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chapter-11-solutions - Chapter 11 Egob 11.1 Explain why two...

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Unformatted text preview: Chapter 11 Egob. 11.1 Explain why two separate 3.] Ts cannot be connected to make a p-n-p-n switch. The p—n-p-n switching action depends on injection of carriers across both base regions (m and p2 in F ig. 11'3) and collection into the base regions of the opposite transistor. Transistor action in the p-n~p feeds majority carrier holes to the base of the nwpvn, for example. This cannot occur with separate transistors, 30 the p-mp-n switching effect does not occur. Eggh. 11.2 How does gate bias provide switching in an SCH ? Switching in the SCR of Fig. 11-4 occurs when holes are supplied to pg at a sufficient rate. Although ja is forward biased with ig == 0, transistor action does not begin until hole injection by ia reaches the critical value for switching. W (a) Sketch the energy band diagrams for the p—n—p-n diode in equilibrium; in the forward-blocking state; and in the forward-conducting state. (b) Sketch the excess minority carrier distributions in regions n; and pg when the p-n-p-n diode is in the forward-conducting state. 138 138 (a) m 5}) 5n EquiliM n: m (b) Each equivalent, transistor is in saturation. Thus the minority carrier distribuzion in each base resembles Fig. 7-1413. W W Forwardvconducting limb. ;1.4 Draw diagrams such as Fig. 7-3 far the: [award-blocking and forward- con- ducting staiss of a pun-pm. Forwardvbiocking state generation m electron flow . wwmbimfio“ 139 In the simplified diagram above we neglect minority carrier transport across each base region. Electrons generated thermally in and about jg recombine in m and j , with injected holes. Similarly, generated holes feed recombination with injected electrons in pg and jg. In the absence of transistor action, I is limited to essentially the reverse saturation current of jz. In the figure be low we neglect. generation compared with transport clue to transistor action. Recombination takes place in m and p2, but many injected carriers are trans- ported through the device by transistor action. More complete diagrams can be found in the book by Gentry e! al‘ (Chapter 11 reading list), p. 72 and 76. Forward-conducting state 3‘1 .52 is W Include avalanche in jg in the coupled transistor model. Referring to Fig. 11—2, ic1 3: (111:1le "l- [cm 1”,, icz = agiMn +1cr321lln i=i01 + 2'02 = “01M; + 012Mn) + ICOlMp + 1022M” 1 = ICOIMp + IC02M1: 1 ~(01Mp + 02M“) The current. becomes large as alM, + eng,l approaches unity‘ 140 140 ...
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