Chapter 4
Semiconducting components
A semiconductor is not a component that is not directly a current conductor, but is an insulator either. In a conductor the current must to the movement of the negative loads (electrons). In the semiconductors produced currents take place so much by the electron movement like of the positive charges (hollow). The semiconductors are those elements pertaining to group IV of the Periodic table (Silicon, Germanium, etc.). To these atoms of other elements, denominated are introduced to him generally impurities, so that the current must to O-electrons to the hollows fundamentally, following the introduced impurity. Another characteristic that them difference talks about its resistivity, being this one included between the one of metals and the one of the insulators.
The semiconductors are very important in electronics since thanks to them we counted today on diverse components very useful in electronics, such as diodes, transistors, thyristors, triac, etc.
The birth of the diode arose from the necessity of transformation of AC voltages in continuous.
The current in a diode presents/displays a sense of circulation of positive charges that go from the anode to the cathode, not allowing the circulation of the current in the opposed sense, which allows the conversion us of AC voltage continuous, procedure known like rectification. This happens because around the diode it will only be able to circulate current when the anode is more positive than the cathode.
They are made up of two regions of semiconducting material that is called union P-N that is the base of all electronic component of active type. Between the two parts of union P-N, and in the zone of contact between both, a denominated region of transition takes place, where zone N to greater tension is generated a small difference of potential, since an electron recombination is satisfied, being than zone P. When it is applied to him a tension to the diode with the positive terminal connected to zone P and the negative to the N will take place a circulation of current between both because a small part of this tension will make level the difference of potential between zones, call threshold voltage, having been these made level in tension, and the rest of the applied tension will produce an electron circulation of zone N to the P.
If that external tension is applied with the interchanged tips, that is to say the positive terminal of the source connected to zone N and the negative to region P, there will be circulation of no current by the diode, because as a result of the applied tension the difference will be increased of existing potential between zones P and N, preventing therefore the circulation of current through same.
With the figure we can have an exact idea something but than it happens in the diode when we applied a tension to him, in either felt (direct and inverse polarization).
The quadrant right superior corresponds to the direct polarization, in the same we can appreciate that a tension exists (VU) from which the diode begins to lead, this tension is the threshold voltage and varies according to is the used semiconducting material in the manufacture of the diode, being of 0.7 V for silicon and 0.3 V for the germanium.
The quadrant left inferior corresponds to an inverse polarization. In her one sees that the current crosses that it (well-known as current inverse) is practically null. It notices that the values smaller than zero in the axis of the current are graduated in uA.
Nótese also that stops greater inverse polarization to VR the inverse current grows indefinitely. An inverse tension of this value or major to him damages to the diode in irreversible form and is known it like tension of rupture or to zéner.
Between the diverse classes of diodes that are in the market, we can mention the following: rectifying diodes (in individual assembly or bridge rectifier), diodes of signal, diodes of commutation, diodes of high frequency, stabilizing diodes of tension, special diodes.
It is the type of diode more used to implement electronic systems of C.C regulation.
A diode of this type works previously in the zone of seen rupture, being called to this tension, tension Zéner VZ. Obvious that the process of manufacture of these varies of the employee for the diodes common given the operation necessity in the zone of rupture. When to zéner a smaller tension to VZ is applied to him this one behaves like a normal diode.
One of the simpler practical applications of zéner is the one of regulator of a continuous tension, whose diagram is in the figure:
| Where: | Tension of entrance sees = | 9 to 12 V |
| Versus = Tension of exit | 7 V | |
| Iz = Current in zéner | 5 mA | |
| Is = Exhaust stream | 20 to 50 mA |
With the use of this circuit we can when coming out assure a maximum tension the circuit, independent of the fluctuations originated in the entrance of the same.
This circuit is very simple to implement, we will only have to do which is the value of the Rlim resistance that will be the limitor resistance that will absorb the tension difference which we want “to trim” in the entrance.
For the calculation of the same we do:
|
Where: |
It sees (min) = Tension of minimum entrance |
| VERSUS = Tension of exit | |
| Iz (min) = Current minim that circulates around the diode (Data collected from the hour of data of the manufacturer). | |
| Is (máx) = Current principle that crosses the load |
If, for example, our source of entrance varies between 9 and 12 V and we want a 7 when coming out tension of V, then Rlim will be:
Rlim £ (9 - 7)/(0.005 + 0.050) = 2/0.055 = 36.36 W
The Iz value (min we obtained) it from the data sheet of zéner.
We see that 36.36 Rlim must be smaller or equal to ohms, but exists in the market this value of resistance? As we already saw in chapter 1, when we spoke of the resistance, that all the values of resistance are not available, we only can find certain values for the resistance. But in this case we will not have to worry since for Rlim we have a level of minor or 36.36 equal to W, then it will be enough with choosing a value next to this one but without passing it. Of the table of 33 values we see that the one that comes near more is W, therefore chose this one.
We already have the ohmic value of the resistance, now needs to see what power is going to dissipate the resistance, for it we multiplied the current crosses that it by the tension that falls in her (It sees - Versus)
The current is: 60.60 Is = 2/33 = mA
then 2 P = V x 60.60 mA = 0.12 W
We choose a Rlim whose power dissipation is 1/2 W.
Another type of diode, perhaps the one of greater diffusion, is the LED, known commonly like LED (Light Emmitting Diode)
The operation of this type of diode is based on the polarization in direct sense of a union P-N. When doing this originates a recombination of electrons and hollows, which originates great amount of energy, that in the case of some semiconductors is translated in a luminous radiation.
Their typical colors are: red, green and amber those that make to the suitable LED to be used in many types of indicators. In addition their durability and to low turn them consumption into almost essential components at the time of wanting to use some type of luminous indicator.
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