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Created it, 05/10/15
Update it, 06/01/13
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SEMICONDUCTORS 3 “5th PART”
We now will observe the behavior of two crystals of semiconductor of the type N and P when one binds one of their end by fusion.
We will examine their properties and the applications which result from this.
We know already that the crystals of the type P contain majority holes as carrying while the crystals of the type N contain free electrons like majority carriers.
Let us see the behavior of the majority carriers when it is formed a junction between two semiconductors of the different types, i.e. between a crystal P and a crystal N (figure 1-a).
In practice, the junction is established by forming by average techniques special a zone P in a monocrystal N or conversely, a zone N in a P. monocrystal On the other hand, it is impossible to link two crystals of the different types to form a junction P.N.
However and this in order to better include/understand what occurs on the level from a junction, we will represent the latter by the union of two plates different of germanium.
When the junction is carried out, part of the free electrons of the crystal NR, under the effect of thermal agitation, starts to be spread in the crystal P and at the same time, always under the push of thermal agitation, part of the holes of the crystal P is propagated in the crystal NR.
Initially, the diffusion of the electrons and the holes is carried out with a certain regularity in the two directions. Theoretically, if one maintains this rate/rhythm for a sufficiently long time, one arrives in a final state in which the free electrons and the holes are uniformly distributed in the two crystals. Actually, the diffusion stops, well before entirely occupying the two crystals and thus, it is formed around the surface of junction only one zone (relatively thin), in which are mixed in equal quantities the free electrons and the holes.
In the final phase, we thus obtain three distinct zones :
- A zone N : constituted by the part N of the crystal, nonoccupied by the holes coming from the crystal P.
- A zone P : constituted by the part of the crystal P nonoccupied by the free electrons coming from the crystal N.
We find finally on the edges of the surface of junction, the new zone which one can call “intrinsic”, by considering that it contains an equal number of free electrons and holes (figure 1-b). One should not however not regard this new zone as rigorously intrinsic. Indeed, the distribution of the free loads is not uniform as in the intrinsic semiconductors. To qualify this phenomenon, one uses English name déplétion area, which one can translate by zone of exhaustion, recalling thus that in the zone in question, the push of diffusion of the free loads coming from the two joined together crystals, becomes exhausted.
By examining the phenomena which immediately occur after the formation of a junction P.N., we limited ourselves to up to now observe displacements of the free electrons and the holes, without taking account of the crystalline reticles in which the diffusion of the loads is carried out. Actually, the two crystalline reticles exert a great influence on the diffusion. Indeed, it is starting from the electric characteristics of the reticles that is born the largest obstacle for completion from the diffusion of the loads in the two crystals.
When the electrons which leave the crystal N enter the reticle of the crystal P and which the holes leaving the crystal P are propagated in the reticle of the crystal N, it occurs at the ends of the two reticles in contact, two new electric states. Indeed, on the end of the crystal N, it is formed an accumulation of positive electricity, due to the loss of electrons and the acquisition of holes, while at the end of the crystal P, we find an accumulation of negative electricity, due to the loss of holes and the acquisition of electrons.
The separation of the loads of opposite signs produces an electric field E, circulating of the positive end of the crystal N at the negative end of the crystal P (figure 1-b and we defer the same diagram in order to simplify you your task and especially to include/understand the process well).
This electric field acts in order to be opposed to the diffusion loads through the junction, insofar as it tends to bring back the holes of the crystal N to the crystal P and the electrons of the crystal P to the crystal N.
With the growth of the intensity of the field E and the diffusion of the loads which continues, the force of recall of the loads also increases. Thus, the push of the diffusion will find opposition more and more, until balance between the opposite forces is reached, then causing the suspension of any diffusion of the loads in the two crystals.
With balance established between the force of the electric field and the push of diffusion, it is formed also a separation between a certain quantity of positive and negative loads on the two edges of the zone of exhaustion. Consequently, at the edges of the latter, a certain potential difference (Vo of the figure 1-b) remains constant and for this reason one calls it usually but improperly, potential of contact or potential of diffusion.
In the future, when we speak about this potential difference Vo, we will use the denomination barrier of potential. This term, more correct than the precedent, points out the obstacle which the Vo potential represents, for the later diffusion of the electric charges from one crystal to another.
It is thus of this barrier of Vo potential which is formed on the level of junction P.N that all the properties depend on the crystal diode.
