SEMI CONDUCTOR - ELECTRONICS

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1 SM ONDUTO - TONS J-Physcs NGY NDS N SODS ased on Paul's excluson prncple n an solated atom electrons present n energy level but n sold, atoms are not solated, there s nteracton among each other, due to ths energy level spltted nto dfferent energy levels. Quantty of these dfferent energy levels depends on the quantty of nteractng atoms. Splttng of sharp and closely compact energy levels result nto energy bands. They are dscrete n nature. Order of energy levels n a band s 10 3 and ther energy dfference 10 3 e. nergy and ange of energy possessed by an electron n a sold s known as energy band. alence a nd () ange of energes possessed by valence electron s known as valence band. (a) Have bonded electrons. (b) No flow of current due to such electrons. (c) lways fulfll by electrons. onducton and () ange of energes possessed by free electron s known as conducton band. (a) t has conductng electrons. (b) urrent flows due to such electrons. (c) f conducton band s fully empty then current conducton s not possble. (d) lectrons may exst or not n t. Forbdden nergy gap (FG) (g) g ( ) mn ( ) max nergy gap between conducton band and valence band, where no free electron can exst. :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 Wdth of forbdden energy gap depends upon the nature of substance. Wdth s more, then valence electrons are strongly attached wth nucleus Wdth of forbdden energy gap s represented n e. s temperature ncreases forbdden energy gap decreases (very slghtly). 1 band energy (e) conducton band forbdden energy gap valence band SSFTON OF ONDUTOS, NSU TOS ND SMONDUTO : - On the bass of the relatve values of electrcal conductvty and energy bands the solds are broadly classfed nto three categores () onductors () Semconductors () nsulator

2 lectron nergy J-Physcs ompar son bet ween conductor, semconductor a nd nsulator : Propertes o nductor Sem conductor n s u l a t o r esstvty m m m onductvty mho/m mho/m mho/m Temp. oeffcent Postve Negatve Negatve (ery slghtly) of resstance () urrent Due to free Due to electrons No current electrons and holes nergy band dagram onducton and No gap alence and Overlappng regon lectron nergy onducton and Forbdden Gap alence and g 1ev lectron nergy onducton and Forbdden Gap 3e g alence and onductor Sem conductor nsulator Forbdden energy gap 0e 1e 3e xample : Pt, l, u, g Ge,, Gas, Wood, plastc, GaF Damond, Mca ONPT OF " HOS" N SMONDUTOS Due to external energy (temp. or radaton) when electron goes from valence band to conducton band (.e. bonded electrons becomes free) a vacancy of free e creats n valence band, hole free e whch has same charge as electron but postve. Ths postvely charged vacancy s termed as hole and shown n fgure. t s defcency of electron n. t's acts as postve charge carrer. t's effectve mass s more than electron. t's moblty s less than electron. Note : Hole acts as vrtual charge carrer, although t has no physcal sgnfcance. GODN KY PONTS Number of electrons reachng from to at temperature T kelvn where g 3/ 3 / g n T e kt T exp kt k oltzmann constant J/K constant T absolute temperature g energy gap between and n slcon at room temperature out of 10 1 atoms only one electron goes from to. n germanum at room temperature out of 10 9 Ge atoms only one electron goes from to. :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

3 J-Physcs FFT OF TMP TU ON SMONDUTO t absolute zero kelvn temperature bove absolute temperature t ths temperautre covalent bonds are very Wth ncrease n temperature few valence strong and there are no free electrons and electrons jump nto conducton band and hence semconductor behaves as perfect nsulator. t behaves as poor conductor. hole free e at 0 K alence band fully flled onducton band fully empty at hgh temperature alence band partally empty onducton band partally flled :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 FFT OF MPUTY N SMONDUTO Dopng s a method of addton of "desrable" mpurty atoms to pure semconductor to ncrease conductvty of semconductor. or Dopng s a process of delberate addton of a desrable mpurty atoms to a pure semconductor to modfy ts propertes n controlled manner. dded mpurty atoms are called dopants. The mpurty added may be 1 part per mllon (ppm). The dopant atom should take the poston of semconductor atom n the lattce. The presence of the dopant atom should not dstort the crystal lattce. The sze of the dopant atom should be almost the same as that of the crystal atom. The concentraton of dopant atoms should not be large (not more than 1% of the crystal atom). t s to be noted that the dopng of a semconductor ncreases ts electrcal conductvty to a great extent. GODN KY PONTS The concentraton of dopant atoms be very low, dopng rato s vary from mpure : pure :: 1 : 10 6 to 1 : n general t s 1 : 10 8 There are two man method of dopng. () lloy method () Dffuson method (The best) The sze of dopant atom (mpurty) should be almost the same as that of crystal atom. So that crystallne structure of sold reman unchanged. SSFTON OF SMONDUTO ntrnsc semconductor (pure form of Ge, ) n n n e h SMONDUTO xtrnsc semconductor (doped semcondutor) N-type pentavalent mpurty (P, s, Sb etc.) donar mpurty (N D) n >> n e h 3 P-type trvalent mpurty (Ga,, n, l) acceptor mpurty (N ) n >> n h e

