REVERSE CHARACTERISTICS OF PN JUNCTION

Transcription

1 EESE CHAACESCS O N JUNCON Whe ucio reverse biased, sace charge regio creaed by ioised doors ad acceors exeds. Elecric field ad volage disribuio ca be fid solvig oisso equaio e( ND NA ) div E div grad ε rε wih corresodig boudary codiios. he abru ucio aroximaio (oe dimesioal case describig arallel lae layers), maximum elecric field i he sace charge regio of he hickess d is give by endd E max ε ε olage across he sace charge regio r ε rε E e max + N D N A

2 High elecric field causes avalache ioisaio Effecive ioisaio coefficie α( E) E A a criical elecric field E α( E) dx or N A >> N D, he N ucio breakdow occurs a he breakdow volage B ε rε E en B D or Si ca be foud B.68 8 N D -3/4 [; m -3 ] Sace charge hickess a breakdow d B N D -7/8 [m; m -3 ] B everse curre desiy is give by d B 7 4 B 3 d B m N -3 m J ( MJ ) M B κ J ed L N i D 3 < κ < 6 eid + τ SC B

3 Limis caused by surface ermiaio laar diffusio a edges resuls i cylidrical ad sherical ars of ucios. Elecric field crowdig occurs a he edge of a cylidrical ucio, he breakdow sars a a volage (B) lower ha B i he case of arallel lae ucio. or he cylidrical ar of a diffused ucio ( B) x.87+.5l d B or x < d B, he edge effec resuls i a cosiderable lowerig breakdow volage.

4 Differe yes of ucio ermiaio are used o decrease elecric field a he edge of ucio - edge bevellig Jucio Sace-charge regio Jucio Sace-charge regio guard rigs semi-isulaig layer field lae imla sacecharge layer sace-ch arge layer field lae guard rigs

5 HEMAL BEAKDOWN A a cosa reverse volage, he ower dissiaed desiy + g 3 g 3 ex ex k W A k W A M A N c κ B M hea geeraed a he ucio is coduced ou of he device a a rae ha is give by h(a) a a J a J cri o Sloe hja cri or hermal sabiliy, we require ha a < for lower emeraues h h a g g B k W k W N cos λ κ < ex 3/ for higher emearaures h h a g g B k W k W cos λ κ < + ex 3 he hermal sabiliy codiio a < is valid as far as < max

6 f emeraure locally becomes higher ha cr, i sars o icrease raidly i ha regio ill he emeraure a some oi reaches he irisic emeraure, i, he - ucio is he effecively shued by a small filame of highly coducig irisic semicoducor kow as a mesolasma resulig i a irreversible degradaio of he - ucio (a) cracks ca develo due o he hermal shock (b) he emeraure ca exceed he melig emeraure a he coac (c) he emeraure ca exceed he melig emeraure of he semicoducor o reve he hermal ruaway maximum ucio emeraure max is allowed for oeraio maximum reeiive reverse volage M < (B) o kee M close o

7 OWE DODES ower semicoducor diodes are mosly used i alicaios for ucorolled recifyig or as free wheelig diodes., A A 5 A 5 M some alicaios, a fas rasie from he o-sae o he off-sae ad coversely. is required

8 OWAD CHAACESCS O OWE DODES orward volage dro a ucio for > k s rereses volage dro across he low doed regio. esisiviy of his regio is modified by carrier iecio. Aroximaio by he -i- diode srucure + + N k e l + + i s + is he volage droed across he -i ucio N is ha droed across he + -i ucio is ha across he i regio + N K αk + e l J Carrier disribuio i he i-regio ca be foud d solvig coiuiy equaio dx L wih boudary codiios d dx d w/ dx w/ J ed d dx a d w/ dx w/ J ed

9 he carrier disribuio is give by ( x) µ µ δ µ + µ Jτ H el a cosh x sih x L a La δ sih w cosh w La La, τ H is he high iecio carrier lifeime ad L a is he ambiolar diffusio legh. he elecric field ca be exressed E ( x) J k d e δ ( µ + µ ) ( x) e ( x) dx carrier coceraio + i + -w/ w/ x egraig elecric field for -w/ x w/ e k + δ l 8b ( b + ) ( W ) ah ( W ) ( W ) ( W ) sih [ δ ] [ + δ ah ] [ ] δ ah a where W {[ ( )] ( )} δ ah W sih W w L w D τ a a H oeial b µ µ w N x

10 he charge corolled model dq d ( ) Q τ he seady sae he average carrier coceraio he resisiviy of he middle regio s Q τ av w ρ A τ ewa e av Q e av w ( µ + µ ) A wa w τ ( µ + µ ) he forward volage dro + k l e s w + τ ( µ + µ ) A high excess carrier coceraio, he carrier mobiliy decreases wih carrier coceraio, i.e. wih icreasig curre desiy Cosequely, he volage dro icreases wih he curre desiy he forward volage dro ca be exressed as J m K l + KJ,5 < m <,75 J H

