The bipolar junction transistor (or BJT) was invented at Bell Laboratories by William Shockley in 1948, the year after he, John Bardeen, and Walter Brattain invented the first working transistor (for which they were awarded the 1956 Nobel Prize in physics). It is constructed from a sandwich of three layers of doped semiconductor material, the thin middle layer being doped oppositely from the other two. Thus there exist two types of BJT: NPN and the PNP, whose schematic symbols are shown at right. The three layers are called the emitter, base, and collector, and their identification with the three schematic device terminals is also illustrated in the figure (note that the emitter is associated with the arrow in the schematic symbols).
The base is the thin middle layer, and it forms one PN junction with the heavily-doped emitter and another with the lightly-doped collector. When used as an amplifier, the base-emitter junction is forward-biased,whereas the collector-base junction is reverse-biased (in the case of an NPN transistor, the collector would be at the most positive voltage and the emitter at the most negative). The resultant charge carrier flows within the NPN transistor.
When the base-emitter PN junction is forward-biased, a current flows into the base and out of the emitter, because this pair of terminals behaves like a typical PN junction diode. The voltage drop from base to emitter is thus that of a typical semiconductor diode, or about 0.6–0.7V (nearly all transistors use silicon as their semiconductor). If the collector’s potential is set more than a few 1/10ths of a volt higher than that of the base, however, an interesting effect occurs: nearly all of the majority charge carriers from the emitter which enter the base continue right on through it and into the collector!
The charge carriers from the emitter that enter the base become minority carriers once they arrive in the base’s semiconductor material. Because the base is thin and the emitter has a large concentration of charge carriers (it is heavily doped), these many carriers from the emitter can approach the collector-base PN junction before they have a chance to recombine with the relatively few base majority charge carriers (holes in the case of an NPN transistor). Since the collector-base PN junction is reverse-biased, most of the charge carriers originally from the emitter can accelerate on through the base-collector junction and enter the collector, where they intermingle with the collector’s lower concentration of similar charge carriers.
As a result, most of the emitter’s charge carriers which enter the forward-biased base-emitter PN junction wind up passing through the base and entering the collector. This charge carrier flow out of the emitter is what comprises the current that flows through the emitter’s device terminal on the transistor (IE). The small fraction of these that recombine in the base then determines the current flow through the base terminal (IB), whereas the much larger fraction passing through the base and entering the collector determines the collector current (IC). If the base-emitter PN junction is not forward-biased, then majority carrier flow from the emitter across the junction does not occur, and the collector-emitter current vanishes (the only currents through the device are now the tiny reverse leakage currents from emitter and collector into the base). EI BICI
The end result is that if we bias the transistor so that its base-emitter PN junction is forward-biased (base about 0.7V more positive than the emitter for an NPN transistor), then a small current flow in the base terminal can stimulate a much larger current flow in the collector terminal: the BJT transistor is a current amplifier (of course, the collector terminal must be connected to a power supply of some sort to complete an external circuit between collector and emitter. The power supply provides the energy required to move the current around this circuit).Transistor (bipolar)-combination of two diodesthat share middle portion, called “base” oftransistor; other two sections: “emitter'' and “collector”.
Usually, base is very thin and lightly doped. Two kinds of bipolar transistors: PnP and NPN transistors“Pnp” means emitter is p-type, base is n-type, and collector is p-type material,in normal operation of PnP transistor, applypositive voltage to emitter, negative voltage to collector. If emitter-base junction is forward biased, “holes flow” from battery into emitter, move into base and some holes annihilate with electrons in n-type base, but base thin and lightly doped ⇒ most holes make it through base into collector.
Holes move through collector into negative terminal of battery i.e. “collector current” flows whose size depends on how many holes have been captured by electrons in the base.This depends on the number of n-type carriers in the base which can be controlled by the size of the current (the “base current”) that is allowed to flow from the base to the emitter; the base current is usually very small; small changes in the base current can cause a big difference in the collector current.
Transistor operation: Transistor acts as amplifier of base current, sincesmall changes in base current cause big changesin collector current. Transistor as switch: if voltage applied to base is suchthat emitter-base junction is reverse-biased, nocurrent flows through transistor (transistor is “off”).Therefore, a transistor can be used as a voltage controlledswitch, computers use transistors in thisway.
Constrution: Transistors face the same conflicting design requirements (i.e. large off state blocking voltage and large on state current density) as that of a diode. Therefore, it is only natural to extend some of the constructional features of diodes to BJT. A BJT has a vertically oriented alternating layer of n type and p type semiconductor materials. The vertical structure is preferred for power transistors because it maximizes the cross sectional area through which the on state current flows. Thus, on state resistance and power lass is minimized.
In order to maintain a large current gain “β” (and hence reduce base drive current) the emitter doping density is made several orders of magnitude higher than the base region. The thickness of the base region is also made as small as possible. In order to block large voltage during “OFF” state a lightly doped “collector drift region” is introduced between the moderately doped base region and the heavily doped collector region. The function of this drift region is similar to that in a Diode. However, the doping density donation of the base region being “moderate” the depletion region does penetrate considerably into the base. Therefore, the width of the base region in a transistor can’t be made as small as that in a signal level transistor. This comparatively larger base width has adverse effect on the current gain (β) of a transistor which typically varies within 5-20. As will be discusses later the collector drift region has significant effect on the output characteristics of a Power BJT.
Practical transistors have their emitters and bases interleaved as narrow fingers. This is necessary to prevent current crowdingand consequent second break down. In addition multiple emitter structure also reduces parasitic ohmic resistance in the base current path.
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