4 J-Physcs N type semconductor When a pure semconductor ( or Ge) s doped by pentavalent mpurty (P, s, Sb, ) then four electrons out of the fve valence electrons of mpurty take part, n covalent bondng, wth four slcon atoms surroundng t and the ffth electron s set free. These mpurty atoms whch donate free e for conducton are called as Donar mpurty (N D ). Due to donar mpurty free e ncreases very much so t s called as "N" type semconductor. y donatng e mpurty atoms get postve charge and hence known as "mmoble Donar postve on". n N-type semconductor free e are called as "majorty" charge carrers and "holes" are called as "mnorty" charge carrers. thermally generated e s thermally generated hole free electrons s s donar e postve donar ons mnorty hole N-type semconductor N-type semconductng crystal P type semconductor When a pure semconductor ( or Ge) s doped by trvalent mpurty (, l, n, Ga) then outer most three electrons of the valence band of mpurty take part, n covalent bondng wth four slcon atoms surroundng t and except one electron from semconductor and make hole n semconductor. These mpurty atoms whch accept bonded e from valance band are called as cceptor mpurty (N ). Here holes ncreases very much so t s called as "P" type semconductor and mpurty ons known as "mmoble cceptor negatve on". n P-type semconductor free e are called as mnorty charge carres and holes are called as majorty charge carrers. xtra hole created by acceptor mpurty atom thermally generated e l l l P-type semconductng crystal thermally generated hole 4 negatve acceptor ons holes P-type semconductor mnorty e :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

5 J-Physcs ntrnsc Semconductor N- type (Pentavalent mpurty) P- t ype(trvalent mpurt y) 1. donor mpurty level acceptor mpurty level. free electron postve donar on hole negatve acceptor on 3. urrent due to Manly due to electrons Manly due to holes electron and hole 4. n e n h n n h << n e (N D ~ n e ) n h >> n e (N ~ n h ) 5. e + h ~ e ~ h 6. ntrely neutral ntrely neutral ntrely neutral 7. Quantty of electrons Majorty - lectrons Majorty - Holes and holes are equal Mnorty - Holes Mnorty - lectrons Mass acton aw n semconductors due to thermal effect, generaton of free e and hole takes place. part from the process of generaton, recombnaton also occurs smultaneously, n whch free e further recombne wth hole. t equlbrum rate of generaton of charge carres s equal to rate of recombnaton of charge carrer. The recombnaton occurs due to e colldng wth a hole, larger value of n e or n h, hgher s the probablty of ther recombnaton. Hence for a gven semconductor rate of recombnaton n e n h so rate of recombnaton n e n h recombnaton coeffcent, The value of remans constant for a sold, accordng to the law of thermodynamcs untl crystallne lattce structure remans same. :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 For ntrnsc semconductor n e n h n so rate of recombnaton n n e n h n n n e n h Under thermal equlbrum, the product of the concentraton 'n e ' of free electrons and the concentraton n h of holes s a constant and t s ndependent of the amount of dopng by acceptor and donor mpurtes. Thus from mass acton law n e n h n1 5

6 J-Physcs lectron- hole ecomb nat on : t s necessarly to complete a bond that electron s shared from neghbourng atoms or t may also be receved from conducton bond. n the second case electron recombnes wth the hole of valnce bond. Ths process s known as electron-hole recombnaton. The breakng of bonds or generaton of electron-hole pars, and completon of bonds due to recombnaton s takng place contnuously. t equlbrum, the rate of generaton becomes equal to the rate of recombnaton, gvng a fxed number of free electrons and holes. x.1 The energy of a photon of sodum lght ( 589 nm) equals the band gap of a semconductng materal. Fnd : (a) the mnmum energy requred to create a hole-electron par. (b) the value of kt at a temperature of 300 K. Sol. (a) hc (n e) so e 1400 ( s n e and s n Å) 5890 Å so e (b) J 3 kt x. P type semconductor has acceptor level 57 me above the valence band. What s maxmum wavelength of lght requred to create a hole? Sol. hc hc Å x. 3 slcon specmen s made nto a p-type semconductor by dopng on an average one ndum atom per slcon atoms. f the number densty of atoms n the slcon specmen s atoms/m 3 ; fnd the number of acceptor atoms n slcon per cubc centmeter. S o l. The dopng of one ndum atom n slcon semconductor wll produce one acceptor atom n p-type semconductor. nce one ndum atom has been dopped per slcon atoms, so number densty of acceptor atoms n 5 10 slcon atom/m atoms/cm 3 x.4 pure Ge specmen s doped wth. The number densty of acceptor atoms s approxmately 10 1 m 3. f densty of electron holes par n an ntrnsuc semconductor s approxmately m 3, the number densty of electrons n the specmen s : Sol. n pure semconductor electron-hole par n m 3 acceptor mpurty N 10 1 m 3 Holes concentraton n h 10 1 m 3 electrons concentraton n e n 10 n h m 3 6 :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

7 J-Physcs x.5 Pure at 300 K has equal electron (n e ) and hole (n h ) concentratons of m 3. Doppng by ndum ncreases n h to 3 10 m 3. alculate n e n the doped. Sol. For a doped sem-conductor n thermal equlebrum n e n h n (aw of mass acton) n n ( ) 16 e h h m 3 SSTTY ND ONDUTTY OF SMONDUTO onducton n conductor elaton between current () and drft velocty (v d ) ne v d current densty n number of electron n unt volume cross sectonal area J amp/m ne v d drft velocty of electron v d J ne J onductvtyne esstvty Moblty v d onducton n Semconductor ntrnsc semconductor P - type N - type n e n h n h >> n e n e >> n h J ne [ v e + v h ] J e n h v h J e n e v e 1 en [ e + h ] 1 e n h h 1 e n e e GODN KY PONTS :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 Due to mpurty the conductvty ncreases approxmately 10 5 tmes sc e + h n e e e + n h e h e(n e µ e + n h µ h ) x.6 What wll be conductance of pure slcon crystal at 300K Temp.. f electron hole pars per cm 3 s 1.07 x at ths Temp, n 1350 cm / volt sec & P 480 cm / volt sec Sol. n e e + n e h n e ( e + h ) mho/cm x.7 Pure at 300 K has equal electron n e and hole n h concentraton of /m 3. Dopng by ndum ncreases n h to /m 3. alculate n e n doped slcon. Sol. n e n n ( ) 16 h ( ) 5 10 m 9 3 7

