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e_| +. Semiconductor Electronics - Class 12 _ ore a 2 ae = “From tiny crystals to powerful circuits — semiconductors drive the modern world.” * s (— ConTENTS (10 Pages) 4. INTRODUCTION QD Introduction & Bases -Py1 | © Semiconductor electronics deals with the study of Q) Intrinsic. Semiconductors ~ Py 2 materials whose electrical conductivity lies between, ® Extrinsic Semiconductors (n-type & p-type) - Py 3 that of conductors and. insulators. GO) fon gation Dinl Sey Fo S controlling. their Biss th dyin, eine fe voltage, temperature or light, we can bul 2 ae ohh Z nS Se devices like diodes, transistors and ICs. ode Applications - Py : : D) Bipolar Junction Transistor (BIT) - Py 7 eee +s thd acevo a odee Transistor Configurations - Py 8 J See ee @ Applications of Transistors -hy4 Definition : Semiconductor *] S £ Points Po 10 A semiconductor is a material whose electrical (ei See i) ealcATAADT atirmadials Mibtaeen that eC | conductor and an insulator and can be controlled. | 2. CONDUCTORS vs SEMICONDUCTORS vs INSULATORS 1 Property Conductors Semicconductors Insulators , : " Very high Moderate Very low Electrical Conduct fms ery ay yD) (a = 108 - 10° S/m) (o = 107° - 10° S/m) (o = 10°” - 107 S/m) ae Very low Moderate Very high Resistivity ( a jo. ery hig) a) (p= 10°* - 10° 2-m) (p = 10°* - 10° +m) (p = 10% - 10” 2-m) E, x0 Energy Band Gap (E,) 4 4 Small Large Wigraed Spe al I (E, © 0.1 - 3eV) (E, > 3ev) Examples Copper (Cu), Silver (Ag), Silicon (Si), Germanium (Ge), Glass, Rubber, Mica, Aluminium (Al) Gallium Arsenide (GaAs) Diamond. 5) | | 3. EXAMPLES OF SEMICONDUCToRS | 4. KEY PROPERTIES oF 1 5. WHY SEMICONDUCTORS ARE Thee alt ots \ SEMICONDUCTORS \ IMPORTANT ? ilicon (Si) = Most wi semiconductor. | ae |i. eT ae 5 eee wee, tlle mie (Si, | ¥ Conductivity Lies betwen conducts; % Form the bass of diades, transistors, # ‘ be afsg < 1 and insulators. 1 ICs and microprocessors, SsGemuniin (Ge) = higer mebly Dan, ih a kL Enable miniaturization and high-spod i aa Canes garb a oe ree | operation of electronic. circuits. % Gallium Avsenide (GaAs) - Compound | "PORE |e Used in solar cells, LEDs, photodetectors semiconductor with high electron mobility; | * Pure semiconductors have both | and sensors. used in Kigh-sped & oplclectronc devices. | tletrans and oles as charge | Found in every modern device ~ from f : \ carriers, \ smartphones to satellites, Crystal structure of | * Conductivity can be controlled by | Conventional current # Sin (Diamond cabic) | daping (adding imparts. '- fepomenmone|_¢ @ Hole Each Salon fms Foe | They are covalent crystalline solids | 9@ 290 Ques. naighbouring atoms. | with a small energy band gop. | La i) i) Both electrons and holes contribute to conduction in semiconductors, aK¢ in kqalcnualt 2 ee ee ec ee ee | Semiconductors have unigue and controllable clectrical properties that make them the heart (wm)! | of all. modern electronic: components and technologies. ' ~. Semiconductor Electronics - Class 12 J hee SS Se “Pure in composition, yet full of promise — intrinsic semiconductors are nature's perfect balance.” * ( 1. INTRINSIC SEMICONDUCTOR - MEANING ) * A pure semiconductor that is not intentionally deped with any impurity is called an intrinsic semiconductor. © Charge carriers are generated only by thermal + Example : Pure Silicon (Si) and Germanium (Ge). ~ PuRE Si AND GE g | 6 Both have four valence. electrons. (Ge pa ae Ch = | © Silicon is a Group 14 element (atomic number = 14), | © Germanium is also Group 14 (atomic number = 32). |-© They