Semiconductor Electronics: Materials, Devices and Simple Circuits: Class 12 Physics Chapter 14 Hinglish mein

14.1 Introduction

Ek phone charger kholo to andar ek chhota transformer, kuch diodes aur ek capacitor milega. Diodes alternating mains supply ko us one-way current mein badalte hain jo battery ko chahiye. Phone ke andar, nakhun se bhi chhoti ek chip mein billions transistors hote hain, aur har ek second mein millions baar on-off switch hota hai. Yeh sab semiconductor devices hain: khaas tarah se taiyaar kiye gaye crystal ke chhote tukde, jinmein hum tay karte hain ki kitne charge carriers honge aur woh kis taraf chalenge.

Twentieth century ke pehle half mein yahi kaam vacuum tubes karti thi, jinhe valves bhi kehte hain. Sabse simple tube, vacuum diode, mein ek metal cathode ko itna garam kiya jaata hai ki uski surface se electrons ubal kar nikalne lagte hain, aur doosra electrode, yaani plate ya anode, unhe evacuated gap ke paar collect karta hai (Fig. 14.1). Dono ke beech ek wire-mesh grid lagane se triode bana, aur aur electrodes jodne se tetrode aur pentode bane; in electrodes par lage voltages electrons ki stream ko steer karte the. Gap mein vacuum hona zaroori tha, kyunki hawa se guzarte electrons baar-baar gas molecules se takraate aur apni energy kho dete.

Kyunki electrons sirf hot cathode se nikalte hain, tube current ko sirf ek direction mein pass kar sakti hai, bilkul paani ke pipe mein lage one-way valve ki tarah; valve naam yahin se aaya. Tubes apna kaam karti thi, lekin woh bhaari aur naazuk thi, cathode garam karne mein power waste karti thi, unhe sau volt ya usse zyada ki supply chahiye hoti thi, aur kuch hazaar ghanton mein woh jal jaati thi. ENIAC, 1945 ka ek mashhoor kamre jitna bada computer, lagbhag 18 000 tubes use karta tha, aur use chalu rakhne ke liye kharab tubes ko lagaataar badalna padta tha.

1930s mein solids ki quantum theory par kaam kar rahe physicists ne samjhaya ki kuch crystals metals se itna alag behave kyun karte hain, aur lagbhag 1940 tak unhe pata chal gaya ki aise do tarah ke crystals ke beech ka junction charge ke liye gate ka kaam kar sakta hai. Solid-state device mein garam metal se kuch ubaal kar nikaalna ya khaali jagah ke paar bhejna nahi padta: charge carriers pehle se crystal ke andar hote hain, aur unki ginti ek chhote voltage se, garam karne se, ya un par light padne se badli ja sakti hai. Heater aur glass hata do, to jo bachta hai woh chhota, thanda aur mazboot hai: ek aisa device jo kuch volts par chalta hai, kam power leta hai aur dashakon tak kaam kar sakta hai. Isiliye purane television sets ki bhaari picture tubes, jo khud vacuum devices thi, bhi gayab ho gayi aur unki jagah chips se chalne wale flat LCD aur LED panels aa gaye.

Yeh chapter ideas ko teen stages mein build karta hai. Pehle, kya cheez kisi material ko semiconductor banati hai aur uske charge carriers kaise behave karte hain (Section 14.2 se Section 14.4 tak). Doosra, jahan p-type material n-type material se milta hai wahan kya hota hai, aur woh junction diode ki tarah kaise kaam karta hai (Section 14.5 aur Section 14.6). Teesra, diodes ac ko dc mein kaise badalte hain (Section 14.7). Transistor, jo do junctions se bana teen-terminal device hai, bilkul inhi ideas par tika hai.

14.2 Metals, conductors aur semiconductors ka classification

Conductivity ke basis par

Part I ke Chapter 3 mein tumne kisi material ki conduct karne ki capacity ko uski resistivity $\rho$ se, ya conductivity $\sigma =1\mathrm{/}\rho$ se describe kiya tha, jo siemens per metre ($\mathrm{S}{\mathrm{m}}^{-1}$) mein naapi jaati hai. Aam solids mein $\rho$ pachchees se zyada powers of ten tak phaili hoti hai (Fig. 14.2). Teen bade groups saaf dikhte hain:

