Created it, 06/03/17
Update it, 06/03/20
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2. - THE DIODE
We described it in the preceding paragraph. It is about a junction. (See theory of the semiconductors).
It consists of two materials, one of conductibility N, the other of P conductibility.
With these materials two electrodes are associated, by ohmic contact (i.e. without creation of a parasitic junction).
One connected to the material P, takes the name of anode, the other with the material N, that of cathode.
The figure 4-a represents the diode as we schematized until now.
The figure 4-b represents the symbol of this diode as you will meet it from now on in the diagrams of electronics.
With what is used the diode ? We noted that, according to direction's of polarization, it had a different behavior. Polarized in reverse, it circulates there a negligible current (due to the minority carriers), while, on line polarized, an important current (due to the majority carriers) can cross it. In short, it lets pass the current only in one direction.
If one were to make an analogy with the fluidic one, one would compare it to a non-return valve.
2. 1. - CHARACTERISTICS OF THE DIODE
Figure 5 represents on the same graphics the complete characteristic of the diode.
Zone 1 corresponds to polarization in the direction passing or direct polarization.
The point located by VF, corresponds to the prevalence of the electric field E, which had with the biasing, on the electric field e, of the barrier of potential and allowing the important passage of the free electrons of the zone N towards the P zone.
The axis of the current represents forward current ID or running of the majority carriers.
The axis of the tensions represents the direct tension applied to the diode.
Zones 2 and 3 correspond to polarization in the blocked direction, or opposite polarization.
Zone 2 is the opposite characteristic usable for a rectifier diode. In this case, one should not reach the tension of avalanche VBR.
The reverse current is due to the minority carriers. In theory in this zone, at fixed temperature, it is theoretically constant, but secondary phenomena tend to increase this one slightly.
Zone 3 is marked by point VBR on the axis of the tensions opposite. It corresponds to an energy transmitted to the minority carrying electrons by the electric field E (generated by the biasing reverses), increased electric field e (generated by the barrier of potential), such as, those, while arriving in the zone N, tear off electrons in the grid of the network, which in their turn, are accelerated by this field and will release from other electrons of the grid before reaching cathode (electrode of the zone N).
This phenomenon is cumulative, which explains this very important increase in the current, for a negligible increase in the opposite tension.
In a rectifier diode, not provided to function in this area, this effect is accompanied by a strong increase of temperature, that dimensions of the diode cannot evacuate. There is, in this case, destruction of the junction. This is why this point takes the name of : tension of reverse breakdown (VBR = breakdown voltage = breaking stress).
2. 2. - ACTION OF THE TEMPERATURE
For any increase in temperature, the donors of the zone N, will lose their surplus electron more easily. Consequently, to make circulate the free electrons of the zone N towards the zone P, the electric field E, generated by the direct biasing, could be weaker.
The biasing will be thus less low (figure 6).
The curve I is relocated into II where it is noticed that VF' < VF for a temperature T' > T.
The remainder of the curve practically does not change. The tension of decreasing threshold, one can interpret that like an improvement of the diode.
In part 2, the increase in temperature supports in a significant way creation of the minority carriers, consequently, an increase in the current IR.
This action is more awkward in this case because it deteriorates the unilateral phenomenon of conduction.
This phenomenon is accentuated even more with germanium, to such a degree that one can use this defect to detect variations in temperature.
The diode about which we spoke until now is that known as of rectification (for important intensities IF) or of signal (when the current IF remains lower than 100 mA).
All the rectifier diodes are produced with silicon as well as most of the diodes of signal. However, one still uses some which is carried out starting from germanium, they are known as : with points. The junction is made differently. The table of figure 6 bis gives the principal characteristics, of a diode with germanium with point, of a diode with silicon with point and of a diode silicon to junction.
One observes, in this table, for virtually identical tensions opposite, reverse currents very different.
The diode with contact points with silicon has a current reverses 60 times weaker than in a diode carried out with germanium. As for the diode with junction, with silicon, its reverse current is 1200 times weaker, but in this case, it is necessary to take into account a technological improvement because, the latter is of more recent realization.
One can also note that this reverse current is 2 times higher for the diode 30 P1 when the temperature passes from 25° C to 55° C (DT of 30° C).
For the diode 17 P2, the reverse current is multiplied by a factor 200 when the temperature passes from 25° C to 150° C (DT = 125° C). With this temperature of 150° C, this current remains comparable with that observed in a diode with point with germanium with 55° C. A note that this diode would not tolerate a temperature of 150° C.
For the diode with junction with silicon, there is an increase in this same current of a factor 2000 for a difference in temperature of 125°. This one remains lower than the reverse current of the diode germanium to 55° C.
Do not be reproduced on this table, the variation of tension VF according to the temperature, because this kind of diode is used for the treatment of the signals (diode of signal). The average current of use being of 50 mA, for the diode 30 P1, tension VF passes from 0,58 V to 0,55 V.
For the diode 1N914B (or 1N4148), tension VF passes from 0,9 V to 0,75 V (for IF = 50 mA) when the temperature passes from 25° C to 150° C.