8 charge densty lectrc Potental J-Physcs x.8 semconductor has equal electron and hole concentraton of /m 3. On dopng wth certan mpurty electron concentraton ncreases to /m 3. () dentfy the new semconductor obtaned after dopng. () alculate the new hole concentraton. Sol. n /m 3 and n e /m 3 () n e > n so t s N-type semconductor () n nen n n h h n e /m 3 P - N JUNTON Technque s for mak ng P- N junct on () lloy Method or lloy Juncton Here a small pece of group mpurty lke ndum s placed over n Ge or n and melted as shown n fgure ultmetely P N juncton form. ndum ndum lqud ndum buttom PN J n N-type N-type N-type arsenc arsenc lqud arsenc buttom PN J n P-type P-type P-type () Dffuson Juncton heated P type semconductor s kept n pentavalent mpurty vapours whch dffuse nto P type semconductor as shown and make P N juncton. () apour deposted juncton or eptaxal juncton f we want to grow a layer of n or p then p wafer s kept n an atmosphere of lane (a slcon compound whch dssocates nto at hgh temperatures) plus phosphorous 8 heat l vapours vacuum N-type Ge/ to vacuum pump P-N J n dffused P-type layer vapours. On crakng of slane at hgh temperature a fresh layer on n grows on p gvng the "P N juncton". nce ths juncton growth s layer by so t s also referred as layer growth or eptaxal juncton formaton of P N juncton. Descrpton of P-N Juncton wthout appled voltage or bas Gven dagram shows a P N juncton mmedately after t s formed. P regon has moble majorty holes and mmoble negatvely charged mpurty ons. p + n N regon has moble majorty free electrons and mmoble postvely charged mpurty ons. Due to concentraton dfference dffuson of holes starts from P to N sde and dffuson of e s starts N to P sde. Due to ths a layer of only postve (n N sde) and negatve (n P sde) started to form whch generate an electrc feld (N to P sde) whch oppose dffuson process, durng dffuson magntude of electrc feld ncreases due to ths dffuson t gradually decreased and ultmately stopes. The layer of mmoble postve and negatve ons, whch have no free electrons and holes called as depleton layer as shown n dagram. hole electrc feld ntensty + dstance dstance dstance free electron deplton layer wdth (10) -6 0 Potental barrer :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

9 GODN KY PONTS Wdth of depleton layer 10-6 m (a) s dopng ncreases depleton layer decreases (b) s temperature s ncreased depleton layer also ncreases. (c) P-N juncton unohmc, due to nonlnear relaton between and. Potental arrer or contact potental G e J-Physcs lectrc feld, produce due to potental barrer d /m Ths feld prevents the respectve majorty carrer from crossng barrer regon DFFUSON ND DFT UNT (1) Dffuson current P to N sde () Drft current N to P sde f there s no basng dffuson current drft current So total current s zero HOU OF P N JUNTON WTH N XTN OTG PPD O S Forward as f we apply a voltage "" such that P sde s postve and N sde s negatve as shown n dagram. P The appled voltage s opposte to the juncton barrer potental.due to ths effectve potental barrer decreases, juncton wdth also decreases, so more majorty carrers wll be allowed to flow across juncton. t means the current flow n prncpally due to majorty charge carrers and t s n the order of m called as forward as. N everse as f we apply a voltage "" such that P sde s negatve and N sde s postve as shown n dagram. P N :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 The appled voltage s n same drecton as the juncton barrer potental. Due to ths effectve potental barrer ncrease juncton, wdth also ncreases, so no majorty carrers wll be allowed to flow across juncton. Only mnorty carrers wll drfted. t means the current flow n prncpally due to mnorty charge carrers and s very small (n the order of µ). Ths bas s called as reversed as. GODN KY PONTS n reverse bas, the current s very small and nearly constant wth bas (termed as reverse saturaton current). However nterestng behavour results n some specal cases f the reverse bas s ncreased further beyond a certan lmt, above partcular hgh voltage breakdown of depleton layer started. reakdown of a dode s of followng two types : () Zener breakdown () valanche breakdown 9

10 J-Physcs omparson between Forward as and everse as Forward as everse as P N postve negatve P N negatve postve P N P N Potental arrer reduces 1. Potental arrer ncreases.. Wdth of depleton layer decreases. Wdth of depleton layer ncreases. 3. P-N jn. provde very small resstance 3. P-N jn. provde hgh resstance 4 Forward current flows n the crcut 4. ery small current flows. 5. Order of forward current s mll ampere. 5. Order of current s mcro ampere for Ge or Neno ampere for. 6. urrent flows manly due to majorty carrers. 6. urrent flows manly due to mnorty carrers. 7. Forward characterstc curves. 7. everse characterstc curve (volt) r f (m) 0 (volt) f knee voltage break down voltage everse saturaton current r ( ) 8. Forward resstance 8. everse resstance f f 100 f r 6 r r 9. Order of knee or cut n voltage 9. reakdown voltage G e 0.3 G e Specal pont : Generally r f 10 3 : 1 for Ge r f 10 4 : 1 for :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