• Metals ki resistivity bahut kam hoti hai, aam taur par ${10}^{-8}$ se ${10}^{-6}\mathrm{\Omega }\mathrm{m}$, isliye $\sigma$ lagbhag ${10}^6$ se ${10}^8\mathrm{S}{\mathrm{m}}^{-1}$ hota hai. (Kai textbook tables metals ke liye zyada wide band dete hain, ${10}^{-2}\mathrm{\Omega }\mathrm{m}$ tak.)
• Semiconductors beech mein aate hain, lagbhag ${10}^{-5}$ se ${10}^6\mathrm{\Omega }\mathrm{m}$, aur unka $\sigma$ lagbhag ${10}^5$ se neeche ${10}^{-6}\mathrm{S}{\mathrm{m}}^{-1}$ tak hota hai.
• Insulators ki resistivity bahut zyada hoti hai, lagbhag ${10}^{11}$ se ${10}^{19}\mathrm{\Omega }\mathrm{m}$, isliye $\sigma$ sirf ${10}^{-11}$ se ${10}^{-19}\mathrm{S}{\mathrm{m}}^{-1}$ hota hai.

In limits ko deewar nahi, balki signpost samjho: kuch materials inke bahar bhi aate hain. Sirf resistivity bhi pakka test nahi hai. Semiconductor asal mein is baat se pehchana jaata hai ki uski resistivity kaise badalti hai. Use garam karo, to uski resistivity tezi se girti hai, jo metal mein hone wale effect ka ulta hai. Us par light daalo, ya sahi impurity ki thodi si maatra milao, to uski resistivity kai powers of ten gir sakti hai. Ise samjhaane ke liye ek gehri picture chahiye: neeche diya energy-band model.

Semiconductors kai families mein aate hain:

Aaj zyaadatar devices silicon, germanium ya inorganic compounds use karte hain. Lekin lagbhag 1990 se organic semiconductors aur semiconducting polymers se bane devices ne naye fields khole hain, polymer electronics aur molecular electronics; 2000 ka Chemistry Nobel Prize is discovery ko mila ki sahi tarah treat kiye gaye polymers conduct kar sakte hain. Is chapter mein hum elemental semiconductors silicon aur germanium par hi rahenge. Yeh ideas thode changes ke saath zyaadatar compound semiconductors par bhi laagu hote hain.

Energy bands ke basis par

Atoms wale chapter mein tumne dekha ki isolated atom mein electron ki sirf kuch sharp energies ho sakti hain. Crystal is baat ko badal deta hai. Uske atoms ek nanometre ke kuch dasven hisse ki doori par paas-paas packed hote hain, itne paas ki padosi atoms ki outer orbits overlap karti hain, isliye outer electron ab ek atom ka nahi rehta: use kai nuclei kheenchte hain aur kai doosre electrons dhakelte hain. Har electron ka mahaul baaki sabse thoda alag hota hai, isliye jahan isolated atom ki ek sharp energy thi, wahan $N$ atoms wale crystal ki $N$ thodi-thodi alag energies hoti hain. $N$ lagbhag ${10}^{23}$ ho to yeh itni paas-paas hoti hain ki milkar lagbhag continuous energy band bana deti hain.

To phir absolute zero par silicon ka lower band bilkul pura bhara kyun hota hai? Baat ginti ki hai. Silicon atom (outer shell $n=3$) ya germanium atom (outer shell $n=4$) crystal mein chaar outer electrons laata hai, do s aur do p, lekin uske outer shell mein aath jagahein hoti hain: do s states aur chhe p states, dono spin directions gin kar. To $N$ atoms $8N$ jagahon ke liye $4N$ electrons laate hain. Jaisa Fig. 14.3 dikhata hai, $8N$ jagahein pehle milkar ek chauda band banati hain aur phir, real crystal ki spacing par, do bands mein regroup ho jaati hain jinmein theek $4N$ jagahein hoti hain, aur unke beech gap hota hai. Absolute zero par $4N$ electrons lower band mein baith kar use poora bhar dete hain, aur upper band khaali reh jaata hai.

Dono bands ke beech ka energy gap band gap hai:

Gap hi kyun tay kare ki solid conduct karega ya nahi? Electric field electron ko thodi extra energy dekar uski speed badhata hai, aur iske liye electron ki current state ke theek upar ek khaali state chahiye. Poore bhare band mein aisi koi state nahi hoti, isliye bhare band ke electrons current carry nahi kar sakte, chahe woh kitne bhi hon. Conduction ke liye aise band mein electrons chahiye jiske paas khaali states hon, ya phir kisi waise bhare band mein khaali jagahein. Teen situations banti hain (Fig. 14.4):

