength (Epr). The voltage at which insulation breakdown occurs is called breakdown voltage (Upr).
The simplest design in the family of semiconductors are diodes, which have only two electrodes between which conductivity exists electric current one way. This type of conductivity in semiconductors is created due to their internal structure.
Device Features
Without knowing the design features of the diode, it is impossible to understand its operating principle. The diode structure consists of two layers with different types of conductivity.
The diode consists of the following main elements:
- Frame. It is made in the form of a vacuum cylinder, the material of which can be ceramics, metal, glass and other durable materials.
- Cathode. It is located inside the balloon and serves to generate electron emission. The simplest cathode device is a thin thread that glows during operation. Modern diodes are equipped with indirectly heated electrodes, which are made in the form of metal cylinders with the property of an active layer that has the ability to emit electrons.
- Heater. This is a special element in the form of a thread that is heated by electric current. The heater is located inside an indirectly heated cathode.
- Anode. This is the second electrode of the diode, which serves to receive electrons emitted from the cathode. The anode has a positive potential compared to the cathode. The shape of the anode is most often the same as the cathode, cylindrical. Both electrodes are similar to the emitter and base of semiconductors.
- Crystal. Its material of manufacture is germanium or silicon. One part of the crystal is p-type with a lack of electrons. The other part of the crystal has n-type conductivity with an excess of electrons. The boundary located between these two parts of the crystal is called р-n junction ohm
These design features of the diode allow it to conduct current in one direction.
Operating principle
The operation of a diode is characterized by its various states, and the properties of the semiconductor when in these states. Let's take a closer look at the main types of diode connections and what processes occur inside the semiconductor.
Diodes at rest
If the diode is not connected to the circuit, then peculiar processes still occur inside it. There is an excess of electrons in the "n" region, which creates a negative potential. The positive charge is concentrated in the “p” region. Together, such charges create an electric field.
Since charges with opposite signs attract, electrons from “n” pass into “p”, filling the holes. As a result of such processes, a very weak current appears in the semiconductor, and the density of the substance in the “p” region increases to a certain value. In this case, the particles disperse uniformly throughout the volume of space, that is, slow diffusion occurs. As a result, the electrons return to the “n” region.
For many electrical devices, the direction of the current does not really matter; everything works fine. For a diode, the direction of current flow is of great importance. The main task of a diode is to pass current in one direction, which is facilitated by the p-n junction.
Reverse switching
If the diodes are connected to the power supply according to the diagram shown, then the current will not pass through the p-n junction. The positive pole of the power supply is connected to the “n” area, and the negative pole is connected to “p”. As a result, electrons from the “n” region move to the positive pole of the power supply. Holes are attracted by the negative pole. A void appears at the transition; there are no charge carriers.
As the voltage increases, holes and electrons attract more strongly, and there are no charge carriers at the junction. When the diode is turned on in reverse, no current flows.
An increase in the density of matter near the poles creates diffusion, that is, the tendency to distribute matter throughout the volume. This occurs when the power is turned off.
Reverse current
Let us recall the work of minority charge carriers. When the diode is turned off, a small amount of reverse current passes through it. It is formed from minority carriers moving in the opposite direction. This movement occurs when the power supply polarity is reversed. The reverse current is usually negligible because the number of minority carriers is very small.
As the temperature of the crystal increases, their number increases and causes an increase in the reverse current, which usually leads to damage to the junction. In order to limit the operating temperature of semiconductors, their housing is mounted on heat-removing cooling radiators.
Direct connection
Let's swap the power poles between the cathode and anode. On the "n" side, electrons will move away from the negative terminal and flow towards the junction. On the “p” side, holes that have a positive charge will be pushed away from the positive power terminal. Therefore, electrons and holes will begin to rapidly move towards each other.
Particles with different charges accumulate near the junction, and an electric field is formed between them. Electrons pass through the p-n junction and move to the “p” region. Some electrons recombine with holes, and the rest pass to the positive pole of the power supply. A forward diode current arises, which is limited by its properties. If this value is exceeded, the diode may fail.
In the direct circuit of the diode, its resistance is insignificant, in contrast to the reverse circuit. It is believed that current does not flow back through the diode. As a result, we found out that diodes operate on the principle of a valve: turn the knob to the left - water flows, to the right - no water. Therefore, they are also called semiconductor valves.
Forward and reverse voltage
When the diode opens, there is forward voltage across it. Reverse voltage is the value when the diode closes and reverse current passes through it. The diode resistance at forward voltage is very small, in contrast to reverse voltage, which increases to thousands of kOhms. This can be verified by measuring with a multimeter.
The resistance of a semiconductor crystal can vary depending on the voltage. As this value increases, the resistance decreases, and vice versa.
If diodes are used in operation with alternating current, then with a positive half-wave of the sine voltage it will be open, and with a negative half-wave it will be closed. This property of diodes is used to rectify voltage. Therefore, such devices are called rectifiers.
Diode characteristics
The characteristics of the diode are expressed by a graph that shows the dependence of current, voltage and its polarity. The vertical coordinate axis in the upper part determines the forward current, in the lower part - the reverse one.
The horizontal axis on the right indicates forward voltage, and the horizontal axis on the left indicates reverse voltage. The straight branch of the graph expresses the diode's passing current and runs close to the vertical axis, as it expresses an increase in the forward current.
The second branch of the graph shows the current when the diode is closed, and runs in parallel horizontal axis. The steeper the graph, the better the diode rectifies current. As the forward voltage increases, the current slowly increases. Having reached the jump region, its magnitude increases sharply.