11 J-Physcs x. 9 The resstance of p-n juncton dode decreases when forward based and ncreases when reverse based. Why? Sol. When p-n juncton s forward based, the wdth of depleton layer decreases and the barrer potental s opposed by the forward bas. n other words, potental barrer decreases. Hence the dffuson of holes and electrons through the juncton ncreases. Due to ths, the dode current ncreases and hence resstance decreases. When p-n juncton s reverse based, the barrer potental s supported and the wdth of depleton layer ncreases. s a result of ths, the dode current becomes almost zero as there s no dffuson of majorty carrers (electrons and holes) through the juncton. Hence the resstance of the juncton dode ncreases when reverse based. x.10 What s an deal dode? Draw the output waveform across the load resstor, f the nput waveform s as shown n the fgure Sol. n deal dode has zero resstance when forward based and nfnter resstance when t s reversed based. Output wave form s shown n fg x.11 potental barrer of 0.5 exsts across a p-n juncton () f the depleton regon s m wde. What s the ntensty of the electrc feld n ths regon? () n electron wth speed m/s approaches the p-n juncton from the n-sde wth what speed wll t enter the p-sde. Sol.: () Wdth of depleton layer m volt/m P N () Work energy theorm 1 1 Mv e Mv f :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 v f Mv e m/s M x.1 Fgure shows a dode connected to an external resstance and an e.m.f. ssumng that the barrer potental developed n dode s obtan the value of current n the crcut n mllampere. Sol voltage drop across p-n juncton 0.5 effectve voltage n the crcut current n the crcut m 40m 100 x.13dfferentate zener and avalanche breakdown. S o l. The dfference between these two are as follows 11

12 f J-Physcs Zener reak down Where covalent bonds of depleton layer, ts self break, due to hgh electrc feld of very hgh everse bas voltage. Ths phenomena predomnant () t lower voltage after "break down" () n P N havng "Hgh dopng" () P N Jn. havng thn depleton layer Here P N not demage paramanently "n D. voltage stablzer zener phenomenan valanche reak down Here covalent bonds of depleton layers are bro ken by collson of "Mnortes" whch aqure hgh knetc energy from hgh electrc feld of very-very hgh reverse bas voltage. Ths phenomena predomnant () t hgh voltage after breakdown () n P N havng "ow dopng" () P N Jn. havng thck depleton layer Here P N damage peramanentaly due to "Heatng effect" due to abruptly ncreament of s used". mnortes durng repeatatve collsons. HTST U OF P-N JUNTON DOD P N P symbol N (0-1) (0-10) + + m + D D h h Forward bas curve r (olt) O (olt) ( ) m everse bas curve ( ) r f TF Forward bas everse bas n forward bas when voltage s ncreased from 0 s steps and correspondng value of current s measured, the curve comes as O of fgure. We may note that current ncrease very sharply after a certan voltage knee voltage. t ths voltage, barrer potental s completely elmnated and dode offers a low resstance. n reverse bas a mcroammeter has been used as current s very very small. When reverse voltage s ncreased from 0 and correspondng values of current measured the plot comes as OD. We may note that reverse current s almost constant hence called reverse saturaton current. t mples that dode resstance s very hgh. s reverse voltage reaches value, called breakdown voltage, current ncreases very sharply. For deal Dode n forward bas 'ON' swtch 0 f 1 n reverse bas 'OFF' swtch t s devce whch s used for convertng alternatng current nto drect current. r :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

13 J-Physcs Half wave rectfer Secondry oltage.. Supply S 1 S D.. Supply S 1 D S nput Output For postve half cycle For negatve half cycle Durng the frst half (postve) of the nput sgnal, let S 1 s at postve and S s at negatve potental. So, the PN juncton dode D s forward based. The current flows through the load resstance and output voltage s obtaned. Durng the second half (negatve) of the nput sgnal, S 1 and S would be negatve and postve respectvely. The PN juncton dode wll be reversed based. n ths case, practcally no current would flow through the load resstance. So, there wll be no output voltage. Thus, correspondng to an alternatng nput sgnal, we get a undrectonal pulsatng output as shown. Peak nverse voltage (P) n half wave rectfer P maxmum voltage across secondary col of transformer ( s ) Peak value of output ( m ) Full wave rectfer When the dode rectfes the whole of the wave, t s called full wave rectfer. Fgure shows the experemental arrangement for usng dode as full wave rectfer. The alternatng sgnal s fed to the prmary a transformer. The output sgnal appears across the load resstance. D 1 D 1 Secondry oltage.. Supply S 1.. Supply S 1 nput S S Output D 1 D D 1 D D 1 D D 1 For postve half cycle D For negatve half cycle D :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 Durng the postve half of the nput sgnal : et S 1 postve and S negatve. n ths case dode D 1 s forward based and D s reverse based. So only D 1 conducts and hence the flow of current n the load resstance s from to. Durng the negatve half of the nput sgnal : Now S 1 s negatve and S s postve. So D 1 s reverse-based and D s forward based. So only D conducts and hence the current flows through the load resstance from to. t s clear that whether the nput sgnal s postve or negatve, the current always flows through the load resstance n the same drecton and full wave rectfcaton s obtaned. 13