form covalent bonds in a diamond cubic erytal | structure. = ic 3. CRYSTAL STRUCTURE AND COVALENT BONDING In Si or Ge, each atom is surrounded tetrahedrally by four nearest neighbours. © Each atom shares its four valence electrons with four neighbouring atoms to complete octet. # Strong covalent bonds hold the crystal together. © At absolute zero (0K), all valence electrons are tied in covalent bonds. x pate ELECTRONS x | « Si and Ge have four valence electrons (configuration ns® np*). : pele eo arsece: = | I Covalent | | covalent. bonding. | | a a ee ot 2s ok ae present at OK. nee ef Silicon (Si) Mer er = 5. BEHAVIOUR AT 0 K AND ROOM TEMPERATURE | 6, ELecTRON-HoLE PAIR GENERATION Condition Behaviour Epa a | 0 ale a a fe eel is exci conduction band. ALOK ee Se pe ermas sap sy PPE |i) eette Leas ichenth a feecaat place (hale), én ithe ) in covalent bonds. bonds. Semiconductor behaves | | ey (Abeolike zero) | Ne rec elactrons, ro. holes. | Ue) an, insalator. | jee | ® Thus, an electron—hole pair is created. # Some covalent bonds break Electrons jump to conduction | | ° © = Electron At Room due to thermal energy. band leaving behind holes | | Om Om Cues =Om O = Hole Temperature (300K)! . Electron —hele pairs ert in valence band. | 7. te generated. Conduction occurs. fe oe ee Oe a eee i} 7. THERMAL GENERATION @ Thermal energy at room temperature continuously generates electron—heole pairs in intrinsic semiconductors © Number of electron —hole pairs generated per unit time per unit volume is called the generation rate (G) © For intrinsic semiconductor, n= p = nj (no, of electrons » no. of holes = intrinsic carrier concentration ). 10. ENERGY BAND DIAGRAM OF INTRINSIC SEMICONDUCTOR | 8. RECOMBINATION | # A free electron may recombine | | with a hole and disappear, | | * The energy released is usually | | | ! I | ' % The two types of charge carriers in intrinsic semiconductors are : (Electrons (negative charge) (ii) Holes (positive charge) Both contribute to conduction. given off as heat (lattice vibrations ). ! * Recombination reduces the | number of free carriers. Conduction. Band. (CB) aa Forbidden Gap (Ey) Valence. Band (VB) 12. INTRINSIC CARRIER CONCENTRATION (ni) conduction band is empty, At room temperature > Some electrons move to conduction band. # Ey (band gap) = E, = 0.66eV (Ge) at 300 K. e pe ie 14. Conbuctivity oF INTRINSIC SEMICONDUCTOR Conductivity, o = g (np, + Pimp) where, n= electron concentration, p = hole concentration = electron mobility, 4p = hole mobil ae electronic charge (1 "x 10" c) For intrinsic semiconductor : *- o Valence band is full, 1.12 eV (Si) n= pan qn (Hn t Mp) ae “Key Points + nz = (NeNy 7 9/247 where, Nc = effective density of states in conduction band Ny = effective density of states in valence band k® Boltemann constant (1.38 «1072 JK") T = absolute temperature (in K) Ey = band gap (in eV) 1 1 * { % As temperature increases, mare electron — hole | pairs are generated. | te nm increases exponentially with temperature | Therefore, o (conductivity) increases rapidly | sith temperature. | te At high temperature, intrinsic semiconductor shows significant conduction Intrinsic = Pure, undoped. semiconductor Carriers are generated thermally. hehe aes > = - Semiconductor Electronics - Class 12 - as hae Z as “From tiny crystals to powerful circuits - semiconductors drive the modern world.” * (* 3. m-Type And p-TyPe SeMIcoNDucToRS +) By introducing a small amount of suitable impurity atoms (doping) into intrinsic silicon or germanium, we can greatly increase conductivity and control the type of majority charge carriers. (A. n-TyPE SEMICONDUCTOR (PENTAVALENT Dortne) } ( B. p-TyPe Semiconbuctor (Trivacent DoPina) ) & Formed by doping Si/Ge with a pentavalent (Group V) impurity such as Phosphorus (P), Arsenic (As) or Antimony (Sb). & The impurity atom has 5 valence electrons * Four electrons form covalent bonds with the four neighbouring Si/Ge atoms. The fifth electron is loosely bound and becomes a free electron at room temperature, % This free electron can move freely in the crystal. and contributes to conduction. 