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ONIT- 1 PART-A Review of Ser conductors “Inkwigsl: Semjconduckows: . Serticgnductors ase matedals whose conductivity Ves hettoeay thot of conductors, such as coppes, and insulatows such os glass. These ave tuo Kinds of seriiconductoos; single-element semiconductor such oS" gesmacium and silicon; and campound semiconductors such BS gallium. asseri de. Cornpoundl semiconductows a2e useful Fn LEDS. Gesmonium eas used in the fabsication of Veoy eaaly +hansistoac, A Sfatom hos 4 vatence electyons, thus it sequirses another + to complete ‘its outesmost shell This is achieved by shaving one of %s valence electrons with eadh of its “fous ndgh bousin toms: Each paisof shared ekchins fooma covalent bond «A rayctal of purse gw has aweguas lattce Stouctuse, whee the kutoms are held In thers position by the cavasent- honds. At sufficiently low tempesatures, appsoaching ok, all the eovatent bonds axe fntact and no electrons aze available to conduct eleckic cuwrent, Thus, at such lov ternpevatu ses; the Ratansic Secon exystal behaves as an insulator. At soom ternpesoture , sufRicient thesmal eneagy exists to break geome of the covalent bond3,a poocess krovon a tthesmal genevsation, As the electaan leaves ft parent atom, %t leoxes behind a net potitive charge, equal to the mragnituae Sf the elect¥on chatge- As tempesatuse ‘incseases, move covalent bonds ase booken and electron - hole Poss ave generate d - Fhe Yncsease 7 the number offeelectrons and holes vesulls A an ‘mesease in the conductivity of Sfl7con, “Thesmal genedation sesults in free electrons and holes fn equal numbes and hence ayucl concentrations, The Bee electrons and holes move sandoroly theough the Sil?con 