14 J-Physcs rdge ectfer D 1 D nput D 1,D 4 D,D 3 D 1,D 4 D,D 3 D 1,D 4 D 3 D 4 Output Durng postve half cycle Durng negatve half cycle D 1 and D 4 are forward based on swtch D and D 3 are forward based on swtch D and D 3 are reverse based off swtch D 1 and D 4 are reverse based offswtch n brdge rectfer peak nverse voltage P s m Form Factor F rms dc or rms dc for full wave rectfer F for half wave rectfer F pple and rpple factor n the output of rectfer some.. components are present. They are called rpple & there measurement s gven by a factor known as rpple factor. For a good rectfer rpple factor must be very low. Total output current rms ac + dc Where ac rms value of component present n output pple factorr ac dc r rms dc 1 F 1 ectfer effcency P P ( + ) dc dc ac rms F Half wave rectfer Full wave rectfer or brdge wave rectfer f 1 f f < < 1, then 40.6% f 0.81 f 1 14 f < < 1, then 81.% Specal Note f f Specal Note f f 0.3% 40.6% Note : n brge full wave rectfer f s two tmes of resstance of P-N jn. dode n F. :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

15 pple Frequency J-Physcs ( ) For half wave rectfer nput nput frequency 50Hz output rpple frequency 50Hz ( ) for full wave rectfer nput nput frequency 50Hz P u l s e ( ) For half wave rectfer output rpple frequency 100Hz nput Pulse frequency 100 Hz ( ) For full wave rectfer output Pulse frequency 50 Hz nput output Pulse frequency 100 Hz Pulse frequency 100 Hz omparson etween verage ectfers Full-wave Half-wave entre-tap r d g e Number of Dodes 1 4 Transformer necessary N o Yes N o Peak secondary voltage s s s :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 Peak nverse oltage s m s m s m (when peak of output m ) Peak load urrent, m MS urrent, rms D current, dc n r d 15 n r d n r pple factor, r ectfcaton effcency (max) 40.6% 81.% 81.% m m pple frequency (when nput 50 Hz) 50 Hz 100 Hz 100 Hz m m d m m

16 J-Physcs x. 1 4 Sol. snusodal voltage of ampltude 5 volts and frequency 50 Hz s appled to a half wave rectfer usng PN dode. No flter s used and the load resstor s The forward resstance f deal dode s 10. alculate () Peak, average and rms values of load currrent. () d.c. power output () a.c. power nput (v) % ectfer effcency (v) pple factor () 5 ( ) m m f 4.75 m m 4.75 dc 7.88m 3.14 m 4.75 rms 1.38m () P dc dc ( ) mw () P ac rms (f + ) ( ) ( ) 155 mw (v) Pdc ectfer effcency P ac % 1 / rms 1.38 (v) pple factor 1 1 ac x.15 The halfwave rectfer supples power to a 1 k load. The nput supply voltage s 0 neglectng forward resstance of the dode, calculate Sol. () dc () dc and () pple voltage (rms value) () () () r or dc 99 dc 99 m 1000 m rms dc 99 ( r ) dc rms volt ( r ) rms r v dc volt. x.16 fullwave rectfer supples a load of 1 K. The a.c. voltage appled to the dodes s 0 volt rms. f dode resstance s neglected, calculate. () verage d.c. voltage () verage d.c. current () pple voltage (rms) m Sol. () verage d.c. voltage dc m where m maxmum across each half of the secondary wndng. f be the rms voltage across each half of the secondary wndng then dc volt. dc () For fullwave rectfer dc m 1000 r (rms ) () r r(rms) r dc r(rms) volt dc 16 m :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

17 J-Physcs x.17 fullwave P.N. dode rectfer used load resstor of No flter s used. ssume each dode to have dealzed charcterstc wth f 10 and r. nce wave voltage appled to each dode has ampltude of 30 volts and frequency 50Hz. alculate. () Peak, d.c. rms load current () d.c. power nput ().. power nput (v) ectfer effcency Sol. () Peak current m m 30volts m f 19.9 m d.c. load current dc m m m 1.66 m. m 19.9 rms 14 m () D.. Power output P dc dc ( ) 1500 Watt mw ().. power nput P n rms (f + ) ( ) ( ) watt mw FT UT To reduce.. omponents apactor Flter D + D output of rectfer output - Flter D + low D :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 + D output of rectfer - Flter (est Flter) + D output of rectfer D + low 1 17 D output output approx pure D.

18 J-Physcs ZN DOD specfcally doped crystal dode whch can work n break down regon s known as Zener dode. t s always connected n reverse based condton manner. Used as a voltage regulator Symbol of Zener dode + Fluctuatng nput S + regulated output n forward based t works as a smple dode. oltage regulatng crcut of Zener dode SOM OTH SP DODS Photod ode juncton dode made from lght or photo senstve semconductor s called a photo dode ts symbol. When lght of energy "h'' falls on the photodode (Here h > energy gap) more electrons move from valence band, to conducton band, due to ths current n crcut of photodode n "everse bas", ncreases. s lght ntensty s ncreased, the current goes on ncreases so photo dode s used, "to detect lght ntensty" for example t s used n "edo camera". ght emttng dode (..D) When a juncton dode s forward based energy s released at juncton n the form of lght due to recombnaton of electrons and holes. n case of or Ge dodes, the energy released s n nfra-red regon. n the juncton dode made of Gas, np etc energy s released n vsble regon such a juncton dode s called "lght emttng dode" (D) ts symbol Solar cell Solar cell s a devce for convertng solar energy nto electrcal. juncton dode n whch one of the P or N sectons s made very thn (So that the lght energy fallng on dode s not greatly asorbed before reachng the juncton) can be used to convert lght energy nto electrc energy such dode called as solar cell. ts symbol () () t s operated nto photo voltac mode.e., generaton of voltage due to the bombardment of optcal photon. No external bas s appled. () ctve juncton area s kept large, because we are ntrested n more power. Materals most commonly used for solar cell s, s, ds, dte, dse, etc. arable capactor (aractor) P N juncton dode can be used as a "apactor" here depleton layer acts as "delectrc materal" and remanng "P" and "N" part acts as metallc plates. ts symbol Dode laser t s nterstng form of D n whch specal constructon helps to produce stmulated radaton as n laser. 18 :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