1% Formed. by doping Si/Ge with a trivalent (Group IIL) impurity such as Boron (B), Aluminium (AL) or Gallium (Ga). tr The impurity atom has 3 valence electrons. % Three electrons form covalent bonds with three neighbouring Si/Ge atoms. + The fourth bond is incomplete, leaving a hole (absence of electron). + A nearby electron can jump into this hole, leaving behind a new hole. Thus, holes appear to move through the lattice, | | | | 1 | | | | | | | | | | 1 1 | i I | | | \ | \ | | | | i | | | | LATTICE DIAGRAM (n-TYPE) 3) LATTICE DIAGRAM (p-TyPE) ) ea © Silicon atom © Stizon atom eer hee j~ Hole (absence t eae ely @) Peatavalent ae) ® * (ree to move) impurity (P) ines (2) fe yColant) SITE BY © Covalent electron © Free electron (majority carrier) ee ea ee ee 37 Ct ft oo mar ae eS = | | & Majority carriers Electrons (-) H 1% Majority carriers Holes (+) ! aor ee Ss | Minty cries: Eleteme (-) | ) *) C. ComMPARISON BETWEEN M-TYPE AND p-TYPE SEMICONDUCTORS * Property n-Type Semiconductor p-Type Semiconductor Dopant used. Pentavalent: atoms (P, As, Sb) Trivalent atoms (B, Al, Ga) Valence electrons of dopant 5 3 Bonding in lattice 4 covalent bonds + 1 extra electron 3 covalent bonds + 1 incomplete bond Majority carriers Electrons (-) Holes (+) | Minority carriers Holes (+) Electrons (=) Charge of ionized impurity Positive donor ion (D*) Negative acceptor ion (Aq) Contribution to conductivity By mobile electrons By mobile holes Example materials Si:P, Si: ahs Ge: Sb Si:B, Si:Al, Ge: Ga Intrinsic C.B. (D. Ferm Lever Suirt (Quauitative) + In an intrinsic. semiconductor, the Fermi level (Ep) lies near the middle of the band gap. * In n-type semiconductor, E, shifts upward towards the conduction band (more electrons) & In p-type semiconductor, Ep shifts downward towards the valence band (more holes) | v_E. Key TAKEAWAY ie Doping transforms intrinsic semiconductors into n-type (electron rich) or p-type (hole rich) materials. | — < Semiconductor Electronics - Class 12 * “From. tiny crystals to powerful circuits - semiconductors drive the modern world.” x (+5. p-n Junction Diope +) 1. FoRmMaTION - How p AND n REGIONS ARE JOINED 6. Junction Dracram A p-type semiconductor is created by doping Si/Ge with c - = acceptor impurities (e.g., B, AL). ¥E Depletion (Space charge) region a ba par aa ‘4 An n-type semiconductor is created by doping with donor P rai a CPs impurities (e.g., P, As, Sb). a © e916 Oieleo,g ° When p-type and n-type materials are brought into intimate hen 6 @! et Socket fa jemi junctions farmed rial © © Ih he 2 © flow A yO Hope © ie 2. DIFFUSION AND RECOMBINATION 25 |f-25 Piol H i eileen ° | e /9 aK) 1@? * Electrons (majority carriers in. n-region) diffuse from n-side ic} G16: ® an iC) to p-side. o0 | 2 oab@x Dis Holes (majority carriers in p-region) diffuse from p-side to ° a | 91@/O1@1o 0 n-side. Near the junction, electrons and holes recombine with each other. © = Hole © = Electron 3. DEPLETION LAYER (SPACE CHARGE REGION) © = Acceptor ion (A) @ = Donor ion (D*) eh 3 at (Wi tet ie recombination occurs near junction > | * Negative acceptor ions (A) on p-side, | immobile ions remain > depletion region + electric field. i + Positive donor ions (D*) on n-side. UW --- = == ---------- Fa) % This region, devoid of free charge carriers, is called the depletion layer. 