Electrical engineering - Basics Page 24

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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)

Diagram 26: Doping sequence P - N - P (= PNP-Transistor):

To each semiconductor layer of the transistor an electrode is attached: external electrodes C (Collector and E (Emitter)), internal electrode B (Basis) with which the current between the emitter and the collector can be contolled. If no control voltage is applied between the basis and the emitter, the distance between the emitter and the collector becomes current permeable since the basis current is overcoming the barrier-effect of the PN-transition and a current can flow between the collector and the emitter.

In this way, it is possible to relatively easily build electrical amplifiers with transistors that have a small electrical input wattage and a large electrical power output since the control current IB is e.g. only 1/100 of the value of the controlled current IC (and/or - IC).

Thyristors and diodes do basically function in the same way.

With thyristors e.g. controllable direct current supplies are designed (-> direct current amplifiers for drives).

IC's can e.g. contain complete amplifier circuits, digital computing modules, memory or memory modules.


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