Bipolar Junction Transistors

Transcription

1 ipolar Juctio Trasistors ipolar juctio trasistor (JT) was iveted i 948 at ell Telephoe Laboratories Sice 97, the high desity ad low power advatage of the MOS techology steadily eroded the JT s early domiace. JTs are still preferred i some high frequecy ad aalog applicatios because of their high speed ad high power output. Questio: What is the meaig of bipolar? The term bipolar refers to the fact that both electros ad holes are ivolved i the operatio of a JT. fact, miority carrier diffusio plays the leadig role just as i the P juctio diode. The word juctio refers to the fact that P juctios are critical to the operatio of the JT (ipolar) Trasistors (bipolar) trasistor = combiatio of two diodes that share middle portio, called base of trasistor; other two sectios: emitter'' ad collector ; usually, base is very thi ad lightly doped. Two kids of bipolar trasistors: pp ad p trasistors pp meas emitter is p type, base is type, ad collector is p type material; i ormal operatio of pp trasistor, apply positive voltage to emitter, egative voltage to collector; p pp

2 . troductio to the JT A JT is made of a heavily doped type emitter, a P type base, ad a type collector. This device is a P JT. (A PP JT would have a P+ emitter, type base, ad P type collector.) P trasistors exhibit higher trascoductace ad speed tha PP trasistors because the electro mobility is larger tha the hole mobility. forward reverse Whe the base emitter juctio is forward biased, electros are ijected ito the more lightly doped base. They diffuse across the base to the reverse biased base collector juctio ad get swept ito the collector. This produces a collector curret,. is idepedet of V as log as V is a reverse bias. Rather, is determied by the rate of electro ijectio from the emitter ito the base, i.e., determied by V. Similar to the P diode theory, the rate of ijectio is proportioal to exp(qv KT). (ipolar) Trasistors or pp(p), if emitter base juctio is forward biased, holes (electros) flow from battery ito emitter, move ito base; some holes(electro) aihilate with electros(hole)s i type(ptype) base. ecause base is thi ad lightly doped, most holes(electros)makeitthroughbaseito collector, holes(electros) move through collector ito egative (positive) termial of battery. Accordigly, collector curret flows whose size depeds o how may holes(electros) have bee captured by electros(holes) i the base. This depeds o the umber of type(p type) carriers i the base which ca be cotrolled by the size of the curret (the base curret ) that is allowed to flow from the base to the emitter. The base curret is usually very small; small chages i the base curret ca cause a big differece i the collector curret. curret holes curret electros forward reverse forward reverse ( 참고 ) 베이스공통회로 ommo ase

3 The emitter is ofte coected to groud. Therefore, the curve is usually plotted agaist V. or V higher tha about.3 V, ig.(b) is with a shift to the right because V = V + V elow V.3 V, the base collector juctio is strogly forward biased ad decreases. ommo-mitter ofiguratio ecause of the parasitic R drops, it is difficult to accurately ascertai the true base emitter juctio voltage. or this reaso, the easily measurable base curret,, is commoly used as the variable parameter i lieu of V Questio: Why is ofte preferred as a parameter over V?. ollector urret The collector curret is the output curret of a JT. Applyig the electro diffusio equatio to the base regio d dx L where, L () t : base recombiatio lifetime : base miority carrier (electro) diffusio costat The boudary coditios are () qv ( e ) qv ( W ) ) () where = i, ad is the base dopig cocetratio. V is ormally a forward bias (positive value) ad V is a reverse bias (egative value). The solutio of q. () is ( x) qv W x sih L ) sih W L (3)

4 . ollector urret Moder JTs have base widths of about. μm. This is much smaller tha the typical diffusio legth of tes of micros. the case of W << L, q. (3) reduces to a straight lie W x sih ( x) ()( x W ) qv kt L ( x) ( e ) i qv sihw L )( x W ) Where i is the itrisic carrier cocetratio of the base material. (4) The miority carrier curret is domiated by the diffusio curret. The sig of is positive. A q d dx A q W i qv ) (5) A is the emitter area of the JT. otice the similarity betwee q. (5) ad the P diode V relatio. oth are proportioal to ad to i. fact, the oly differece is that d'dx has produced the W term i q. (5) due to the liear ' profile. quatio (5) ca be codesed to S A q qv i ) where S is the saturatio curret. quatio (5) ca be rewritte as qv ) W i (6) i p dx (s cm 4 ) is the base ummel umber where p is the majority carrier cocetratio i the base. 8.3 ase urret Wheever the base emitter juctio is forward biased, some holes are ijected from the P type base ito the + emitter. These holes are provided by the base curret,. is a udesirable but ievitable side effect of producig by forward biasig the juctio. At a ideal ohmic cotact such as the cotact of the emitter, the equilibrium coditio holds ad p' = similar to q. (). Aalogous to q. (6), the base curret ca be expressed as A W q i i i qv dx ) (7) where i, (emitter dopig cocetratio) ad are ot fuctios of x A q W i qv ) (8) (b) s a large desirable? Why?

5 8.4 urret ai How ca be maximized? i i W W Perhaps the most importat parameter of a JT is its commo emitter curret gai, β. Aother curret ratio, the commo base curret gai, is defied by α is typically very close to uity, such as.99, because β is large. is a load o the iput sigal source, a udesirable side effect of forward biasig the juctio. should be miimized (i.e., β should be maximized). (9) () () () (6)(7) (9) XAMPL: urret ai A JT has = ma ad = ma. What are, b ad a? Solutio:.99 ma.ma ma μa. ma μa ma We ca cofirm ad

6 (ipolar) Trasistors Amplifier of base curret : small chages i base curret cause big chages i collector curret. Switch: if voltage applied to base is such that emitter base juctio is reverse biased, o curret flows through trasistor, i.e., trasistor is off. Therefore, a trasistor ca be used as a voltage cotrolled switch. : 베이스공통단락회로증폭인자 (commo base short circuit amplificatio factor) : =.95 ~.99 (always, < ) ad ac ac V costat : 이미터공통순방향전류증폭률 (commo emitter forwarurret amplificatio factor) : = ~ or larger. V costat curret coservatio ( ) & O, O ( ) haracteristics of ipolar Trasistors V R V R V V Saturatio regio Active regio reakdow regio utoff regio ase mitter Voltage(forward): V.7V VoltageatR :V R =V V Ohm s law for R :V R = R the, R =V V =(V V )R or, ollector mitter voltage V V V R Ohm s law for R :V R = R the, V =V R (where, = β ) ollector ase voltage : V =V V