7, Wey DIFFUSION CurRENT BALANCES DRIFT CURRENT ? % Tt acts as an insulating region between p and n. Diffusion current (due to concentration gradient) drives majority carriers across the junction. Electric field in depletion region causes drift current in the opposite. direction. % Ab equilibrium, both eurrents are equal in magnitude and opposite in direction -» Net current = O. % This equilibrium is stable; any small disturbance restores it. 4. Barrier PoTeNTIAL (JUNCTION PoTENTIAL), Vo The immobile ions create an electric field (E) directed from reside (+) to p-side (-). x This field sets up a potential difference across the junction, called barrier potential Vo. * It opposes further diffusion of majority carriers. 5. EQuiLisRium CONDITION * Diffusion continues until the electric field becomes strong enough that drift of carriers due te this field balances diffusion. & At equilibrium: Net current across the junction = 0. [Dynamic equilibrium exists: diffusion current = drift current] 10. PoTeNTIAL BARRIER (TyPICAL GRAPH) i 9. FORMATION STEPS (SUMMARY) | © p-type and n-type semiconductors are joined, Pp n | @® Electrons and holes diffuse across the junction. i | \ i) | 8. SymBoLic REPRESENTATION I I I | I I I I I | | | I I + i 7 @ Recombination occurs near the junction, p-side Depletion pth, ! u @® Uncovered ions form depletion layer. region Anode (p) ® Electric field & barrier potential develop. © Equilibrium established: Iyige = Lgriee - ¢ 11. Errect oF TEMPERATURE ON BARRIER POTENTIAL + CONCEPT HIGHLIGHT — DEPLETION REGION * = 4 As temperature increases, more carriers are generated, The depletion region is a thin space-charge region around the junction ~ Sof ~ This increases minority carrier concentration and reduces that is free from mobile charge carriers (electrons and holes). the width of depletion region. Tk contains only fixed ions — AW on p-side and D* on n-side. Therefore, barrier potential Vo decreases slightly with increase | It behaves like an insulator and is responsible for the barrier potential in temperature (= -2 mV/'C for Si). that prevents further diffusion of majority carriers. PDO, 9 sqraancors *. Semiconductor Electronics - Class 12" rue 470 > a = = “ From. tiny crystals to powerful circuits — semiconductors drive the arse world.” Miodegr)) —p- Cathode (-) <+ 4, DIODE APPLICATIONS + » —_—— SE ("Diodes are the most widely used semiconductor devices. They find applications in _ | 7 S 1. RECTIFIER CONCEPT 3x © A rectifier converts AC (alternating current) AC Input o— into DC (direct current). ANG. |» DC Output * Diodes allow current in one direction only. o—J | (Dione Circuit) | _------ © AC negative half-cycle is blocked, leavir ‘ Wan tee a Lut ve) Block diagram of a rectifier circuit ] 2. HALF-WAVE RECTIFIER 7 D | 3. FuuL-WAvE (BripGe) RECTIFIER ¥ © Uses a single diode. ‘ 12 Uses four diodes in bridge configuration. 