Section C2: BJT Structure and Operational Modes

Transcription

1 Secton 2: JT Structure and Operatonal Modes Recall that the semconductor dode s smply a pn juncton. Dependng on how the juncton s based, current may easly flow between the dode termnals (forward bas, v D > V ON ) or the current s essentally zero (reverse bas, v D < V ON ). Keepng ths n mnd, we are gong to ntroduce the bpolar juncton transstor as nothng more than two dodes placed back to back wth the center regon common to both dodes. The JT comes n two flavors the npn and the pnp the schematc representaton and crcut symbols for whch are llustrated n Fgure 4.1(a) of your text and reproduced below. As you can see, the dode sandwch that creates the JT results n three dstnct regons, each of whch s connected to the outsde world (hence the three-termnal desgnaton) and are labeled emtter (), base (), and collector (). ach of these regons has a specfc purpose and general desgn consderatons as follows: The emtter s a medum-szed, heavly doped regon whose prmary purpose s to nject ts majorty carrers (electrons for n-type, holes for p-type), through the base and nto the collector. The collector s a thck, lghtly doped regon desgned to collect the majorty carrers njected from the emtter. The base s a thn, medum doped layer whose prmary purpose s to provde the control of the current between the emtter and collector. Notce that the base s of the opposte materal type from the collector and emtter. Ths property s what allows the current control of the JT through the njecton of mnorty carrers nto the base regon (Note: What s majorty n the emtter s mnorty n the base.). The nvolvement of both carrer types, electrons and holes, n transstor operaton s what gves the bpolar juncton transstor ts name. As a prevew of comng attractons, ths may be contrasted wth the feld-effect transstor (FT), whch s a unpolar devce n that t utlzes a sngle carrer type ether electrons or holes for operaton.

2 The operatonal mode of the JT depends on how the junctons between the regons are based. Snce there are two junctons, the emtter-base juncton (J) and the collector-base juncton (J), and each of these junctons may be ether forward- or reverse-based, there are four possble modes of operaton. Mode J as J as Actve (or Normal Actve) Forward Reverse utoff Reverse Reverse Saturaton Forward Forward Inverted Actve Reverse Forward In our studes, we are gong to be lookng at three of these modes: The normal actve mode s used when the JT s employed as an amplfer n analog applcatons and s what we wll be concentratng on n ths course sequence. A combnaton of cutoff and saturaton modes are used n logc applcatons and other requrements that nvolve devce swtchng that may be addressed n other courses or electves you may choose to nvestgate. For our purposes (analog small-sgnal amplfcaton), cutoff and saturaton wll be dscussed n terms of ntroduced nonlnearetes.e., regons we want to stay away from! NOT: In the followng dscussons, we wll be concentratng on the npn JT. The concepts and dervatons for the pnp JT are complmentary, whch smply means that the devce behaves the same as the npn wth the followng changes: The materal types are nterchanged (.e., n-to-p and p-to-n). The majorty and mnorty carrer types are nterchanged (npn: electrons are majorty carrers and holes are mnorty carrers; pnp: holes are majorty carrers and electrons are mnorty carrers). The bas voltage polartes are reversed and the current drectons are swtched. The npn JT n the Normal Actve Mode As ndcated n the table above, the normal actve mode occurs when the J s forward based and the J s reverse based. Recall from our dscusson of the pn juncton that a forward based juncton has a lowered potental barrer, whle the barrer for a reverse based juncton s much

3 larger than equlbrum. Ths s sketched n the fgure below, where the (a) shows the potental hll dagram assocated wth an unbased JT and (b) s representatve of the potental hll dagram of a JT n normal actve mode. In both of these fgures, the juncton depleton regons are shown n yellow. For the J, the depleton regon becomes smaller for the appled forward bas and for the J, the depleton regon s larger wth reverse bas. Note that the unbased depleton regons are representatve of the relatve dopng between the two regons of a juncton. Recall that the lower the dopng, the greater the extent of the depleton regon n the materal. So the fgure below s n agreement wth the earler dscusson of JT regons;.e., the dopng n the emtter s greater than the dopng n the base, and the dopng n the base s much greater than the dopng n the collector. To lose the multtudnous words and slap ths stuff n representatve form: N > N ; N >> N. D A A D an you see t? The current flow of the npn JT s shown n Fgure 4.2(a) and s reproduced below n a modfed form to agree wth the fgure above. Keep n mnd that we re talkng about an npn JT so the majorty of the current s gong to be due to electrons. However, conventonal current drecton assumes postve charge movement and that s why the and current drectons look counterntutve (The carrers move n