1. Metals. Conduction band valence band se overlap karta hai, ya sabse upar wala occupied band sirf thoda bhara hota hai. Bahut saare electrons khaali states ke bilkul paas hote hain, isliye kamzor field bhi bada current chala deta hai: resistivity kam hoti hai.
2. Insulators. Gap chauda hota hai, $E_g>3\mathrm{e}\mathrm{V}$; diamond ke liye yeh lagbhag $5.4\mathrm{e}\mathrm{V}$ hai. Room temperature par typical thermal energy sirf $kT\approx 0.026\mathrm{e}\mathrm{V}$ hoti hai, isliye lagbhag koi bhi electron kabhi paar nahi uthta. Conduction band khaali rehta hai aur current nahi behta.
3. Semiconductors. Gap patla hota hai, $E_g<3\mathrm{e}\mathrm{V}$: silicon ke liye lagbhag $1.1\mathrm{e}\mathrm{V}$ aur germanium ke liye $0.7\mathrm{e}\mathrm{V}$. Room temperature par thode se electrons paar pahunch jaate hain. Woh conduction band mein aazaadi se chalte hain, aur unki chhodi khaali jagahein baaki valence electrons ko bhi chalne deti hain. Conductivity medium hoti hai, aur temperature ke saath tezi se badhti hai kyunki zyada electrons paar jaate hain.

14.3 Intrinsic semiconductor

Silicon aur germanium usi arrangement mein crystallise hote hain jismein diamond ke carbon atoms hote hain, yaani diamond lattice. Har atom ek tetrahedron ke centre mein hota hai jiske chaar corners par uske nearest neighbours hote hain. Har atom ke chaar valence electrons hote hain, aur har ek kisi alag neighbour ke ek electron ke saath pair banata hai. Aisa shared pair do atoms ko jode rakhta hai: yeh covalent bond hai. Is tarah har atom chaar covalent bonds se apni jagah tika rehta hai, aur har valence electron kisi na kisi bond mein bandha hota hai.

Asli lattice three-dimensional hai, lekin flat sketch mein bonds zyada saaf dikhte hain: square grid par atoms, har ek apne chaar neighbours se do electrons wale bond se juda (Fig. 14.5). Bahut kam temperature par har bond poora hota hai. Koi electron chalne ke liye free nahi hota, isliye crystal conduct nahi kar sakta: $0\mathrm{K}$ par perfect semiconductor insulator hai.

Crystal ko garam karo to uske atoms apni jagah par hilne-dulne lagte hain. Kabhi-kabhi yeh hilna kisi bonding electron ko kam se kam $E_g$ energy de deta hai, aur woh bond se chhoot kar free electron ki tarah crystal mein ghoomne lagta hai. Peeche woh ek adhoora bond chhodta hai, jise hole (होल) kehte hain. Yeh khaali jagah neutral nahi hoti. Electron ke jaane se pehle aas-paas ke atoms neutral the, isliye $-e$ charge khone ke baad us jagah par $+e$ charge hota hai. Band ki bhasha mein, ek electron valence band se conduction band mein uth gaya hai, aur neeche ek khaali state chhod gaya hai.

Pure crystal mein har free electron apne hole ke saath hi paida hota hai, isliye per unit volume dono ki ginti barabar hoti hai. Itna pure semiconductor jiske carriers sirf thermal excitation se aate hain, intrinsic semiconductor (नैज अर्धचालक) kehlata hai. Iske liye,

Ab do bilkul alag motions possible hain. Free hua electron kisi bhi conduction electron ki tarah conduction band mein ghoomta hai. Alag se, hole relay se chalta hai: padosi bond ka ek electron side se khaali jagah mein khisak jaata hai, jisse woh bond poora ho jaata hai aur jahan se electron aaya tha wahan nayi khaali jagah ban jaati hai. Electron ek step chala; hole ek step ulti taraf chala (Fig. 14.5, step 3). Relay ke baad relay, yeh khaali jagah crystal mein aise ghoomti hai jaise koi positive particle ho.

Electric field lagao to dono tarah ke carriers drift karte hain. Free electrons negative hone ki wajah se field ke against chalte hain. Holes positive charges ki tarah behave karte hain aur field ke saath, negative terminal ki taraf, chalte hain. Ek taraf jaata negative charge aur doosri taraf jaata positive charge, dono field ki direction mein conventional current hain. Electrons se carry hone wale hisse ko $I_e$ aur holes se carry hone wale hisse ko $I_h$ likho, to total hai

Pairs ki ginti hamesha ke liye fixed nahi hoti. Jab bhi bond tootta hai, ek pair paida hota hai, aur jab bhi ghoomta electron kisi hole mein gir kar bond ko dobara bana deta hai, pair khatam ho jaata hai; is process ko recombination kehte hain. Jaise kisi shahar ki aabaadi tab steady rehti hai jab janm aur maut balance karte hain, waise hi constant temperature par crystal us haalat mein aa jaata hai jahan pairs barabar rate se bante aur mitte hain, aur $n_i$ us balance ki aabaadi hai. Crystal ko garam karo to janm ki rate badhti hai, isliye balance bade $n_i$ par chala jaata hai. Isiliye pure semiconductor ka resistance garam hone par girta hai, jabki metal ka badhta hai.