4 © Only positive half-cycles ac 1 * Current flows through the load in the pass through the diode, Input R i same direction in both half-cgcles. AC aw []R. vo © Output is pulsating DC Y 1 Output is smoother DC with ripple Input ke x, | with ripple Frequency = f. {Frequency = 2f. 22 Input (AC) Output (Pulsating DC) H Input (AC) Output (Full-wave DC) i) 1 Nae t => AAA, t j t => : L : ork t \ - 4. LED (Light Emitting Diode) 5. PHOTODIODE 6. SOLAR CELL (Photovoltaic Cell) F. ZENER DIODE AS ww -~@- VOLTAGE. REGULATOR © pi ° ° [> ° “ay FO + & A 2 Anode (+) Cathode (-) |} Anode Cathode 3 z) © Emits light when forward biased. | | * Works in reverse bias. * Converts sunlight directly * Operated in reverse breakdown. © alors depend a mabriel used | | Comrarts iced Ugh nb carr, | | ‘0 elstteatlenegy: 4 Maintains nearly constant volage. Lighting + LEDS in bulbs, indicators, displays, street lights. | & Low power consumption. | > Communication + Photodiodes in fibre epics, IR remotes ! i Fost tla ad ah affcaae | > Energy + Solar calls ie panels, Zener diodes in regulators. GQ | % Can be made te work for a wide range of aplication. | % Electronics + Clipping/clamping in audio systems, oscilloscopes. YZ | Essential building blocks of modern electronic circuits | > Protection + Diodes protect circuits from reverse polarity. Ww} ru | Key Takeaway From rectifying current to glowing LEDs and harvesting sunlight ~ diodes are tiny devices —()— | \ with a huge impact on our daily lives! | Semiconductor Electronics - Class 12 yee “From tiny crystals to powerful circuits - semiconductors drive the modern world.” * 8. BrroLak JuNcTION TRANSISTOR (BJT) + + A BJT is a three-layer, two-junction semiconductor device in which current is controlled by both electrons and holes. 1. Types oF TRANSISTORS 7 2. TERMINALS +r + npn Transistor : The two outer layers are n-type | © Emitter (E) : Heavily doped region. p-type (Base). * pnp Transistor : The two outer layers are p-type (Emitter & Collector) and the middle layer is n-type (Base). ¢ i | | i | | | i | | cS ' (Emitter & Collector) and the middle layer is | 6 Base (B) : Very thin and lightly doped region. 1 * Collector (C) : Moderately doped and physically larger region. 4. Basic ConstRucTIOoN & DopING +r © Consists of two p-n junctions placed back to back, * Emitter : Heavily doped > injects charge carriers. * Base : Very thin (= 1-10 um) and lightly doped > allows only a small portion of carriers. 3. SymBots npn Transistor pnp Transistor | | e € | | | | & LB | | E | E | : Emitter current (largest) : Base current (smallest) : Collector current (nearly as large as Te) } cr. #.__BIASING IN ACTIVE REGION 7 ° E-B junction : Forward biased (lowers the barrier, carriers injected) , \ | if | I * C-B junction : Reverse biased \ H ' | | | (increases barrier, carriers collected) © Result : A small change in Ip controls a large change in Ic. 8. CurReNT RELATION xy } ine eh pee | | | | | 6. TRANSISTOR ACTION +r © When the emitter-base junction is forward biased and the collector-base junction is reverse biased, the transistor operates in. the active region. © Electrons (npn) or holes (pnp) are injected from the emitter into the base. Because the base is very thin and lightly doped, most of these carriers do not recombine and are swept into the collector by the reverse biased C-B junction. ° Te : total carriers injected by emitter Tg : small portion that recombines in base Ic : remaining carriers collected by collector a » TYPICAL USES OF TRANSISTOR +r Amplification of signals (audio, RF, video). Switching applications in digital circuits. Oscillators and waveform generators. Voltage regulators and current sources. Used in computers, communication systems, television, radios, smartphones, etc. CBE * a (alpha) : Current gain in common-base configuration. a= oa (040 <1, typically ~ 0.95 to 0.998) Gl +e ee ele]? (eo ee, eae Wi ===> = =a, | 11. Structure DIAGRAMS yr ' 1 | Sheet Seong) | | i | ! ne Pp n fo \ | | ! | | f i Hi \ t i | | Heavily Very