2_1_f-b pn junction pt1 Lecture 15.1

2_1_f-b pn junction pt1 Lecture 15.1 - Forward biased p —...

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Unformatted text preview: Forward biased p — n junction A voltage source VI . atteh p . I] as a battery 15-. eenneeted with the punitive tenninal to the p -- side and the negative terminal to the n — side. Forward biased p — n junction I] A voltage source VI . aneh p as a battery is. eennected with the punitive tenninal to the p -- side and the negative terminal tn the n — side. As per our earlier discussion, the Fermi {eve} in the neutral n — region is shifted upward by an amount equal to qVF with respeot to the Fermi level in the neutral p e region neutral n — region Ilfilltl'fll p — region Ea. under fijrward bias. is shifted upwards by an 31113th q‘VF in that neutral :1 — ruginn uncle-r fnnwsrd hiss with FESpuct tn where, it was in the p — rsginm Under anvsrd Elias? harrisr hsight is q{vhi _ VF} The space shsrgs rsginn ssrrsspsnds is s rcducsd barrier hsight q[‘w’hi — VF], The width of" this spacs Chargi: rcginn can be determined by solving Phisssn squalisn 01‘ [mm Gauss Thssrsm ss hcthrs- Barrier height in thermal Equilibrium is qvhi : [min + Li 95.1 The expression fer the widths cf the space charge regicn will essentially be cf the same form except VH1 will be replaced by Vb. —VF The physical reason why the barrier height is less is because less amount of charge is needed in the space Charge region to support a reduced barrier height. Thermal Equilibrium Forward Bias Electrons in the 11 side 1arith energy above the barrier height will diffuse to the 13 side. Under forward bias more electrons have higher energy and will be able to dif'Fuse to the p — side resulting in an increase in the di fusion current. Similarly mere hules will diffuse frem the neutral p — regien tn the neutral n — reginn. Due tn the dif‘Fusi-en ef eleetrnns tn the p — side, the minnrity earrier {:lensitj.r in the [3 —side inereases. Si-imilarl}r due tn the difl‘usien ef hnles, the minerit},r earrier density in the n — side inereases frem its thermal equilibrium value. This prneess is ealled minnrity carrier injection. In thermal equilibrium, there was a diffusien eurrent and a drift eurrent flewing in eppesite elireetiens and the twn eempenents were equal resulting in aere net eurrent. But these twe eenlpnnents are very large in eeniparisfln with the diffusiun eurrent under Femat‘d bias. This a]lews us te use Beltaman relatien even under femard bias whereas Beltanian relatien is valid nnl}r under eenditiens ef aere eurrent flew. As befnre 1w: “will Chfl-flflfl the flrigin K = D at lhfi metallurgical junctifln. I _ l , I“:.—1"""—:7"I :f—‘HH—L‘FI flflllll‘ill Ilttutl‘fll [ 3.1“ + 11'“ ] np(E:—EP)=HH(K:KII)e—q RT Jul—v.) pints; Efl)=pn(x=—xp)fiq kT £p(x:xn):pn(x=fln —pn{} an(E:—xp)=flp(x:_xp)_flnfl pp(K=—J{p app” [Jud-album I'I'Ifldflt‘fltfl inj Edi-n: _q I _ _ _ _ ILT nn{x— xrj—nn{x —H”]fi 15- 'u =I] E—q fl+flqfi L'a-l'll 1|." El _... k'l' qr: — —_ _ _... I-c'l' _ fin—nn{1— xp flm—I'In” I: ] Similarly pn{x=xn]=pp[x=_xp}flq kT _qxu HIE = pail-E: ET: kT qfl = E it'l' PH” , I“? q fipix = x“): I-:'I'I [x : En}_pnfll = pm] E kt _1 Example : Calculate the width at" the space eherge regien. uf an abrupt p—n junctien 1with a net impurity density elf' "MA = lfl'fiem 3 and NU z lfl'iem 3-311 the n —side. a] in thermal equilibrium and h} under a ibrward him: (if [1.4 Vie-It. |-I'- IS em. =.U259[II1[%H = fleeev 3] Thermal eqmlibrium —1-1 lfi i xp: wxfim =.fl336xm‘cm 1.5mm x-flfix(1n”+mm) Similarly x“ =.336x MEI—‘1 cm xd = Kn + Er. =[.fl335+.336]x1fl"cm =.9T5Hlfl'”cm b} Forward Bias; {1f (1.41.? The bflTTiEI‘ height is Vhi — VF = 0.65 — 0+4 = 0-256%? “HF and 3;“ calculated in part a) has 0.256 0.6513 tr:- he multipliad by .05533x104cm +553H1EF4£=m K n- 3': l1. ...
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2_1_f-b pn junction pt1 Lecture 15.1 - Forward biased p —...

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