Capacity of the junction
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Properties of junctions
P.N. |
The diode with point (or with
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Created it, 05/10/15
Update it, 06/01/14
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SEMICONDUCTORS 3 “6th PART”
Figure 9 represents the typical curve characteristic of a diode with junction. By examining it attentively, one notices that the curve always passes by the axes, i.e. in the absence of tension applied at the boundaries of the diode, the power is not on any more.
The left part of the characteristic, corresponding to the opposite polarization of the diode, shows that by increasing the negative tension applied, the current becomes very weak, even if one arrives at tensions of several volts. Indeed, the characteristic always remains with the same distance from the horizontal axis (figure 9).
Taking into account this, one can thus think that the opposite tension can increase without limitation since the current, having reached the limit of saturation (corresponding to the concentration of the minority carriers) does not increase any more. Actually, the power is constant only up to one well defined point of the tension, after which it starts to increase very quickly, to such a degree that the opposite characteristic is on from the almost horizontal position to the driving position (figure 10).
One thus notes a considerable increase in the minority carriers and consequently, the reverse current becomes much more intense.
One calls this phenomenon ZENER effect, according to the name of the scientist who discovered it and succeeds in giving the explanation of it.
The tension to which the phenomenon appears is known as tension of ZENER (Vz of figure 10).
To explain the ZENER effect, it is necessary to recall that when a diode is polarized in reverse, we have a reverse current, made up of minority carriers, which had with the formation of couples electron-positron pairs, caused by the rupture of the bonds existing between the atoms of the semiconductor.
The rupture of the bonds is caused by the vibration of the atoms under the effect of the temperature. There also exists of other causes likely to generate the formation of a couple electron-positron pair.
By studying junction P.N we saw that near this junction, it is formed a zone of free loads (zone of exhaustion), to which corresponds a potential difference, called barrier of potential.
Since this zone is excessively mean, the fact of increasing the opposite biasing of the diode causes an increase in the potential difference close to the junction. This increase reaches such a proportion (Vz = tension of ZENER), which it generates a force able to move away from their orbit a great number of outer-shell electrons, pertaining to the atoms being near the junction.
However, for each electron which moves away from its orbit, it appears a couple electron-positron pair, therefore a considerable increase in the minority carriers.
The reverse current becomes relatively intense then.
The diodes designed to benefit from this phenomenon are called zener diodes (figure 11). In those, there is also another phenomenon called the avalanche effect.
The avalanche effect occurs when the minority carriers present near the junction accelerate at a point such, that they run up against the outer-shell electrons of the atoms, at a sufficient speed to detach them from their orbit.
The free loads thus obtained are accelerated and produce in their turn (always by shocks) of new loads and so on. Thus, the number of free loads increase very quickly, from where the name of effect of avalanche given to this phenomenon.
The loads made free in this manner make amplify the current due to the ZENER effect. That means that according to the tension of ZENER (Vz), the reverse current passes very quickly from a value reduced to a remarkably high value.
Figure 12 illustrates this phenomenon very well. It indeed represents the pace of the opposite characteristic of a zener diode.
As one can note it, the reverse current is initially so small (a few microamperes) that the characteristic is almost on the horizontal axis, of which it is detached then rather abruptly to practically pass in a vertical state. By looking at the almost vertical part of the characteristic, tending to show that the tension remains constant although the current varies within broad limits, one understands that one can use the zener diodes as stabilizers of tension.
Indeed, after a suitable doping of the semiconductors, it is possible to vary within broad limits, the tension for which one checks the Zener effect. With such processes, it is possible to manufacture zener diodes, able to stabilize tensions of a different nature, energy of a few volts until several tens of volts.
Figure 13 represents some types of zener diodes usually used.
5. - CAPACITY
OF THE JUNCTION
A junction P.N has a resistance depending on the type of polarization which is applied to him :
- In direct polarity, this resistance has a low value.
- In opposite polarity, it is very large (put aside the zener diodes where the effect of avalanche was caused).