14.4 Extrinsic semiconductor

Room temperature par pure silicon kamzor conductor hai. Usmein $5.0\times {10}^{28}$ atoms ke beech lagbhag $1.0\times {10}^{16}$ free electrons per cubic metre hote hain, yaani $5\times {10}^{12}$ atoms mein se sirf lagbhag ek ne electron khoya hai. Isse bhi bura, $n_i$ temperature ke saath tezi se badalta hai, isliye pure silicon ka device thandi subah aur garam dopahar mein alag-alag behave karega. Kaam ke devices ko kahin zyada carriers chahiye, aur woh bhi utne jitne hum chun sakein.

Ilaaj hairaan karne jitna halka hai. Har million mein kuch silicon atoms, ya usse bhi kam, ko dhyaan se chune gaye element ke atoms se badal do, to conductivity ek million guna ya usse zyada badh sakti hai. Is jaan-boojh kar ki gayi milawat ko doping kehte hain; milaye gaye atoms dopants hain, aur is tarah treat kiya gaya crystal extrinsic, yaani doped, semiconductor (बाह्य अर्धचालक) hai. Dopant ko fit hona chahiye: uske atoms ka size silicon ke atoms ke kareeb hona chahiye, taaki har ek bas waise perfect lattice mein ek silicon atom ki jagah le le.

Silicon aur germanium group 14 ke hain aur unke chaar valence electrons hain. Kaam ke dopants padosi groups se aate hain: pentavalent atoms, jinke paanch valence electrons hote hain, jaise group 15 ke phosphorus (P), arsenic (As) aur antimony (Sb); aur trivalent atoms, jinke teen hote hain, jaise group 13 ke boron (B), aluminium (Al), gallium (Ga) aur indium (In). Yeh do families do bilkul alag tarah ke semiconductors banati hain.

n-type semiconductor

Ek silicon atom ki jagah phosphorus atom rakho (Fig. 14.6). Uske paanch valence electrons maujood chaar bonds ke liye ek zyada hain, isliye chaar bonds mein chale jaate hain aur ek bach jaata hai. Is extra electron ko kaun pakde rakhta hai? Sirf phosphorus atom ka bacha hua charge $+e$, isliye yeh jodi hydrogen atom jaisi hai, lekin bahut kamzor: aas-paas ka silicon attraction ko itna kamzor kar deta hai ki electron ki orbit kai atoms par phail jaati hai. Use alag karne mein silicon mein sirf lagbhag $0.05\mathrm{e}\mathrm{V}$ aur germanium mein $0.01\mathrm{e}\mathrm{V}$ lagta hai, jabki asli hydrogen ke liye $13.6\mathrm{e}\mathrm{V}$ aur bond todne ke liye $1.1\mathrm{e}\mathrm{V}$ (silicon) ya $0.7\mathrm{e}\mathrm{V}$ (germanium) lagta hai. Room temperature ki thermal energy, lagbhag $0.026\mathrm{e}\mathrm{V}$, lagbhag har extra electron ko chhudane ke liye kaafi hai, aur peeche ek fixed $\mathrm{P}{\phantom{X}}^{+}$ ion reh jaata hai. Jo impurity is tarah crystal ko electron deti hai, woh donor (दाता) hai.

Typical levels tak doped silicon mein donor electrons thermally free hue electrons se kai powers of ten zyada hote hain. Holes ka haal pure silicon se bhi bura hota hai: donor electrons ke samundar se ghire hone ki wajah se har hole jaldi dhoondh kar bhar diya jaata hai, isliye hole concentration $n_i$ se bhi neeche gir jaata hai. Isliye electrons ko majority carriers aur holes ko minority carriers kehte hain, aur material n-type semiconductor hai, n uske negative majority carriers ke liye:

p-type semiconductor

Boron iska ulta karta hai. Sirf teen valence electrons ke saath woh apne chaar mein se teen bonds poore kar sakta hai; chautha bond ek electron kam reh jaata hai, matlab boron atom ke theek bagal mein ek hole hota hai (Fig. 14.6, step 3). Paas ke Si–Si bond ke kisi electron ko us khaali jagah mein girne ke liye bahut kam energy chahiye. Jab aisa hota hai, to boron atom poori tarah bonded ho jaata hai lekin uske protons jitna balance kar sakte hain usse ek electron zyada rakhta hai, isliye woh fixed $\mathrm{B}{\phantom{X}}^{-}$ ion ban jaata hai, jabki khaali jagah free hole ban kar crystal mein chali jaati hai. Jo impurity is tarah lattice se electron leti hai aur har atom par ek hole banati hai, woh acceptor (ग्राही) hai.