With this value of current, one is still far from the allowed maximum value ; this is why in this use, the variation of VF is not alarming.
To finish, will know that certain diodes support tensions opposite of several thousands of volts (VRM = 40 000 V ; IF = 3 A ; VF = 44 V).
Others are planned for forward currents several thousands of amps (VRM = 2 000 V ; IF = 2 000 A ; VF = 1,35 V ; IF during 10 ms = 15 000 A).
2. 3. - DIODES WORKED OUT FROM REMARKABLE PHENOMENA OF THE JUNCTION
a) The diode varicap
We know that the barrier of potential widens when the junction is polarized in reverse. This variation is proportional to tension VR applied. If one compares this thickness of the barrier to dielectric of a condenser, one can say :
- that the more isolated the electrodes are,
the lower the value of the condenser is (on equal surface and identical material).
In the case of the junction, plus the opposite tension will increase, plus the barrier of potential widens and more its capacity will decrease.
This corresponds for a value of VR of 30 V, for example, with a capacity of about 3 pF.
If VR is equal to 3 V, this same capacity takes the value of 30 pF.
This phenomenon is made profitable with the diodes varicap (variation of capacity).
In theory, this variation of capacity can go from a few picofarads to a thousand of picofarads.
In practice, it is limited of a few picofarads to about fifty picofarads. One can reach a ratio of 6 between the minimal value and the maximum value.
One uses these diodes in the systems of electronic agreement (circuits PLL with oscillator ordered by tension) or the systems multiplier of frequency.
b) The zener diode
The tension of avalanche, that one know from now on, is made profitable in certain diodes to be used as reference of tension.
This phenomenon is reversible, provided that the cumulative effect does not involve the destruction of the junction by heating.
The physicists knew to limit this heating by a suitable doping and a particular geometry of the junctions.
It should be noted that for these diodes, two effects are used :
the Zener effect for the very weak tensions (lower than 6 V)
the effect of avalanche beyond 6 V.
If one injects into such a diode a current reverses corresponding to the crossing of the threshold of avalanche, of the important variations of this current will result in a practically constant tension at the boundaries of the diode as long as one will remain beyond the tension of avalanche.
These diodes, consequently, will be used as elements of reference in the food of controlled continuous tensions.
They will be used in the clipping systems. The signal applied to these diodes is not affected as long as it does not reach the threshold or bends of Zener. Beyond that, the impedance of the diodes becoming very low, the signal is chopped.
The Zener tension of some of these diodes can reach several hundreds of volts. Others are carried out for reverse currents of several amps.
c) photodiodes - photovoltaic cells
In the semiconductors, thermal agitation is creative source of pairs electrons / holes. Another energy can lead to the same result : the light.
A diode, polarized in reverse, is the seat of a very weak current due to the minority carriers. If, in the case of this diode, one practices a window, the light which will reach the junction increases the number of minority carriers. This increase results in a rise in the reverse current.
We are in the presence of a detector of light radiation. It is the photodiode.
The reverse current and the current of conduction due to the electric field e generated by the displacement of the majority carriers.
In a junction, the distribution of the ionized atoms creates an electric field e.
They thus act as the tension of a pile would do it. If one places at the electric terminals a voltmeter whose impedance is raised enough, one can appreciate this tension.
When a light radiation comes to strike the junction, if the reverse current tends to increase, all occurs as if the creative tension of the field e became larger. This increase is highlighted by the voltmeter. If the circuit external with the junction has a sufficiently large impedance, we can note the effect of photovoltaic cell.
d) Diode LED
Polarization in the direct direction is accompanied by an emission of light.
The diffusion current of the majority carriers, results in the displacement of free electrons of the zone N into the zone P where they recombine with the holes of this zone, while entering the grid.
According to materials', these recombinations are accompanied by an emission of protons, from of which the frequency of vibration, specific to material, conduit with emissions of light of colors different. It is the principle of the diode LED (Light Emetting Diode which means diode à émission de lumière).
e) The diode tunnel
It is about a very doped semiconductor and which has a very mean junction.
They lead in reverse and on line up to a certain value of IF. Then when VF continues to increase, IF decrease. It is this beach of negative resistance which is used, inter alia, in the amplifiers UHF or in very fast commutation.
f) The Schottky diode
It consists of a semiconductor and a metal. It has a very low capacity of junction, it finds its use in the circuits at very high frequency.
g) Diode P.I.N.
It is characterized by a tension of high avalanche VBR. The capacity of the junction remains very low and parasitic resistance series is a function of the period of the signal applied. It is used for this phenomenon in the circuits of modulation in high frequency.
All these components are practically used in numerical electronics, they were even at the origin of certain complete logical systems (logic with diodes).
The photodiodes associated with diodes LED are very much used in the systems known as : opto-couplers.
Those make it possible to isolate galvanically from the circuits from which the potentials are very different.
The optical fiber associated these two elements makes it possible to transport numerical information in very disturbed or high-risk mediums (powder mills, collieries…).
We now will see the transistor effect.
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