Transisitores

In the section diodes we saw the union of two glazing, P and N, properly doped. Now we are going to investigate that it happens, from the electrical point of view, if we join two diodes if, that is to say if we join a set P-N with another N-P, back against back; or if we join a N-P with another P-N, in the same conditions.
Because two central sections possess same doped, they make a mistake if, so that we have left a real union that is equivalent, in the first case, to P-N-P and in the second one to N-P-N.
In 1949, someone realizing tests (these tests will be realized in special articles) realized that semiconductor of enormous possibilities was before a new device, and it was baptized with the name of extracted transistor of transfer by resister (transference resistance, in English) because the transistor offers a variable resistance.

Theory of the Transistor

Considering this realized test this device has to take three electrodes or terminals, one as each of the glazing of which it consists. To the crystal that receives the current, the first one of the three, it differs with the issuer's name; the crystal of the center like base, and to the crystal of exit of the current, collector. Then, in a transistor of type NPN, the first one N will be the issuer, P will be the base, and another N, the collector. They usually abridge these names with the letters E, B and C respectively.
To understand well the functioning of the transistor we must remember the atomic theory, where the crystal N is a crystal that has electrons excess, and the crystal P, it is a crystal with hollows excess. For example a transistor of type NPN, following the image in which a power supply (B) provides with current the issuer connected to the negative pole in the crystal N, negatively also. Under these circumstances they form like a few barriers Z1 and Z2 in the unions with the crystal P of base, which they block the way of the current. The base is full of hollows that happen to be occupied by the most next electrons of the contiguous glazing, forming these atoms barriers in balance that blocks the way of the current (except the current very weak one of escape of the most scarce value).
But if there polarizes the source of the same sign that she, that is to say, with a positive tension with regard to the issuer, what is called in the opposite direction, the barrier Z1 eliminates because the positive potential applied to the base repels the hollows towards the glazing N and they penetrate in the resistance area. The free electrons of the issuer cross it being attracted by the positive potentials of the base and of the collector. Since the positive potential of the collector is much higher than that of the base, the electrons will feel more attracted by the first one, by what will obtain a high current of the collector (that we will abridge IC) and a small current of base (IB). The current of the issuer (IE) will be therefore equal to the sum of the collector's current and the base current, as it is deduced of the Kirchhoff laws. That is to say:

IE = IC + IB

Of this it is deduced that the current that goes out for the collector is not increased by the base current. In fact, the current that passes for an issuer and that IE designates consists of the current of the base and of the collector that then they will circulate in different sense. In the image we see a scheme of elementary circuit of a transistor in which (V) designates also the name of the tensions. This way we have that VBE is the tension base - issuer, VCE is the tension collector - issuer. Since it can turn, in the issuer the base currents collector join, as the Kirchhoff law says.

 

Keep on reading on transistors:

Functioning of the Transistor.
Principal functions of the Transistor.
Code of designation of Transistors.