Ab sab kuch mirror image mein chalta hai. Acceptor holes thermal holes se bahut zyada hote hain, aur thermally free hue thode se electrons ko bahut saare holes jaldi saaf kar dete hain. Holes majority carriers hain aur electrons minority carriers, aur material p-type semiconductor hai, p uske positive majority carriers ke liye:

Doped crystal ab bhi electrically neutral hota hai. n-type material mein har extra free electron ke saath $\mathrm{P}{\phantom{X}}^{+}$ jaisa ek fixed donor ion hota hai; p-type material mein har extra hole ke saath $\mathrm{B}{\phantom{X}}^{-}$ jaisa ek fixed acceptor ion hota hai. Doping yeh badalti hai ki kaun se charges chalne ke liye free hain, total charge nahi.

Doped semiconductors ke energy bands

Band ki bhasha mein (Fig. 14.7), donor ka extra electron gap ke andar ek donor level $E_D$ par hota hai, jo silicon mein $E_C$ se sirf lagbhag $0.05\mathrm{e}\mathrm{V}$ neeche hai. Ise $kT\approx 0.026\mathrm{e}\mathrm{V}$ ke saath rakho to koi hairaani nahi ki room temperature par lagbhag har donor electron conduction band mein uth chuka hota hai; n-type silicon mein conduction band lagbhag poora inhi se bhara hota hai. Acceptor $E_V$ ke theek upar ek acceptor level $E_A$ jodta hai. Valence electrons utni hi aasani se in levels tak uchhal jaate hain, acceptors se jud jaate hain aur valence band mein holes chhod dete hain.

Band diagrams ki ek khaasiyat hai. Upar ka matlab hai electron ke liye zyada energy; hole ek gayab electron hai, isliye hole ki energy neeche ki taraf badhti hai. Energy paane wale electrons upar uthte hain; energy paane wale holes neeche doobte hain. Isiliye tum padh sakte ho ki hole acceptor level se valence band mein 'girta' hai. Yeh wahi ghatna hai jismein electron valence band se acceptor level tak chadhta hai.

14.5 p-n junction

Lagbhag har semiconductor device, charger ke diode se lekar processor ke transistors, torch ki LED aur chhat ke solar cell tak, ek hi structure par bana hai: p-n junction, yaani ek hi crystal mein p-type region aur n-type region ke beech ki boundary. In devices ko samajhne ke liye pehle humein dekhna hoga ki banate waqt is boundary par kya hota hai, aur phir lagaya gaya voltage, jise bias kehte hain, iske saath kya karta hai.

14.5.1 p-n junction formation

Junction do tukdon ko daba kar nahi banaya ja sakta (Example 14.3 samjhata hai kyun); use ek hi crystal ke andar ugaana padta hai. Ek common recipe p-type silicon ke ek patle slice, yaani wafer, se shuru hoti hai. Uske ek hisse mein donor atoms daale jaate hain, high temperature par surface se diffusion karke ya unhe ions ki beam ki tarah daag kar, jab tak woh wahan acceptors se zyada na ho jaayein aur woh hissa n-type na ban jaaye. Jis plane par doping badalti hai use metallurgical junction kehte hain.

Ab do tarah ki carrier motion mein muqabla hota hai (Fig. 14.8). Pehli hai diffusion: carriers bheed wali jagah se kam wali jagah ki taraf phailte hain, jaise ruke paani mein ink ki boond phailti hai. p-side holes se aur n-side electrons se bhari hai, isliye holes p se n ki taraf aur electrons n se p ki taraf diffuse karte hain. Dono motions p se n ki taraf conventional current ke barabar hain, jise diffusion current kehte hain.

Diffusion zyada der nahi chal sakta, kyunki har crossing peeche kuch chhod jaati hai. Jis donor atom ka electron chala gaya, woh lattice mein fasa positive ion hai; jis acceptor atom ka hole chala gaya, woh negative ion hai, jo utna hi fixed hai. Paar gaye carriers doosri taraf intezaar kar rahe majority carriers se milte hain aur unke saath recombine ho jaate hain. Nateeja hai junction ke dono taraf phaili ek patli slab jismein se mobile carriers khaali ho gaye hain aur sirf nange ions bache hain, p-side par negative aur n-side par positive. Yeh slab depletion region (अवक्षय क्षेत्र) hai, jo aam taur par lagbhag $0.1\mu \mathrm{m}$ moti hoti hai, yaani sirf kuch sau atomic spacings.