The reverse branch of the graph shows that as the reverse voltage increases, the current practically does not increase. But, when reaching the boundaries acceptable standards there is a sharp jump in the reverse current. As a result, the diode will overheat and fail.
Basic parameters of diodes- this is the forward current of the diode (I pr) and the maximum reverse voltage of the diode (U rev). These are the ones you need to know if the task is to develop a new rectifier for a power source.
Forward diode current
Forward diode current can be easily calculated if the total current that the load of the new power supply will draw is known. Then, to ensure reliability, it is necessary to slightly increase this value and you will get a current for which you need to select a diode for the rectifier. For example, the power supply must withstand a current of 800 mA. Therefore, we choose a diode whose forward diode current is 1A.
Diode Reverse Voltage
Maximum diode reverse voltage- this is a parameter that depends not only on the value of the alternating voltage at the input, but also on the type of rectifier. To explain this statement, consider the following figures. They show all the basic rectifier circuits.
Rice. 1
Rice. 2
Figure 2 shows a full-wave rectifier with a midpoint output. In it, as in the previous one, diodes must be selected with a reverse voltage 3 times higher than the effective input value.
Published Date: 12/23/2017
Do you know what reverse voltage is?
Reverse voltage
Reverse voltage is a type of energy signal created when the polarity of an electrical current is reversed. This voltage often occurs when reverse polarity is applied to a diode, causing the diode to react by operating in the opposite direction. This reverse function can also create a breakdown voltage within the diode, as this often breaks the circuit to which the voltage is applied.
Reverse voltage occurs when the power signal connection source to a circuit is applied in an inverted manner. This means that the positive lead source is connected to the ground or negative conductor of the circuit and vice versa. This voltage transfer is often not intended, since most electrical diagrams unable to handle stress.
When minimum voltage is applied to a circuit or diode, it may cause the circuit or diode to operate in reverse. This may cause a reaction such as the box fan motor turning incorrectly. The element will continue to function in such cases.
When the amount of voltage applied to a circuit is too large, the signal for the receiving circuit, however, is called breakdown voltage. If the input signal that has been reversed exceeds the allowable voltage for the circuit to maintain, the circuit may be damaged beyond the rest of the usable. The point at which the circuit is damaged refers to the breakdown voltage value. This breakdown voltage has a couple of other names, reverse peak voltage or reverse breakdown voltage.
Reverse voltage can cause breakdown voltage, which also affects the operation of other circuit components. Beyond the damaging diodes and reverse voltage circuit functions, it can also become a reverse voltage peak. In such cases, the circuit cannot contain the amount of input power from the signal that has been reversed, and may create a breakdown voltage between the insulators.
This breakdown voltage, which can occur across circuit components, can cause breakdown of components or wire insulators. This can turn them into signal conductors and damage the circuit by conducting voltage to different parts of the circuit that should not receive it, causing instability throughout the circuit. This can cause voltage arcs from component to component, which can also be powerful enough to ignite various circuit components and cause a fire.
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What is forward and reverse voltage? I'm trying to understand the principle of operation of a field-effect transistor. and got the best answer
Answer from Vovik[active]
Direct - a plus is applied to a plus, a minus is applied to a minus. The opposite is true - to a plus - a minus, to a minus - a plus.
In relation to a field-effect transistor - between the source and the gate.
A bipolar transistor has a base and an emitter, not a field effect transistor.
A bipolar transistor consists of two back-to-back r-n transition and with one common output - emitter - base (common type) - collector, like two diodes, only the common “layer” is thin and conducts current if you apply a direct voltage, which is called opening, between the emitter and the base.
The greater the forward voltage between the base and emitter, the more open the transistor is and the lower its emitter-collector resistance, i.e., there is an inverse relationship between the emitter-base voltage and the resistance of the bipolar transistor.
If a reverse voltage is applied between the base and emitter, the transistor will turn off completely and will not conduct current.
If you apply voltage only to the base and emitter or base and collector, you get a regular diode.
The field-effect transistor is designed somewhat differently. There are also three terminals, but they are called drain, source and gate. There is only one pn junction, gate -> drain-source or gate<- сток-исток в зависимости от полярности транзистора. Затвор находится между истоком и стоком и к нему (измеряется относительно истока) всегда прикладывается только обратное напряжение, которое создаёт поле в промежутке между истоком и стоком, в зависимости от напряжённости больше или меньше препятствующее движению электронов (следовательно, изменяя сопротивление транзистора) , и, таким образом, создающую обратную зависимость между напряжением исток-затвор и сопротивлением полевого транзистора.
Answer from ALEX R[guru]
On the 1st question, direct and reverse direc- tion occurs in a semiconductor (diode), i.e., the diode passes current in the direct direction, but if the current flows in the opposite direction, everything is closed. For clarity, the nipple of a bicycle tire goes there, there is no way back. Field tr-r, just for the sake of understanding, there is no electronic connection between the gate and the drain-source, but the current passes due to the evil field created at the gate. Something like that.
Answer from Alexander Egorov[guru]
direct - minus to the region with n-conductivity, plus to the region k with p-conductivity
the opposite is the opposite
by supplying only the emitter and collector, no current will pass, since the ionized atoms of the base will repel the free charges of the emitter from the pn junction (which are already difficult to jump over the pn junction, since it is a dielectric). And if you apply voltage to the base, it will “suck” free charges from the base and they will no longer repel the emitter charges, preventing them from crossing the pn junction. The transistor will open.
By the way, the emitter, collector and base are not a field-effect transistor, but a bipolar transistor.
If you apply voltage only to the base and emitter or base and collector, then it will be a simple diode (each pn junction is a diode).
Answer from User user[guru]
The field-effect transistor has a p or n type field-controlled channel. transistor terminals gate drain source
