Transistors - An introduction

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TRANSISTORS

The transistor is an arrangement of semiconductor materials that share common physical boundaries. Materials most commonly used are silicon, gallium-arsenide, and germanium, into which impurities have been introduced by a process called "doping." In n -type semiconductors the impurities or dopants result in an excess of electrons, or negative charges; in p -type semiconductors the dopants lead to a deficiency of electrons and therefore an excess of positive charge carriers or "holes."

The Junction Transistor

The n-p-n junction transistor consists of two n -type semiconductors (called the emitter and collector) separated by a thin layer of p -type semiconductor (called the base). The transistor action is such that if the electric potentials on the segments are properly determined, a small current between the base and emitter connections results in a large current between the emitter and collector connections, thus producing current amplification. Some circuits are designed to use the transistor as a switching device; current in the base-emitter junction creates a low-resistance path between the collector and emitter. The p-n-p junction transistor, consisting of a thin layer of n -type semiconductor lying between two p -type semiconductors, works in the same manner, except that all polarities are reversed.

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TRANSISTORS

Transistors can be regarded as a type of switch, as can many electronic components. They are used in a variety of circuits and you will find that it is rare that a circuit built in a school Technology Department does not contain at least one transistor. They are central to electronics and there are two main types; NPN and PNP. Most circuits tend to use NPN. There are hundreds of transistors which work at different voltages but all of them fall into these two categories.
			TWO EXAMPLES OF DIFFERENT SHAPES OF TRANSISTOR
Transistors are manufactured in different shapes but they have three leads (legs). The BASE - which is the lead responsible for activating the transistor. The COLLECTOR - which is the positive lead. The EMITTER - which is the negative lead. The diagram below shows the symbol of an NPN transistor. They are not always set out as shown in the diagrams to the left and right, although the ‘tab’ on the type shown to the left is usually next to the ‘emitter’.
The leads on a transistor may not always be in this arrangement. When buying a transistor, directions will normally state clearly which lead is the BASE, EMITTER or COLLECTOR.
		SIMPLE USE OF A TRANSISTOR DIAGRAM 'A' DIAGRAM 'B' Diagram 'A' shows an NPN transistor which is often used as a type of switch. A small current or voltage at the base allows a larger voltage to flow through the other two leads (from the collector to the emitter). The circuit shown in diagram B is based on an NPN transistor. When the switch is pressed a current passes through the resistor into the base of the transistor. The transistor then allows current to flow from the +9 volts to the 0vs, and the lamp comes on. The transistor has to receive a voltage at its ‘base’ and until this happens the lamp does not light. The resistor is present to protect the transistor as they can be damaged easily by too high a voltage / current. Transistors are an essential component in many circuits and are sometimes used to amplify a signal. http://www.citycollegiate.com/electronicsXb.htm The construction and working of transistor TRANSISTORS   A three terminal semiconductor electronic device is called transistor. Transistors are widely used in electronic appliances such as computers, radio, audio video equipment, bio medical instrument etc. CONSTRUCTION   A transistor is a three layer semiconductor which consist a very thin central layer of one type of semiconductor material sandwiched between two relatively thick layer of second type. TYPES OF TRANSISTORS     pnp-TRANSISTORS   In this type of transistors n-type semi-condutcor piece is sandwiched between two piece of p-type semiconductor layers. npn-TRANSISTORS   In this type of transistors p-type semi conductor piece is sandwiched between two piece of n-type semiconductor layers. ESSENTIAL PARTS OF TRANSISTORS   There are three essentials parts of a transistor  Base: It is the central layer denoted by b.  Emitter: It is the outer layer denoted by e.  Collector: It is the outer layer denoted by c. WORKING   Consider any one of the transistors for example a pnp-transistor. Let the two p-end are connected to two batteries as shown in the diagram. The forward bias causes the holes in the p-type emitter to flow towards the base which constituent Ie current. These holes cross into the n-type base, they try to combine with electrons but the base is lightly doped and is very thin. Therefore only few holes combine with electrons and the remaining holes cross into the collector and generates collector current Ic. In this way almost the entire emitter current flows in the collector circuit. From the above description it is clear that: Ie = Ib + Ic Thus there are two current paths through a transistor. One is the base-emitter path or input and the other is the collector-emitter path or output.