Nange ions ka matlab unbalanced charge, aur unbalanced charge ka matlab field. Yahan yeh positive ions se negative ions ki taraf, yaani n se p ki taraf, point karta hai, aur slab ke andar har mobile carrier par lagta hai. Jo minority carriers bhatak kar andar aate hain, p-side se electrons ya n-side se holes, woh paar baha diye jaate hain. Field se chalne wali is motion ko drift kehte hain, aur iska current, yaani drift current, n se p ki taraf, diffusion current ke against behta hai.

Yeh process khud ko limit kar leta hai, jaise koi bheed apni hi baad khadi kar le. Jitne zyada carriers diffuse karte hain, utne zyada ions khulte hain, field utna mazboot hota hai aur wapas dhakelne wala drift current utna bada hota hai. Badhna tab rukta hai jab dono currents barabar ho jaate hain. Dono behte rehte hain, lekin akele chhode gaye junction mein net current zero hota hai.

Potential ki nazar se dekho to khule charge ne n-side ko p-side se $V_0$ upar utha diya hai. n se p ki taraf diffuse karne ki koshish karte electron ko yeh step chadhna padta hai, aur p se n jaane ki koshish karte hole ko bhi, isiliye $V_0$ ko barrier potential (रोधिका विभव) kehte hain. Yeh volt ka kuch dasvan hissa hota hai: silicon junctions ke liye lagbhag $0.6$ se $0.8\mathrm{V}$ aur germanium ke liye $0.2$ se $0.4\mathrm{V}$. Chhota hone ke bawajood yeh itni patli layer ke across hota hai ki andar ka field bahut bada hota hai (Example).

14.6 Semiconductor diode

p-n junction ke dono ends par metal contacts lagao to tumhare paas ek semiconductor diode hai, ek two-terminal device (Fig. 14.9). p-side wala terminal anode hai aur n-side wala cathode. Circuit diagrams mein diode ko ek bar ki taraf point karte triangle ki tarah banaya jaata hai. Triangle us direction mein point karta hai jismein forward biased diode mein conventional current aasani se behta hai, yaani p se n; bar n-side dikhata hai. Asli diode par ek painted band cathode wala end dikhata hai.

Diode ke across judi battery apna voltage $V$ barrier mein jod deti hai ya usse ghata deti hai, is par depend karta hai ki use kis taraf se joda gaya hai. Bina battery ke junction Section 14.5.1 wale equilibrium mein hota hai: barrier $V_0$ aur koi net current nahi.

14.6.1 Forward bias mein p-n junction diode

p-side ko battery ke positive terminal se aur n-side ko negative terminal se jodo, to diode forward biased (अग्रदिशिक बायस) hai. Battery ka voltage kahan jaata hai? Diode series mein teen resistors ki tarah behave karta hai: p-region, depletion region aur n-region. Beech wala, jismein carriers nahi hain, sabse bada hai, isliye lagbhag saara voltage usi ke across girta hai. Wahan yeh built-in step ke against kaam karta hai: barrier $V_0$ se ghat kar $V_0-V$ ho jaata hai, use banaye rakhne ke liye kam khule ions chahiye, aur depletion region patla ho jaata hai (Fig. 14.10).

To phir current itna zyada kyun badhta hai? Barrier ke neeche khade carriers ko alag-alag kaabiliyat wale athletes socho, jo sab ek deewar phaandna chahte hain. Zyaadatar thoda hi kood sakte hain; bahut kam ooncha kood paate hain. Oonchi deewar ho to lagbhag koi paar nahi jaata. Ise thoda neecha karo to sirf kuch tagde koodne wale paar jaate hain, isliye current chhota rehta hai. Aur neecha karo to paar jaane walon ki ginti sirf badhti nahi, kai guna ho jaati hai, kyunki har step neeche aam koodne walon ka ek kahin bada group andar le aata hai. Isiliye forward current $V$ ke saath lagbhag exponentially badhta hai.