11 Carrier-carrier scaerig becomes sigifica a curre desiies over. A/mm ad leads o a reducio i he carrier mobiliies. he ambiolar diffusio legh L a decreases as he carrier coceraios rise wih curre desiy m K + K l J + KJ.5 < m <.75 for w > L K w ~ ex L J m Liear aroximaio + ( O ) r A is he ye (omial) curre r π A differeial resisace is he hreshold volage

12 ANSEN OCESSES N OWE DODES here are imora dyamic rocesses a work durig he rasiios bewee he o-sae ad he off-sae, which lead o areciable differeces bewee he seady-sae ad he dyamic characerisics. o-sae characerised by floodig ier layers wih high iecio of excess carriers off-sae characerised by sace charge regio of ioised imuriies o ucio, / m x / µm N A -N D /m -3 ] Solvig rasie rocesses eeds simulaous soluio he coiuiy equaio ad he oisso equaio D x x + x ( µ E ) diff x τ div E div grad d w N x / µm e( + N ε ε r D NA )

13 he rasiio from he everse- o he orward-biased Sae A he begiig of he diode ur o, o excess carriers are i he high resisace regio. resuls i a relaively high forward volage dro whe a forward curre ulse wih a high di /d is alied. Excess carrier coceraio i he middle regio of he diode icreases wih ime ad hece is resisace, dimiishes. orward volage droed across he diode ormally rises o a maximum, afer which i decreases wih ime o he seady-sae value. his akes a ime of he order of w /8D a, which is yically i he rage. µs o µs, for values of w bewee 5 µm ad 5 µm. M.9 M. M r M. fr

14 everse ecovery rocess wo ses of he rasiio rocess:. emovig excess carriers from ier layers. Buildig u sace charge regio o he ucio i he firs he curre falls o zero ad reverses a rae deermied by he exeral circui excess carriers are swe ou ad recombie s f carrier coceraios a he - ucio reach he levels of hermodyamic equilibrium.5 rrm he formaio of a sace-charge regio ad reverse volage icreases. he reverse curre sars o fall i a way ha is corolled by he diffusio ad recombiaio rocesses i he base regio ciruis wih serie iducace he chage i di/d iduces a overshoo of he reverse volage ( di d) + s rrm + di d f rrm s rrm o serie iducace

15 he sored charge aroximaio he chage of he excess mobile carrier charge Q dq d ( ) τ Q i( ) τ H H i( ) Q( ) τ H τ H ex τ H + he curre reaches is maximum value rrm whe sored charge reached value Q A he same ime sace charge regio a he ucio was develoed Suosig exoeial decay of he curre i + s ( ) rrm ex τ eff + s ( ) Q ex τ eff Q

16 everse recovery charge is coeced wih he rocess Q rr rrmrr s di d rr A deailed aalysis, based o he coiuiy equaio, shows ha τ eff wτ τ Cπ D * H * H a + w where w is he hickess of he regio sill flooded wih carriers rom his aalysis follows ha N diodes have fas decrease of reverse curre hard recovery.

17 M - rr rade-off for fas reverse recovery diodes

18 o avoid oscillaios i circuis, sof reverse recovery ( f s ) is desirable; i.e. o shore s ad rolog f. o solve his roblem, i is ecessary decrease carrier coceraio a -layer carrier coceraio i o-sae τ cos axial lifeime gradie -w/ w/ x elecro irradiaed Axial carrier lifeime gradie (a he shores carrier lifeime) - roo ad helium imlaaio - iridium diffusio? r diffused

19 ower Schoky Diodes Charge rasor i he Schoky diode is by maoriy carriers. ime cosas associaed wih he ur-o ad ur-off rocesses are limied oly by he ime required o charge ad discharge he caaciace of he sace charge regio a he meal-semicoducor coac. A - + K arameers of he diode are fucios of he resisiviy of he -ye layer, ρ N 3 4 B ρ N S ρ N C ρ N - W BN W c wih icreasig breakdow volage, he forward characerisics deeriorae ad he ime cosa, C S, ges loger W v B N N+ c A a Schoky coac of a fiie area, (B) - + A Eiaxial layer Subsrae K or M > he diffused guard-rig is ofe used + A K

20 Merged ower Schoky (MS) diode A fas ower diode ca be formed by he arallel iegraio of a M-- + Schoky diode ad a -- + ucio diode (described as MS, SN, JBS diodes) J A/mm - MS diode -- + diode Schoky diode - A + Schoky diode A (a) K 5 µm (c) K i Schoky MS N Schoky diode (b)