19 J-Physcs x. 1 8 zener dode of voltage Z (6) s used to mantan a constant voltage across a load resstance (1000) by usng a seres resstance S (100). f the e.m.f. of source s ( 9 ), calculate the value of current through seres resstance, Zener dode and load resstance. What s the power beng dsspated n Zener dode. Sol. Here, 9 ; Z 6 ; 1000 and S 100, Potental drop across seres resstor Z urrent through seres resstance S s urrent through load resstance s 6 Z urrent through Zener dode s Z amp. Power dsspated n Zener dode s P Z Z Z 6 x Watt x.19 Zener dode s specfed havng a breakdown voltage of 9.1 wth a maxmum power dsspaton of 364 mw. What s the maxmum current that the dode can handle. Sol. Maxmum current that the gven dode can handle s e., 40 m. TNSSTO nventor Wllam radford Shockley, John ardeen and Walter Houser rattan. Transstor s a three termnal devce whch transfers a sngal from low resstance crcut to hgh resstance crcut. :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 t s formed when a thn layer of one type of extrnsc semconductor (P or N type) s sandwtched between two thck layers of other type of extrnsc semconductor. ach transstor have three termnals whch are :- () mtter () ase () ollector mtter : t s the left most part of the transstor. t emt the majorty carrer towards base. t s hghly doped and medum n sze. ase : t s the mddle part of transstor whch s sandwtched by emtter () and collector (). t s lghtly doped and very thn n sze. ollector : t s rght part of the transstor whch collect the majorty carrers emtted by emtter. t has large sze and moderate dopng. There are two semconductng PN- junctons n a transstor () The juncton between emtter and base s known as emtter-base juncton (J ). () The juncton between base and collecter s known as base-collector juncton (J ). 19

20 J-Physcs TNSSTO OF TWO TYPS N-P-N Transstor f a thn layer of P-type semconductor s sandwtched between two thck layers of N-type semconductor s known as NPN transstor. NPN N P N P-N-P Transstor f a thn layer of N-type of semconductor s sandwtched between two thck layer of P-type semconductor s known as PNP transstor. PNP P N P GODN KY PONTS Transstor have two P-N Juncton J and J, therefore t can be based n four followng ways as gven below: N P N mtter-ase ollector-ase egon of workng Forward based everse based ctve everse based Forward based nverse ctve everse based everse based ut off Forward based Forward based Saturaton omparson between, and mtter Medum sze Hgh doppng ase Smallest sze ow doppng ollector argest sze Medum doppng The collector regon s made physcally larger than the emtter. ecause collector has to dsspate much greater power. Transstor all mostly work n actve regon n electronc devces & transstor work as amplfer n ctve regon only. Transstor.e. t s a short form of two words "Transfer resstors". gnal s ntroduced at low resstance crcut and out put s taken at hgh resstance crcut. ase s lghtly doped. Otherwse the most of the charge carrer from the emtter recombne n base regon and not reaches at collector. Transstor s a current operated devce.e. the acton of transstor s controlled by the moton of charge carrers..e. current 0 :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

21 WOKNG OF NPN TNSSTO J-Physcs The emtter ase juncton s forward bas and collector base juncton s reversed based of n-p-n transstor n crcut () and symbolc representaton s shown n Fgure. emtter-base juncton N collectoremtter-base juncton P N e h m m When emtter base juncton s forward bas, electrons (majorty carrers) n emtter are repelled toward base. The barrer of emtter base juncton s reduced and the electron enter the base, about 5% of these electron recombne wth hole n base regon result n small current ( b ). The remanng electron ( 95%) enter the collector regon because they are attracted towards the postve termnal of battery. For each electron enterng the postve termnal of the battery s connected wth collector base juncton an electron from negatve termnal of the battery connected wth emtter base juncton enters the regon. The emtter current ( e ) s more than the collector ( c ). The base current s the dfference between e and c and proportonal to the number of electron hole recombnaton n the base. WOKNG OF PNP TNSSTO e b + c When emtter-base juncton s forward based holes (majorty carrers) n the emtter are repelled towards the base and dffuse through the emtter base juncton. The barrer potental of emtter-base juncton decreases and hole enter the n-regon (.e. base). small number of holes ( 5%) combne wth electron of base-regon resultng small current ( b ). The remanng hole ( 95%) enter nto the collector regon because they are attracted towards negatve termnal of the battery connected wth the collector-base juncton. These hole consttute the collector current ( c ). :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 m emtter-base juncton P collectoremtter-base juncton N P m s one hole reaches the collector, t s neutralzed by the battery. s soon as one electron and a hole s neutralzed n collector a covalent bond s broken n emtter regon. The electron hole par s produced. The released electron enter the postve termnal of bettary and hole more towards the collector. 1 +