Paar pahunch kar carrier kya karta hai? p-side par pahuncha electron ab holes ke samundar mein ek minority carrier hai; n-side par pahuncha hole bhi isi tarah ginti mein kam hai. Ise minority carrier injection kehte hain. Agar tum diode ke saath-saath minority carriers gin pao, to woh depletion region ke theek bahar dher mein milenge aur doori ke saath kam hote jaayenge (Fig. 14.11), kyunki woh edge se door diffuse karte hain aur dheere-dheere recombination mein kho jaate hain. Yeh do diffusion streams, p-region mein phailte injected electrons aur n-region mein phailte injected holes, milkar forward current banati hain, jo aam taur par milliamperes ya usse zyada hota hai.

14.6.2 Reverse bias mein p-n junction diode

Ab battery ko ulta karo, taaki n-side positive ho: diode reverse biased (पश्चदिशिक बायस) hai. Voltage ab bhi lagbhag poora depletion region par hi girta hai, lekin is baar yeh built-in step mein jud jaata hai. Barrier badhkar $V_0+V$ ho jaata hai, use sambhaalne ke liye zyada ions khulne chahiye, aur depletion region chauda ho jaata hai. Majority carriers ke liye deewar ab bahut oonchi hai, aur diffusion current lagbhag gayab ho jaata hai.

Current ki ek patli dhaar bachi rehti hai, jise minority carriers carry karte hain. p-side se junction tak bhatak kar aaya koi bhi electron, ya n-side se koi bhi hole, field mein pakda jaata hai aur paar le jaaya jaata hai. Ek waterfall socho: har second girne wala paani upar ki nadi se tay hota hai, cliff ki oonchai se nahi. Isi tarah yeh current is baat se tay hota hai ki har second kitne minority carriers junction tak pahunchte hain, battery se nahi. Chhota sa reverse voltage bhi un sabko collect kar leta hai, aur bada voltage un carriers ko collect nahi kar sakta jo hain hi nahi, isliye reverse current voltage ke saath lagbhag nahi badalta. Yeh bahut chhota bhi hota hai: germanium diodes mein microamperes, silicon diodes mein nanoamperes. (Yahi drift forward bias mein bhi behta hai, lekin wahan injected carriers ke saamne kho jaata hai.) Garam karne se zyada minority carriers bante hain, isliye reverse current temperature ke saath badhta hai.

Har diode ki ek limit hoti hai. Ek khaas reverse voltage ke baad, jise breakdown voltage $V_{br}$ kehte hain, reverse current chhota nahi rehta aur tezi se upar bhaagta hai, yahan tak ki thoda sa zyada voltage bahut bada current chala deta hai. Woh current junction ko garam karta hai, aur agar circuit mein koi aur cheez use manufacturer ki di hui maximum value se neeche nahi rakhti, to diode kharab ho jaata hai. Apne rated current se aage dhakela gaya forward-biased diode bhi isi tarah fail hota hai. Isliye aam diodes ko breakdown se kaafi door rakha jaata hai. (Zener diodes khaas diodes hain jo breakdown mein kaam karne ke liye design kiye jaate hain; inhe voltage ko steady rakhne ke liye use kiya jaata hai.)

Current ka voltage ke against graph, yaani V–I characteristic, Fig. 14.12 ke circuit se naapa jaata hai. Ek rheostat ya potentiometer diode ke across voltage set karta hai, jise uske across juda voltmeter padhta hai, jabki series mein laga meter current padhta hai. Forward currents milliamperes mein hote hain, isliye milliammeter kaafi hai. Reverse half naapne ke liye diode ko ulta kiya jaata hai aur milliammeter ki jagah microammeter lagaya jaata hai, kyunki currents hazaar ya usse zyada guna chhote hote hain.

Nateeja Fig. 14.13 jaisa dikhta hai. Forward side par shuru mein lagbhag kuch nahi hota; phir voltage ki ek patli range mein current uchhal kar lagbhag exponentially badhta hai, itna ki volt ke kuch sauven hisse aur badhane se woh double ho sakta hai. Jis voltage par yeh uchhaal hota hai use cut-in (ya threshold) voltage kehte hain: silicon diodes ke liye lagbhag $0.7\mathrm{V}$ aur germanium diodes ke liye $0.2\mathrm{V}$. Reverse side par current breakdown tak bahut chhota aur lagbhag flat rehta hai, jise reverse saturation current kehte hain. Dono halves lagbhag hamesha bahut alag current scales par banaye jaate hain.

