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Semiconductor Diode Circuits

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  Diode Circuits Brief introduction to Semiconductors Conductivity [ − 1 m − 1 ]= 1  Resistivity  Note: Resistance =   L A (conductivity for Si depends on doping, Cu ~ 6E7 Sm 1 ! #in$ of a crysta% matri& of si%icon atoms (Si #as ' va%ence e%ectrons!1 )* a%% e%ectrons are +ound inva%ence +ondsno carriers avai%a+%e for conductioneg Si%icon, ermanium aretetrava%ent e%ements-or . * t#erma% f%uctuations can +rea$ e%ectrons free creatinge%ectron#o%e pairs/ot# can move t#roug#out t#e %atticeand t#erefore conduct current  Diode CircuitsEnergy %eve%s are discrete for a sing%e, iso%ated atom0n a crysta%, many atoms are in c%ose pro&imity #e energy %eve%s aresp%it ~nfo%d #e discrete %eve%s +ecome near%y continuous, it#in +ands Conduction band  : ~composed of e&cited states of t#e sing%e atoms Valence band  : ~ ground state of t#e sing%e atom Forbidden Energy Gap (Band Gap)  e&ists +eteen va%ence and conduction +ands 2(1e3! inSemiconductors #is is t#e energy re4uired to +rea$ one of t#e +onds Insulators : 5arge gap, difficu%t to to ump to conduction +andpass Semiconductors : #erma% e&citations at room temp, moderate app%ied potentia%s can +ridge gap Conductor : Conduction and va%ence +ands over%ap c#arge f%os free%y  app%ied potentia%s Intrinsic semiconductors : no impurities Extrinsic semiconductors : impurities introduced it# different va%ence t#at under%ying crysta%8  Diode Circuits Doped semiconductors o e&amp%es of doped (e&trinsic! semiconductors it# impurities of different va%ence va%uest#an t#e si%icon atoms #e addition of dopants can great%y increase t#e conductivity of t#emateria% (a%%os for adustment of distance +eteen t#e va%ence and conduction +ands!Donors put energy %eve%s ust +e%o conduction +and (in t#e gap!9cceptors put energy %eve%s ust a+ove t#e va%ence +and9 #o%e umping into t#e va%ence +and is %i$e an e%ectron umping into t#e conduction +andE%ectrons are easi%y e&cited (t#erma%%y! from E D  to t#e conduction +and for ntype materia% 0n ptype materia%, t#ey are easi%y e&cited from t#e va%ence +and to E 9   Diode Circuits P! unction C#arges diffuse due to t#erma% effects ;o%es diffuse from psideto nside, e%ectrons diffuse from nside to pside #is diffusive f%o creates an e4ui%i+rium stateE%ectron#o%e pairs created it#in t#e gap f%o opposite to t#ediffusion from < to N9t e4ui%i+rium: ∣  I  d  ∣−∣  I   g  ∣= * =  I  (no net current f%o=!define: ∣  I  d  ∣=∣  I   g  ∣=  I  * 9pp%ying a forard vo%tage (3 <  . 3  N ! to t#e diode causes c#argesto diffuse from t#e < to N nodes of t#e device  I  =  I  d  −  I   g  =  I  d  −  I  *  Boltzman factor  :  N  N  * = e&p [ −  E  −  E  *  k  T   ] ratio of c#arge carriers in #ig#er state compared to t#ose in state  E  *  E  −  E  * represents a step in potentia% energy  E  −  E  * =− qV  N  * ) >carriers at state  E  *  N  ) > carriers at state  E  current is proportiona% to c#arge carriers:  N  N  * =  I  d   I  *   I  d   I  * = e&p  qV   / kT    I  =  I  d  −  I  * =  I  * e&p  qV   / kT  −  I  * t#us e #ave t#e diode e4uation:  I  =  I  * [ e&p  qV   / kT  − 1 ] (a%most correct!9dd additiona% materia%dependent factor to #and%e various recom+ination effects  I  =  I  * [ e&p  qV   / nkT  − 1 ] '
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