2. - POLARIZATION
OF THE JUNCTION
After having examined the formation of a junction between two semiconductors of the different types, let us see the behavior of this one now when it is polarized, i.e. when the tension delivered by a source of continuous food is applied to the two zones of the crystal.
The tension can be applied in the two directions i.e., by connecting the positive pole of the pile to the zone N and the negative pole at the zone P or conversely, by connecting the positive one to the zone P and the negative one at the zone N.
In the first case, one says that the junction (i.e. the diode) is polarized in reverse while in the second case, the diode is on line polarized.
What does it occur on the level from junction P.N when this one is polarized in reverse ?
Figure 2 shows that at the moment when is completed, part of the free electrons is detached from the zone N of the crystal and moves towards the positive pole of the battery of food.
At the same moment, a certain quantity of electrons emitted by the negative one of the battery, joined the zone P of the crystal, where they will make disappear part of the holes.
So now we admit that in the zone P there are no free electrons which can join the zone N to replace those which are pushed back towards positive pile and which in the zone N there are not holes which can be propagated to the zone P in order to replace those which disappeared, we could show the suspension from the movement of the loads circulating from the crystal to the battery and the battery to the crystal. Indeed, the number of the free electrons present in the zone N of the crystal is incontestably very large, but nonunlimited ; it is the same for the holes present in the P. crystal.
The cancellation of the current produced by the pile, at once after the closing of the circuit, is justified by the fact that the electrons and the holes are in a number limited in one or the other of the crystal and by impossibility of replacing them when they move away and that they disappear.
In reality, the displacement of the loads and consequently, the current produced by the pile, ceases before even as the zone N was not released from its electrons and the zone P of its holes.
To explain this phenomenon, let us know that the barrier of potential is reinforced quickly with the reduction in the free electrons and by the holes in the respective zones and its amplitude increases while passing from Vo with Vo' (compare on this subject the figures 1-b and 2).
The new potential difference Vo' can thus cancel the effect of the external tension Vi, before all the electrons of the zone N are not pushed back towards positive pile and before all the holes of the zone P did not disappear.
Tension Vi applied at the boundaries of the diode (figure 2) is known as opposite tension. If account is held of what was known as previously, the current circulating in the diode (at the boundaries of which one applied a tension reverses) should be cancelled quickly. Actually, the current is not cancelled completely because of presence of the minority carriers, i.e. of the presence of holes in the zone N of the crystal and of free electrons in the P. zone.
A certain number of minority carriers always succeeds in crossing the junction, thus causing a replacement partial of the free electrons in the zone N and of the holes in the P. zone One thus notes the presence of a very weak current, circulating of the end N at the end P of the crystal. This current is called current opposite (li).
Let us see the opposite phenomenon now, i.e. when junction P.N is on line polarized (figure 3-a).
When is completed, the electromotive force of the battery puts moving the free electrons of the zone N and the holes of the zone P, which converge both towards the junction (figure 3-a), inside which the electrons fall into the holes, which involves the disappearance of the ones and others. However, the free electrons which fall into the holes, are continuously replaced by others, coming from negative from the source from food.
Thus, all the disappeared holes are replaced by others, which are formed side of the zone P, towards the positive one of the battery. The flow of the loads thus reproduces perpetually, forming a D.C. current. Besides one notes it by measuring the direct resistance of the diode.
The Id D.C. current is known as forward current, the external tension, Vd which is at the origin of the formation of the Id current, is known as direct tension.
The figure 3-b illustrates the case where the tension continues Vd is lower than the potential difference Vo (figure 1-b), which constitutes the barrier of potential. Thus, as long as the Vd tension is lower or equal to Vo, the current is practically null. This current exists practically only when the Vd tension exceeds the value of Vo. This value is different according to whether the junction is consisted by a germanium crystal or a silicon crystal : for germanium, this value is normally 0,2 to 0,3 V whereas for silicon it is 0,6 to 0,7 V.
A junction P.N allows the passage of a current when this one traverses the semiconductor in the direction of the crystal doped P towards that doped N. It is opposed to the circulation of a current in the opposite direction.
3. - DIODE
A JUNCTIONJunction P.N out of germanium or silicon can be used to carry out a device called diode whose graphic symbol is represented figure 4. The conduction of the diode is materialized by the direction of the arrow.