$V\mathrm{/}I$ jaisa ek number aise device ko describe nahi kar sakta. Tez forward hisse par yeh das-baees ohms hai, reverse hisse par kai megaohms, aur yeh point se point badalta hai. Circuit mein aam taur par yeh maayne rakhta hai ki voltage thoda badalne par current kitna badalta hai, isliye diode ke liye hum working point par characteristic ka slope use karte hain, jise dynamic resistance kehte hain:

Ohmic resistor ki characteristic origin se guzarti seedhi line hai, isliye uske har point par $\mathrm{\Delta }V\mathrm{/}\mathrm{\Delta }I$ aur $V\mathrm{/}I$ ek hi number hain. Neeche ke simulation se ise check karo, phir junction explorer par wapas jao, jahan curve ke saath chalne par $r_d$ ek million ya usse zyada ke factor se badalta hai.

14.7 Junction diode ka rectifier ki tarah use

Mains electricity alternate karti hai, lekin batteries, LED lamps aur zyaadatar electronic circuits ko aisa voltage chahiye jo kabhi ulta na ho. Fig. 14.13 ki characteristic dikhati hai ki diode kaise madad kar sakta hai: yeh ek taraf current aasani se pass karta hai aur doosri taraf lagbhag bilkul nahi. Ac source aur load ke beech diode lagao, to charge load tak sirf un half cycles mein pahunchta hai jo diode ko forward-bias karte hain. Jo circuit diodes ko is tarah use karke ac ko one-way current mein badalta hai use rectifier (दिष्टकारी) kehte hain.

Half-wave rectifier

Sabse simple rectifier load $R_L$ ke saath series mein laga ek single diode hai, jise transformer ke secondary terminals A aur B se supply milti hai, jo suvidhajanak voltage deta hai (Fig. 14.16). Kya hoga, yeh sirf is par depend karta hai ki secondary voltage kis taraf point karta hai:

Is tarah load ko gaps se alag positive humps ki ek row milti hai, har input cycle par ek hump. Uska voltage ab bhi badalta hai, lekin kabhi sign nahi badalta, aur rectified ka yahi matlab hai; aur kyunki har cycle ka sirf aadha hissa load tak pahunchta hai, circuit ko half-wave rectifier kehte hain. Block karte waqt iska bahut chhota reverse current ignore kiya ja sakta hai. Table ki doosri row mein ek design rule bhi chhupa hai: jab diode block karta hai, to poora peak secondary voltage uske across reverse direction mein hota hai, isliye uska breakdown voltage us peak se aaram se zyada hona chahiye.

Full-wave rectifier

Cycle ke dono halves use karne ke liye do diodes aur aisa transformer chahiye jiske secondary ke beech mein ek connection ho, yaani centre tap C (Fig. 14.17). Diode $D_1$ secondary ke end A se aur diode $D_2$ end B se chalta hai, unki p-sides (anodes) A aur B par hain; unke cathodes jude hain, aur load us jod se wapas C tak jaata hai. C se naapne par ends A aur B ke voltages hamesha barabar aur ulte hote hain, isliye dono diodes baari-baari kaam karte hain:

Dono hi case mein $R_L$ se current ki direction same rehti hai, isliye load ko har half cycle mein ek hump milta hai aur output pulses input frequency se double frequency par aate hain. Har diode secondary winding ka sirf aadha hissa use karta hai aur sirf aadhe time kaam karta hai, lekin input cycle ka koi hissa waste nahi hota, isliye yeh full-wave rectifier half-wave circuit se kahin behtar tarah supply ka use karta hai.

Output ko filter karna

Rectified output ek taraf behta hai lekin ab bhi oobad-khaabad hota hai; mains ke full-wave rectification mein yeh ek second mein sau baar zero se peak tak jaata aur wapas aata hai. Iske badalte hisse ko ripple kehte hain. Ise smooth karna filter ka kaam hai: sabse simple, output ke across, $R_L$ ke parallel mein laga ek capacitor, halaanki $R_L$ ke series mein laga inductor bhi yahi kaam kar sakta hai.

Capacitor ko ek paani ki tanki socho jise ek tap rukk-rukk kar bharta hai jabki uske tale ka ek chhed use lagaataar khaali karta hai (Fig. 14.18). Har rectified pulse tap ka ek jhonka hai jo capacitor ko peak voltage tak bhar deta hai. Pulses ke beech diode off hota hai aur load capacitor ko khaali karta hai, isliye uska voltage girta hai. Badi tanki ko gaps ka pata hi nahi chalta; chhoti tanki lagbhag khaali ho jaati hai. Circuit ki bhasha mein khaali hone ki rate time constant $\tau =R_{L}C$ se tay hoti hai, aur ripple tab chhota hota hai jab $\tau$ pulses ke beech ke time se kaafi lamba ho. Isliye power supplies bade capacitors use karti hain, aur aise capacitor-input filter ka output peak voltage ke kareeb rehta hai. Practice mein yahi sabse common filter hai.