4 opposte drectons, but they also have opposte charge remember from our current flow dscussons n Secton A5?). NOT: In the followng, only the dffuson current component s consdered. Under normal actve mode bas condtons, the drft current due to thermally generated mnorty carrers s essentally neglgble. omponents of, and contrbutons to, the total devce current may be descrbed as: Under forward bas (V > 0), majorty carrers (electrons) are pushed away from the negatve termnal of V over the lowered potental barrer and njected nto the base regon. Snce the J s forward based, the depleton regon s relatvely narrow and most of the carrers njected from the emtter wll dffuse across the J nto the base regon. Once n the base regon, there are two possbltes for the njected majorty carrers. 1. They may make t through the base regon nto the J depleton regon, attracted by the postve potental of the collector connecton. Snce the base regon s very thn, ths s ndeed what happens to most of the carrers njected from the emtter. 2. A small fracton of the njected majorty carrers are lost to the base. There are two ways to look at ths: the carrers are attracted to the postve termnal of V and are lost through dffuson or, the carrers njected nto the base from the emtter (electrons) are annhlated by the free carrers n the base (holes) through recombnaton. However t

5 s more comfortable for you to look at t, ths porton of the majorty carrers are lost and no longer partcpate n the current flow of the devce. Ths s where the control comes n manpulaton of ths loss effect determnes the amount of charge (and therefore the current) that makes t from the emtter to the collector! The majorty carrers that make t to the J depleton regon (opton 1 above) are rapdly swept through the depleton regon and nto the collector contact. Ths may also be looked at as f the electrons fall down the very large potental hll created by the J reverse bas as shown n Fgure 4.1(b) of your text or as sketched n the fgure above. Lookng at the above fgure, and keepng n mnd that even though electrons and holes are shown movng n opposte drectons, they both contrbute to postve current (defned n the fgure as nto the devce) snce they carry opposte charge, we can defne the currents for the npn JT as follows: I O : leakage current across the J, due to reverse bas of the juncton. : base current due to postve base potental of V (ths accounts for the holes that are njected across the forward based J and the recombnaton losses n opton 2 above). : collector current due to majorty carrers that made t through the base regon. : emtter current that can be expressed by KL at the top node as: = + (quaton 4.1) Snce the devce currents depend on several factors, AND we re gong to have to have models for the transstor n order to desgn and analyze crcuts, the nterrelatonshps that occur n the transstor wll be modeled by dependent sources. To establsh these dependences for devce models, several gan constants are defned. The frst of these gan constants, the common-base current gan α, s defned as the rate of change of the collector current wth the change n the emtter current wth the voltage between the collector and base held constant, or = α (quaton 4.2) v = const

6 The nternal devce current flows, as detaled n Fgure 4.2(a) or the fgure above, are smplfed and redefned n Fgure 4.2(b) (presented to the rght) n terms of the gan term α. Ideally, all of the emtter current would be collector current and α would be unty (=1). quaton 4.2 allows us to defne a proportonal relatonshp between the emtter and collector currents n terms of ths gan or, α. From Fgure 4.2(b), t may be seen that the collector current s the sum of ths contrbuton and the leakage current across the reverse based J, or = α + I α (quaton 4.3, modfed) O Practcally, the values for α usually range between 0.9 and y combnng quatons 4.1 and 4.3, solvng for, and approxmatng 1/α as equal to one, an approxmaton of the base current may be expressed as 1 α 1 α IO α α (quaton 4.5, modfed) The coeffcent of n the smplfcaton (also found by takng the partal dervatve of the frst expresson wth respect to ), s the second gan constant we wll be dscussng. y defnton, the large sgnal amplfcaton factor also known as the drect current amplfcaton factor, s gven the symbol β and s defned as the rate of change of the collector current wth the change n the base current, or: β = = α 1 α (quaton 4.6) Rewrtng the smplfed verson of quaton 4.5 and makng the assumpton that α s approxmately equal to one n quaton 4.3, we can express the relatonshps between the termnal currents as: β (quatons 4.8 & 4.9)

7 Although there s some nherent naccuracy n the above approxmatons, n most practcal crcumstances (.e., f β>>1 or, equvalently, α.1) these smple relatonshps wll be more than adequate to effectvely desgn and analyze amplfer crcuts. However and, although your author does dscuss t too much, f β s not much greater than one we must use = β β + 1 α ; = = = β = α = β + 1 β β + 1 β α β ( + ) ; ; = = 1 Nothng has changed; these are the real relatonshps between the gan constants and the devce currents. Keep ths n mnd though no approxmaton s 100% vald! Usually, we wll be dealng wth β on the order of 100 or greater. For these cases, the smplfed approxmatons are suffcent and any dscrepancy s usually lost n the round off. Prove t to yourself!.