thin, Moderately | Heavily Very thin, Moderately | | doped. Lightly doped doped i doped. Lightly doped doped. | a) npn Transistor Ng aN FT *. Semiconductor Electronics — Class n* Dito: Jt Se Logic Gates, Summary and Quick Revision ~~ Ns “Think Binary. Build Logic. Create the Future.” * 4. DIGITAL ELECTRONICS & BINARY IDEA 2. LOGIC GATES : SYMBOLS, TRUTH TABLES & FUNCTIONS © Digital electronics works with two discrete states: 0 and 1. = Low (OFF) © Any information (data, sound, image) 1 HIGH (oN) is represented in binary form. © Logic gates are the basic building blocks that process binary inputs to produce a binary outputs. 3. CHAPTER SUMMARY (At a Glance) & Conductors, Semiconductors & Insulators - basic ideas. x Intrinsic semiconductors - pure Si/Ge, equal electrons & holes. Extrinsic semiconductors ~ doping to make n-type (donor) and p-type (acceptor) % p-n Junction Dicde - formation, depletion region, biasing, V-I characteristics & applications. Bipolar Junction Transistor (BJT) — structure, operation in CB, CE, CC configurations, current. relations % Transistors as amplifiers & switches - amplification factor, cut-off & saturation. Logic gates - basic gates, truth tables & applications, if Truth Table One-line Function Gate Symbol 7 ae NOT = eae 4: A Y - 1 0 (Output is. complement (Inverter) OR Digital Si 0 0 0 mh A Oo 4 4 A+B OR ls Yo) 4 0 | 4 | (Cutput te 4 of ony 11 1 input is 4) t> 0 0 0 AND |e Pape wee Ape B— io 0 (Output is 1 only if 11 1 both inputs are 1) r 0 0 1 TE al 04 4 : seu B ap= u 10 1 (NOT of AND) 14 0 n 0 0 1 ast 041 0 + NOR DY 1-04) —9 (NOT of OR) 1 0 A Pele AOB XOR |, Yet t (4 when inputs ore 14 0 different) 00 1 —— xor | 4 pd as We 0 ae 1 B 10 () (4 when inputs are el 1 same) IMPORTANT DEFINITIONS * We 4 = (@®) Intrinsic Semiconductor : Pure semiconductor without @ Extrinsic Semiconductor : Semiconductor doped with any intentional impurity (e.g., pure Si or Ge). 5. Key FORMULAS & RELATIONS 6. Very LIKELY BoaRD QUESTIONS @® Ie=IstIe impurities to increase conductivity (n-type or p-type). aD @® Blbeta) = 3 Ic = Blp I,= ale, G) Depletion Region + Region around a p-n junction ® devoid of mobile charge carriers ‘a ® Diode + A two-terminal semiconductor device that allows .C) current mainly in one direction. ® © Transistor (BIT) : A three-terminal device used for | | @) amplification or switching (Emitter, Base, Collector). @ = oS SSeS = 7. Quick Revision CHECKLIST (KCL in transistor) (Current gain) (Active region) ie ce) Vee (Si) 0.7 V, Vege (Ge)*0.3V T= 1, (e%r%- 1) (Diode Eqn) Vy = ne 26 mV at 300K 26 mV : ir’ ae T fmA) (Small signal) & Draw V-I characteristics of pn junction diode and explain its working. Explain formation of depletion region. Differentiate between intrinsic and extrinsic semiconductors. Draw and explain BJT in active region. Derive relationship between Iz, Ig and Ig Explain transistor as a switch with suitable circuit. % Draw symbols and truth tables of basic logic gates. Which gates are universal ? © Can I distinguish between n-type and p-type? © Can I draw V-I curve of a diode? DD Do I remember transistor configurations? DO Can I write current relations in BJT? Di Can I draw all logic gate symbols de tables? DD Am I comfortable with formulas & definitions ? Tf YES to all, You're Exam Ready! “> FINAL TAKEAWAY 5 8. FINAL TAKEAWAY /+2@ Semiconductor electronics is the brain behind modern technology. i | | | From ting diodes to powerful processors, everything works with logic, precision and imagination. Keep Practicing, Keep Improving! | | + Believe in Yourself - You've Got This! 