22 J-Physcs asc Tra nsstor rcut onfgurat ons :- To study about the characterstcs of transstor we have to make a crcut [n whch four termnals are requred. ut the transstor have three termnals, so one of the termnal of transstor s made common n nput and output both. Thus, we have three possble confguraton of transstor crcut. () ommon base confguraton () ommon emtter confguraton () ommon collector confguraton n these three common emtter s wdely used and common collector s rarely used. ommon emtter characterstcs of a transstor rcut Dagram : 1 ( ) ( ) rcut dagram for characterstc curve of n-p-n transstor n mode nput characterstcs The varaton of base current ( b ) (nput) wth base emtter voltage ( ) at constant-emtter voltage ( ) s called nput characterstc. () Keep the collector-emtter voltage ( ) constant (say 1) () Now change emtter base voltage by 1 and note the correspondng value of base current ( b ). () Plot the graph between and b. (v) set of such curves can be plotted at dfferent ( ) Output characterstcs The varaton of collector current c (output) wth collector-emtter voltage ( ) at constant base current ( b ) s called output characterstc. () Keep the base current ( b ) constant (say b 10) () Now change the collector-emtter voltage ( ) usng varable resstance and note the correspondng values of collector current ( c ). () Plot the graph between ( versus c ) b( ) nput characterstc curves 0 (m) 0 ce 5 ce be(volt) 100 b 75 b 50 b 5 b (volt) (v) set of such curves can be plotted at dfferent fxed values of base current (say 0, 0, 30 etc.) TNSSTO S N MPF The process of ncreasng the ampltude of nput sgnal wthout dstortng ts wave shape and wthout changng ts frequency s known as amplfcaton. devce whch ncreases the ampltude of the nput sgnal s called amplfer. transstor can be used as an amplfer n actve state. basc crcut of a common emtter transstor amplfer s shown b :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

23 J-Physcs ntput sgnal + + common emtter amplfer NPN transstor o + amplfed output sgnal amplfer omparatve study of transstor confguratons 1. ommon ase ( ). ommon mtter () 3. ommon ollector () nput esstance ow (100 ) Hgh (750 ) ery Hgh 750 k Output resstance ery Hgh Hgh ow urrent Gan ( or ) ( or ) ( or ) oltage Ga n o o o :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 Power Gan v p 3 v v less than 1 p Po P Phase dfference same phase opposte phase same phase (between output and nput) pplcaton For Hgh Fequency For udble fequency For mpedance Matchng p p Po P p p Po P

24 J-Physcs elaton etween and dvde by dvde by GODN KY PONTS n transstor charge carrers move from emtter to collector. mtter send the charge carrers and collector collect them ths happen only when emtter-base juncton s forward bas and collector-base juncton s reverse bas (base of amplfer) n transstor reverse bas s hgh as compared to forward bas so that the charge carrers move from emtter to base exert a large attractve force to enter n collector regon so base current s very less. confguraton s wdely used becasue t have large voltage and power gan as compared to other amplfers. n amplfer negatve feed back s used to stablzed the gan. s used for mpdence matchng for connectng two transstors n cascade. Q. 0 transstor s a current operated devce. xplan why? ns. The acton of a transstor s controlled by the charge carrers (electrons or holes). That s why a transstor s a current operated devce. Q. 1 n a transstor, reverse bas s qute hgh as compared to the forward bas. Why? n s. n a transstor, charge carrers (electrons or holes) move from emtter to collector through the base. The reverse bas on collector s made qute hgh so that t may exert a large attractve force on the charge carrers to enter the collector regon. These movng carrers n the collector consttute a collector current. Q. transstor s a temperature senstve devce. xplan. ns. n a transstor, conducton s due to the movement of current carrers electrons and holes. When temperature of the transstor ncreases, many covalent bonds may break up, resultng n the formaton of more electrons and holes. Thus, the current wll ncrease n the transstor. Ths current gves rse to the producton of more heat energy. the excess heat causes complete breakdown of the transstor. Q. 3 The use of a transstor n common-emtter confguraton s preferred over the common-base confguraton. xplan why? ns. The current gan and hence voltage gan n the common-emtter confguraton s much more than of commonbase confguraton. Hence the former s preferred over the later. 4 :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

25 J-Physcs Q. 4 Why do we prefer transstor over the vacuum tubes n the portable rado recevers? ns. Ths s because of two reasons : () () Transstor s compact and small n sze than the vacuum tube. Transstor can operate even at low voltage whch can be suppled wth two or three dry cells. Q. 5 Why a transstor cannot be used as a rectfer? ns. f transstor s to be used as a rectfer the ether emtter-base or base-collector has to used as dode. For equated workng of the sad set of dodes, the number densty of charge carrers n emtter and base or base and collector must be approxmately same. s base s lghtly doped and comparatvely thn, so emtter cannot work as a rectfer. x.6 n a transstor, the value of s 50. alculate the value of. Sol x.7 alculate the collector and emtter current for whch b 0, 100 Sol. 100, b 0 c b e b + c m x.8 For a common emtter amplfer, current gan 50. f the emtter current s 6.6 m, calculate the collector and base current. lso calculate current gan, When emtter s workng as common base amplfer. Sol. 50 ; e 6.6 m c c b 50 b...() b e c + b usng equaton () we get b + b 51 b or b m 51 Hence c m and :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 x.9 Transstor wth 75 s connected to common-base confguraton. What wll be the maxmu collector current Sol. for an emtter current of 5 m? 75, e 5 m or or c c e 75 e m

