Whtes, EE 320 Lecture 10 Page 1 of 9 Lecture 10: Bpolar Juncton Transstor Constructon. NPN Physcal Operaton. For the remander of ths semester we wll be studyng transstors and transstor crcuts. The transstor s a three termnal devce. It s probably only the second such devce you ve encountered n electrcal engneerng, after the op amp. Roughly speakng, the transstor acts an electronc valve: the node current or voltage at one termnal controls the current enterng the second termnal and extng the thrd. It would be dffcult to overstate the mportance of the transstor to electroncs. Some of ts uses are: Dgtal logc Memory crcuts Amplfers Electronc swtches. The two basc famles of transstors are bpolar juncton transstors (BJTs) and feld effect transstors (FETs). We ll start by dscussng BJTs for approxmately 13 lectures followed by FETs for the remanng 12 lectures. BJTs are covered n Chapter 5 of your text. 2009 Keth W. Whtes
Whtes, EE 320 Lecture 10 Page 2 of 9 BJT Physcal Structure BJTs are formed from three doped regons on a slcon crystal. These can ether be npn doped regons or pnp. A smplfed sketch of an npn transstor s: (Fg. 5.1) As can be seen, the BJT s formed from two back-to-back pn junctons: Emtter-base juncton (EBJ) Collector-base juncton (CBJ). Ths specfc way of drawng the BJT has been around from the very begnnng of these transstors. Ths fgure: s from Wllam Shockley s U.S. patent 2,569,347 ssued n 1951.
Whtes, EE 320 Lecture 10 Page 3 of 9 BJTs can also be fabrcated from two p-type regons and one n- type. Ths s called a pnp transstor: (Fg. 5.2) Whle the BJT mght appear to be symmetrcal by lookng at Fg. 5.1, the actual devces are not. For example, the cross secton below of an npn transstor clearly shows that the EBJ and CBJ, for example, have very dfferently szed surface contact areas, whch wll greatly change ther relatve behavors. (Fg. 5.6) There are four basc modes of operaton for a BJT dependng on the states of the two pn junctons of the transstor:
Whtes, EE 320 Lecture 10 Page 4 of 9 Mode Emtter-Base Jct. Collector-Base Jct. Cutoff Reverse Reverse Actve Forward Reverse Saturaton Forward Forward Reverse Actve Reverse Forward In dgtal logc applcatons, the transstor swtches between the cutoff and saturaton modes. As a lnear amplfer n a communcaton crcut, the transstor would operate n the actve mode. Because of the asymmetrcal physcal constructon, the reverse actve mode s not the same as nterchangng the collector and emtter leads. The states of the two pn junctons can be altered by the external crcutry connected to the transstor. Ths s called basng the transstor. NPN Transstor n the Actve Mode We ll begn the dscusson of the BJT physcal operaton by consderng an npn transstor n the actve mode. To bas t n the actve mode, we need to forward bas the EBJ and reverse bas
Whtes, EE 320 Lecture 10 Page 5 of 9 the CBJ (notce that the emtter and collectors have swapped postons from Fg. 5.1 shown earler): (Fg. 1) The overall objectve of ths crcut s to create a current flowng from the collector to the emtter termnals n the transstor that s controlled, so to speak, by the base voltage V BB. How does ths transstor operate n ths crcut? Because of the forward bas on the EBJ, charges can flow across ths juncton gvng rse to E. Ths current s prmarly electrons that are njected from n to p. The electrons njected n the base dffuse across the thn base regon towards the collector. Some of the e - recombne n the base, but ths regon s manufactured to be thn and lghtly doped compared to the emtter so ths recombnaton s kept small. Otherwse, the BJT would just operate as two back-to-back dodes and no current would flow.
Whtes, EE 320 Lecture 10 Page 6 of 9 A representatve mnorty carrer concentraton profle s shown below n Fg. 5.4. (Note that C and E are swtched wth reference to Fg. 1 above.) (Fg. 5.4) The e - that reach the reverse-based CBJ encounter a large electrc feld. Ths E sweeps them nto the collector formng the collector current C as shown n Fg. 1 above. A small base current B s present largely due to recombnaton n the base wth the small amount of njected holes from the base to the emtter. Ths s an mportant current, though. Dscusson About BJT n the Actve Mode 1. The proporton of electrons from the emtter that make t to the base s called the collector effcency, α:
Whtes, EE 320 Lecture 10 Page 7 of 9 C α or C E E = α (5.16),(1) Typcally t has values of near 0.99. Note that α s called the common-base current gan n the text. From KCL n the crcut of Fg. 1: E = B + C (5.13),(2) = = 1 α (3) or ( ) B E C E We can deduce from (3) that wth α 1, then B wll be much, much smaller than E. 2. The rato of collector current to base current s called the current gan, β: C β or C = βb (5.10),(4) B 3. Dvdng (1) by (4) we fnd that C = α B 1 α Equatng ths to (4) we fnd α β = (5.19),(5) 1 α and solvng ths equaton for α β α = (5.17),(6) β + 1
Whtes, EE 320 Lecture 10 Page 8 of 9 Wth α 0.99 β 100. 4. One can thnk of the base current n the BJT as controllng the collector current: C = βb. Snce β s large, then a small change n B produces a large change n C. If the base were an nput sgnal and the collector the output, then ths would be sgnal amplfcaton! Awesome! 5. The crcut symbol and current conventons for the npn BJT are The arrows ndcate the assumed drectons for postve current for the npn BJT. The flled arrow s always located on the emtter and helps us to remember the drecton of the emtter current. 6. For basng n the actve mode as shown n Fg. 1, one basng crcut mght be
Whtes, EE 320 Lecture 10 Page 9 of 9 (Fg. 5.14a)