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MODULE V |
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MODULE V |
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4.1 Transistors as electric components
Transistors belong to the semiconductors. Semiconductors posses special electrical
characteristics and are e.g. used in electronic controllers.
Besides the transistors other important semiconductors are diodes, thyristors
and integrated circuits (IC's).
The basic materials for semiconductor components are germanium and silicon in
pure crystalline form. Both materials only have a very limited electrical conductivity
and furthermore have an electrical 4-(quadruple) valency.
They have to be intentionally and well dosed "contaminated" in order to become
electrically conductive. The "contamination" is either done with materials that
have a chemical 3-(triple) valency like e.g. Indium - in this case one speaks
of a so called "P-doping" - or with materials that have a chemical 5-(quintuple)
valency like e.g. Antimony. In the latter case one speaks of an "N-doping".
The electrical conductivity results from an electron surplus of the N-doping
and an electron deficiency in the crystal grid of the P-doping. If one joins
the two materials and applies a voltage (negative pole at the N-semiconductor
- positive pole at the P-semiconductor) an electric current is flowing from
the N-semiconductor to the P-semiconductor caused by the movement of the electrons
from the electron surplus side to the electron deficiency side. In the case
of a reverse poling no current can flow since then there is an electron deficiency
in the P-semiconductor.
A transistor consists of three semiconductor layers and is fabricated in two
different doping sequences:
Diagram 25: Doping sequence N - P - N (= NPN-Transistor)