26 J-Physcs x.30 The base current s 100 and collector current s 3 m. (a) alculate the values of, e and Sol. (b) change of 0 n the base current produces a change of 0.5 m n the collector current. alculate a.c.. b m, c 3 m (a) c b and e c m (b) b m, c 0.5 m 0.5 c ac b x.31 n npn transstor crcut, the collector current s 10 m. f 95% of the electrons emtted reach the collector, what s the base current? Sol. c 95% c 0.95 e e c 10m m ( c 10 m) Now e c + b b e c m x.3 n an NPN transstor electrons enter the emtter n 10 6 s and % electrons recombne wth holes n base, then current gan and are : Sol. mtter current e Ne t m ase current b m but e c + b c e b m c e 0.98 and F D K c b 49 Feedback are two types : Postve feedback When nput and output are n the same phase then postve feedback s there. t s used n oscllators. Negatve feedback f nput and output are out of phase and some part of that s feedback to nput s known as negatve feedback. t s used to get constant gan amplfer. 6 :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65

27 J-Physcs T NSSTO S N OS TO Oscllator s devce whch delvers a.c. output wave form of desred frequency from d.c. power even wthout nput sngal exctaton. The electrc oscllatons are produced by crcut (.e. tank crcut contanng nductor and capactor). These oscllatons are damped one.e. ther ampltude decrease wth the passage of tme due to the small resstance of the nductor. n other words, the energy of the oscllatons decreases. f ths loss of energy s compensated from outsde, then undamped oscllatons (of constant ampltude) can be obtaned. Ths can be done by usng feed back arrangement and a transstor n the crcut. n-p-n T 1 1 T 3 4 S 1 (swtch) crcut producng oscllatons conssts of an nductor of nductance and capactor of mutual nductance (couplng through magnetc feld) varable capactance nductor of nductance ' s connected n the collector-emtter crcut through a battery and a tappng key (K). nductors and ' are nductvely coupled (Fgure) Work ng When key K s closed, collector current begns to flow through the col. s ths current grows, magnetc flux lnked wth col ncrease (.e. changes). nce col s nductvely coupled wth, so magnetc flux lnked wth col ' also changes. Due to change n magnetc flux, nduced e.m.f. s set up across the col '. The drecton of nduced e.m.f. s such that the emtter-base juncton s forward based. s a result of ths basng, emtter current e ncreases whch n turn ncreases the collector current c [ e b + c ]. Wth the ncrease n collector current, magnetc flux lnked wth col also ncreases. Ths ncreases the e.m.f. nduced n the col '. The ncreased nduced e.m.f. ncreases the forward bas of emtter-base juncton. Hence emtter current s further ncreased whch n turn ncreases the collector current. The process of ncreasng the collector current contnues tll the magnetc flux lnked wth col ' becomess maxmum (.e. constant). t ths stage, the nduced e.m.f. n col ' becomes zero. The upper plate of the capactor gets postvely charged durng ths process. T' output :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65 When nduced e.m.f. becomes zero, the capactor starts dschargng through the nductor. The emtter current starts decreasng resultng n the collector current. Wth decreasng collector current whch flows through ', e.m.f. s agan nduced n the col ' but n the opposte drecton. t opposes the emtter current and hence collector current ultmately decreases to zero. The change n magnetc flux lnked wth col ' stops and hence nduced e.m.f. n the col becomes zero. t ths stage, the capactor gets dscharged through col but now n the opposte drecton. Now the emtter current and hence collector current ncrease but now n the opposte drecton. Ths process repeats and the collector current oscllates between maxmum and mnmum values. 1 f 7

28 J-Physcs DNTGS OF SMONDUTO DS O UUM TUS d v a ntages Semconductor devces are very small n sze as compared to the vacuum tubes. Hence the crcuts usng semconductor devces are more compact. n vacuum tubes, current flows when the flament s heated and starts emttng electrons. So, we have to wat for some tme for the operaton of the crcut. On the other hand, n semconductor devces no heatng s requred and the crcut begns to operate as soon as t swtched on. Semconductor devces requre low voltage for ther operaton as compared to the vacuum tube. So a lot of electrcal power s saved. Semconductor devces do not produce any hummng nose whch s large n case of vacuum tube. Semconductor devces have longer lfe than the vacuum tube. acuum tube gets damaged when ts flament s burnt. Semconductor devces are shock proof. The cost of producton of semconductor-devces s very small as compared to the vacuum tubes. Semconductor devces can be easly transported as compared to vacuum tube. D s a d v a n t a g e s Semconductor devces are heat senstve. They get damaged due to overheatng and hgh voltages. So they have to be housed n a controlled temperature room. The nose level n semconductor devces s very hgh. Semconductor devces have poor response n hgh frequency range. Q. 3 3 Why s a transstor so called? ns. The word Transstor can be treated as short form of two words 'transfer resstor'. n a transstor, a sgnal s ntroduced n the low resstance crcut and output s taken across the hgh resstance crcut. Thus, a transstor helps to transfer the current from low resstance part to the hgh resstance part. Q. 3 4 The base regon of a transstor s lghtly doped. xplan why? n s. n a transstor, the majorty carrers (holes or electrons) from emtter regon move towards the collector regon through base. f base s made thck and hghly doped, then majorty of carrers from emtter wll combne wth the carrers n the base and only small number of carrers wll reach the collector. Thus the output or collector current wll be consderably small. To get large output or collector current, base s made thn and lghtly doped so that only few electron-hole combnaton may take place n the base regon. Q. 3 5 xplan why the emtter s forward based and the collector s reverse based n a transstor? ns. n a transstor, the charge carrers move from emtter to collector. The emtter sends the charge carrers and collector collects them. Ths can happen only f emtter s forward based and the collector s reverse based so that t may attract the carrers. 8 :\Data\014\Kota\J-dvanced\SMP\Phy\lectroncs\ng\1. lectroncs-sem onductor Theory.p65