An attentive examination of a junction shows the similarity with a condenser. Indeed, if we consider a nonpolarized junction P.N, three parts are detached (figure 14) :
A material containing of the live loads can be compared to a driver whereas another, deprived of these same loads, behaves like an insulator. Thus, the junction appears made up of two parts out of metal (A and C) separated by another insulating (B), similar to dielectric.
It is known that two metal plates separated by an insulator (dielectric) constitute a condenser and the junction of a diode in is an example.
So now, one applies to the junction an opposite polarization, the plates A and B are respectively made negative and positive; the barrier of potential increases and the capacity is some modified (figure 15-a).
Thus, according to the value of the opposite tension applied to the diode, the intrinsic capacity of this one varies in some limit (figure 15-b). The equivalent diagram of the diode in reverse is given figure 15-c.
A variable capacity in parallel on a resistance of very great value to simulate the escape of current (a few microamperes) of the dielectric one.
The property of the junction in reverse made it possible to carry out circuits checking automatic of frequency of oscillators (for example, oscillating buildings of receivers FM or TV).
Figure 16 represents two types of diodes with variable capacity (VARICAP) of which some with silicon offer a capacity active of a few picofarads to a hundred picofarads approximately according to the opposite tension applied at the boundaries.
Rectifier the diodes to silicon, the most used, can also be used in variable capacity; parallel resistance (very large) does not disturb the oscillating circuits.
In direct polarization, the capacity of the diode “is masked” by the resistance which is low in this case. The capacity of the diode can play a harmful part in the assemblies at very high frequencies where it is used for its properties of rectification. Indeed, its reactance can become very weak and thus offer a short-circuit to the opposite tension. Its employment is thus to proscribe in these assemblies.
6. - PROPERTIES
OF JUNCTION P.N.According to the type of doping carried out in a junction P.N., the use of the diode is quite specific :
7. - DIODE
A POINTS (OR A CRYSTAL)Conceived before the diode with junction, one uses it for one-way conduction in direct polarization. It is composed of a specific contact, on the level of the semiconductor pastille, by a very fine tungsten wire. One used, at the time, to detect the electromagnetic waves (waves radios for example), a detector with crystal crystal, consisted a piece of lead sulfide ore (crystal) on which was posed what one called the “moustache of the cat”. Figure 17 illustrates the manufacture of a diode with point of which the provision of the various elements hardly changes that already seen for the diode with junction.
After closing with the drinking glass holder (figure 17-a and 17-b) one makes pass, during a short moment, a current of very great value through the tungsten point and the silicon or germanium pastille doped N, in order to create, by fusion at the point of contact of these two elements, a final junction (figure 17-c).
The pace of the characteristic of the diode with point is thus appreciably the same one as that of the diode with junction as shown in the figure 18.
However, some differences are noticed : in direct polarization (dial 1, figure 18), for the same current in the two components, the tension applied at the boundaries of the diode with point is higher than that of the diode with junction. All in all, the diode with point has a direct resistance higher than the diode to junction.
In opposite polarization (dial 3, figure 18), the current (Ii) increases with the tension (Vi) and does not present, as for the diode with junction, a stage characterized by a current reverses practically constant when the tension (Vi) varies up to a maximum value where the effect of breakdown occurs.
The shape of the curve in reverse for the diode with point rises from the heating created in the microphone-junction (tungsten point, semiconductor), involving the increase in the minority carriers responsible for the reverse current.
This diode, having a very low capacity of microphone-junction, is indicated for the assemblies demodulators of high frequencies.
It should be noted that in this type of diode, the ZENER effect does not exist and the property to rectify is better with a semiconductor of the type N. Therefore the diodes with point are made of a semiconductor doped N on which the microphone-junction with the tungsten point is established.
After having spoken about the diode, we will start, in the next lesson (semiconductor 4), the examination of the transistor where its application in the electronic assemblies is diversified.
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