This assembly is then heated to about 500 0C in an atmosphere of hydrogen. The collector pallet is about 3 times larger than the emitter pallet. It basically consists of a thin small wafer of Ge (or Si), with thin pallets of some p-type dopant typically indium alloyed on either side forming the emitter and collector. Alloy Type (or Fused Junction) Transistor | Types of Transistor on the Method of Constructionįigure 4 gives the structure of a PNP fused junction transistor. Further sometimes the entire symbol is enclosed within a circle as shown. Sometimes letter E, B and C are added as shown in figure 15 to designate emitter, base and collector respectively. Hence, in the symbol for NPN transistor, arrowhead on the emitter electrode points away from the base as shown in figure 3(b). In an NPN transistor, on the other hand, electron flow from emitter into the base and the conventional electric current flows from base into the emitter. Hence, in the symbol for PNP transistor, arrowhead on the emitter electrode points into the base as shown in figure 3 (a). The direction of the conventional electric current is also from emitter into the base. Thus, in a PNP transistor, holes flow from emitter into the base region. The direction of the arrow head gives the direction of the conventional electric current with emitter given forward bias. The emitter is identified by an arrow head placed on the inclined line. The thick horizontal line represents the base while the two inclined lines represent the emitter and the collector. Opaque paint is usually coated on the outside of glass envelope to exclude incident light.įigure 3 shows the circuit symbols for PNP and NPN transistors applicable to all types of transistors irrespective of the method of construction. The whole assembly is then etched and covered with moisture proof grease, mounted in a suitable mechanical structure and then hermetically sealed within a small glass envelope with leads passing through the foot. Similarly, leads are attached to the thin base reign. Non-rectifying electrodes are deposited at each end of the bar and wires or leads are attached to the end electrodes to form the emitter and the collector leads. Each such bar contains all the three regions and the two junctions and ultimately forms a grown junction transistor. Next each slice is cut normal to the slice surface thereby resulting in a number of semiconductor bars of rectangular sections. The continuously large crystal formed from the melt is cut into a large number of small thin constant width slices by cutting along the planes at right angles to the planes of junctions. Two junctions are formed: the emitter-to-base junction or simply the emitter junction J E and Collector-to-base junction or simply the collector junction J C. The base region is extremely thin, or less. The thin central section is called the base whereas and end sections constitute the emitter and the collector. Thus, there get formed a very thin layer of n-type semiconductor sandwiched between p-type materials on either side. Later on, the type of impurity is changed to n-type by adding to the melt, n-type impurity in sufficient large quantity and after a very small-time interval, again the impurity is changed to p-type by adding sufficient p-type impurity. Such a transistor is formed by first growing a Ge (or Si) crystal of p-type from a melt of Ge (or Si). Grown Junction Transistor | Types of Transistor on the Method of Constructionįigure 1 and 2 show respectively the three- and two-dimensional views of basic grown junction PNP transistor. Alloy type of Fused Junction Transistors.Type of Transistors: Depending on the method of construction transistors may be classified in following five categories: Types of Transistor on the Method of Construction An NPN transistor, on the other hand, consists of a silicon (or germanium) crystal in which a thin layer of p-type silicon is sandwiched between two layers of n-type silicon. A PNP transistor consists of a crystal of silicon (or germanium) in which a thin layer of n-type silicon is sandwiched between two layers of p-type silicon. A junction transistor may be of PNP type or NPN type and may be made of germanium or silicon.
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