Bipolar Junction Transistors(BJTs)Lecture 2107th May 2013
Brief Introduction of Transistor• Invented by a team of– Three men at Bell Laboratories in 1947.– Although it was not a BJT but it was the beginningof a technological revolution that is stillcontinuing.– Almost all the electronic devices and systemstoday are an outgrowth of early developments insemiconductor transistors.
Two Basic Types• Bipolar Junction Transistor (BJT)• Field-Effect Transistor (FET)• Applications of BJTs– Used in two broad areas• As a linear amplifier to boost or amplify an electricalsignal• As an electronic switch
Transistor Structure• BJT is constructed with three dopedsemiconductor regions separated by two pnjunction.• The three regions are called emitter, base andcollector.• One type consists of two n-regions separatedby a p region (npn) and the other type consistsof two p regions separated by an n region(pnp).
Junctions in BJTs• The pn junction joining the base region andthe emitter region is called the base-emitterjunction.• The pn junction joining the base region andthe collector region is called the base-collectorjunction.
EBC - Terminals• A wire lead connects to each of the three regions.•• These leads are labeled E, B and C for Emitter, Base andCollector respectively.• The base region is lightly doped and very thincompared to the heavily doped emitter and themoderately doped collector regions.– (Assignment Qs, Why is it so?)– The term bipolar refers to the use of both holes andelectrons as carriers in the transistor structure.
Transistor operation•The proper bias arrangement for both npn and pnp transistors for activeoperation as an amplifier is shown above.•In both cases the base-emitter (BE) junction is forward-biased and the base-collector (BC) junction is reverse-biased.
Working …• The forward bias from base to emitter narrows the BE depletionregion, and the reverse bias from base to collector widens the BCdepletion region, as depicted in figure.• The heavily doped n type emitter region is teeming withconduction-band (free) electrons that easily diffuse through theforward-biased BE junction into the p-type base region where theybecome minority carriers.• The base region is lightly doped and very thin so that it has alimited number of holes. Thus, only a small percentage of all theelectrons flowing through the BE junction can combine with theavailable holes in the base. These relatively few recombinedelectrons flow out of the base lead as valence electrons causingsmall base electron current as shown above.
Working …• Once in this region electrons are pulled through the reversebiased BC junction by the electric field set up by the forceof at- traction between the positive and negative ions.Actually you can think of the electrons as being pulledacross the reverse-biased BC junction by the attraction ofthe collector supply voltage.• The electrons now move through the collector region, outthrough the collector lead, and into the positive terminal ofthe collector voltage source. This forms the collectorelectron current, as shown in figure above. The collectorcurrent is much larger than the base current. This is thereason transistors exhibit current gain.