The anode (A) corresponds to the zone P of the junction and cathode (K) at the zone N ; end “A” requires a positive tension compared to the other end “K”.
The junction is obtained by the installation, on a pastille of semiconductor N, of a certain quantity of aluminum (figure 5-a) or indium; one heats the unit in order to obtain the fusion of aluminum or indium, and fusion partial of the semiconductor (figure 5-b). After cooling, these bodies are solidified forming a zone P for aluminum and junction P.N in the pastille N (figure 5-c).
The whole is then introduced into a tube of glass (figure 5-d) and the driver is welded with aluminum or indium (figure 5-d on the right). One closes again the tube of glass to form the case of the diode (figure 5th). There also exists of other manufactoring processes of the diodes. For example, to obtain junction P.N. one can refer to the method of the diffusion which consists in evaporating impurities so that they penetrate in the pastille P in order to form a zone N.
The diffusion is used in the manufacture of the diodes with silicon which can support great tensions and strong currents. Figure 6 illustrates some types of diodes with semiconductors. The cathode of the diodes of the figures 6-a and 6th is indicated by a ring or a point of color on the case of the component.
On the figures 6-c and 6-d, we have two other types of diode to silicon playing the part of rectifier. These rectifier diodes are on these figures out of metal case or plastic case.
In spite of their reduced dimensions, certain diodes provide high currents (more than 10 amps) and even manage to function correctly at a very high ambient temperature (150°C).
The diode of the figure 6-c, very much used in the food of radio operator receivers and T.V., can be fixed on the frame of an apparatus by means of nuts and discs as that of the figure 6-f employed for the very high powers where a heat sink is necessary.
The electric characteristics of the component, given by the manufacturer, are valid only for one given ambient temperature bus if the latter varies, the values of the diode change appreciably.
To obtain the curve characteristic tension-current of a diode, one uses two electronic assemblies:
- The first (figure 7-a) makes it possible to obtain the direct characteristic of the diode. For that, using a potentiometer which one varies, one applies a direct tension Vd, measured by the voltmeter (V) and one records the corresponding values of the forward current Id on the milliammeter (mA).
- The second (figure 7-b) allows to obtain the opposite characteristic of the diode. For that, the assembly remains the same one with the only difference as the pile and the two apparatus measurings is connected in opposite direction. It should be noted that the use of a microammeter (µA) facilitates measurements of reverse currents (Ii) weak.
Each couple of measured values (tension and current) can be deferred on a graph having two graduated orthogonal axes (figure 8), the first horizontal one in volts, the second vertical one in milliamperes.
By taking as example the couple of values : Vd = 1,5 V and Id = 5 mA, one obtains on the graph a point A corresponding to the intersection of the two lines into dotted leaving the place perpendicularly even where these values are carried.
By point by point deferring each couple of values obtained in the corresponding dials on the graph (tensions and forward currents in the dial “1”, tensions and reverse currents in the dial “3” and by connecting by a feature all these points, one obtains the curve characteristic of a type of diodes (figure 8).
One can notice that the curve passes by the origin of the axes (point 0) ; a null tension involves the absence of current.
In the dial “1”, one observes that the current and the tension increase or decrease together.
For each type of diode, the manufacturer fixes a maximum value (Idmax) not to exceed without risking to damage it. He also indicates the repetitive current of point that the diode can support during short moment with intervals of given times and the nonrepetitive current of point that the diode receives once from time to time, for example during a powering.
These two last values are higher than that of the current (Idmax) which accounts for 20 to 30 times minus the value of a nonrepetitive current of point.
In the dial “3”, one notes that the reverse current (Ii) is practically constant and that it does not depend on the opposite tension (Vi) when this one varies on a certain beach. The opposite tension of a diode can reach a few hundreds of volts up to a limiting value (Vimax) imposed by the manufacturer beyond whom the diode is destroyed.
As long as this value is not reached, the reverse current of the diode remains weak and consequently it cannot occur a breakdown of the junction of the diode.
In short, the manufacturer provides the following characteristics :
A diode of power which provides high a Idmax current cannot have a current reverses IR as weak as that which supports only a few tens of milliamperes and however quality is not therefore affected. It is wise to know at which temperature is the junction of the diode when one determines the value of the current reverses IR because this last results from the concentration of the minority carriers which are sensitive to the thermal variations.
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