I-V and Switching Characteristics of Back Illuminated OPFET using Finite Difference Methods

Transcription

1 tratoal Joural of Coutr Alcatos ( Volu 3 No.4, Octobr -V ad Swtchg Charactrstcs of Back lluatd OPFET usg Ft Dffrc Mthods Rajsh B. oha Dt of Elctrocs ad Tlcoucato Goa Collg of Egrg Faragud, Poda, Goa-da Jaa V. Gatod Dt of Elctrocs ad Tlcoucato Goa Collg of Egrg Faragud, Poda, Goa-da ABSTRACT OPFET (Otcal Fld Effct Trasstor s a usful c for otcal coucato ad as hoto dtctor. ths ar, th -V charactrstcs of th back lluatd OPFET ar lottd usg ft dffrc thods b solvg th wthout t ddt cotut quatos whch th cdt radato s allowd to tr through th substrat b srtg a fbr artall to th substrat. Th swtchg t of th c has also b lottd. Gral Trs Prforac, Thor. Kwords OPFET, hoto dtctor, ft dffrc thods, back lluato.. NTRODUCTON At rst, otcal fbr coucato las a ortat rol cabl coucato tcholog for wdbad, ultda ad hgh-sd alcatos []. Th fudatal hscal chas bhd otcal lluato s th absorto of hotos th valc bad of th atral thrb cratg lctros ad hols. Th hoto-gratd lctros cotrbut to th dra-sourc currt wh a dra-sourc voltag s ald ad th hols lo a hoto-voltag at th Schottk jucto ad th - jucto of th c rsultg th odulato of th chal coductac []. A cotut quato s a dffrtal quato that dscrbs th trasort of so kd of cosrvd quatt; ths cas, t s th carrr dst. Solvg th cotut quato to obta th currt-voltag charactrstcs s a ajor ara of rsarch. Estg rsarchs clud rturbato thod [3] ad aaltcal thods [4]-[7] to solv th cotut quato of th OPFET. ths ar, th ft dffrc thods ar usd to solv th cotut quatos usg ft dffrc quatos to aroat drvatvs.. THEORY Th schatc structur of th o-latd GaAs OPFET wth back lluato s show Fg., havg th fbr srtd artall to th substrat so that th absorto taks lac both substrat ad actv rgo. Th dra-sourc currt flows alog th -drcto ad th lluato s cdt alog th -drcto of th c. Elctro-hol ars ar gratd du to absorto of hotos th utral substrat rgo, th Sourc + r s Gat -GaAs S-sulatg substrat hv hv Dra + Fg : Schatc structur of th c wth fbr srtd artall to th substrat []. actv lar-substrat dlto rgo, th utral chal rgo ad th Schottk jucto dlto rgo. Th otcall gratd lctros ov toward th chal ad cotrbut to th dra-sourc currt wh a dra-sourc voltag s ald whl th hols ov th oost drcto. Wh ths hols cross th jucto a hotovoltag s lod. Ths voltag bg forward basd rducs th dlto wdth of both th juctos []. To us a ft dffrc thod to solv a robl, o ust frst dscrtz th robl's doa. Ths s do b dg th doa to a ft ubr of quall sacd ots as show fgur [8]. r d z d=.µ 5

2 tratoal Joural of Coutr Alcatos ( Volu 3 No.4, Octobr J s th lctro currt dst gv b [] J qv qd (8 J s th hol currt dst gv b [] J qv qd (9 Fg : Ft dffrc dscrtzato of th robl doa alog th drcto wth quall sacd ots or ods [8]. Th drvatv / ca b aroatd at th ot svral was. Th forward dffrcg tchqu uss ( Th backward dffrcg tchqu uss ( Th ctral dffrcg tchqu uss Aroatos of dffrtal quatos of ordr ca b gv as: Th t ddt cotut quato for lctros ca b rrstd o dso as [] (3 (4 (, t J (, t (, t Rs G t q S Th t ddt cotut quato for hols ca b rrstd o dso as [] (, t J (, t (, t Rs G t q S G s th volu grato rat of carrrs ad s assud to var otall wth dstac. Takg th surfac as th rfrc sc th radato s cdt fro th surfac sd, follows th followg rlato []: G ( d (7 whr d s th surfac to substrat thckss, α s th absorto coffct r ut lgth ad Φ s th radato flu dst r ut ara r scod. (5 (6 th abov quatos, τ s th lctro lft, s th ubr of lctros, s th ubr of hols, R s s th surfac rcobato rat, S s th surfac rcobato vloct for lctros ad S s th surfac rcobato vloct for hols. q s th lctroc charg, v s th carrr vloct th drcto, D s th dffuso costat for lctros ad D s th dffuso costat for hols. Th tr R s s gv b [], [9] R N k k ( T s s t t s ( k ( s t k ( s t whr s = αφτ ad s = αφτ. N T s th tra dst, k s th catur factor for hols, k s th catur factor for lctros, s ad s ar th surfac carrr coctratos for lctros ad hols rsctvl, whch tak th valus t ad t rsctvl wh th Fr lvl ls th tras. τ s th hol lft. Th cdt radato s assud to b odulatd b a sgal of frquc ω. Thus udr sall sgal codto [] j t ( whr zro dcats th dc valu ad o dcats th ac valu. Thr ar two ffcts ducd b th otcal lluato th c. Ths ar th hotocoductv ffct ad hotovoltac ffct. Th absorto of hoto rg wth th -t lar ad th s-sulatg substrat grats css carrrs ad crass th coductvt of ths rgos, thus sttg u a hotocoductv currt addto to a dra currt. Ths rocss ars fro th sarato ad collcto of gratd lctro-hol ars alog th logtudal drcto of th chal, ad thrfor hav thr strogst ddc o dra to sourc voltag rathr tha o gat to sourc voltag. Ths s th hotocoductv ffct. Th hotovoltac rocsss ars fro th collcto of hotogratd carrrs to th sac charg rgos. Thus thr ar two dffrt kds of hotovoltac ffcts: th tral hotovoltac ffct ad th tral hotovoltac ffct. Sc a dffrc dog lvls sts btw th -t lar ad th s-sulatg substrat a ottal barrr, or dlto rgo, s st u at thr trfac as occurs wth - juctos. Wh hoto absorto occurs at ths trfac barrr, th gratd carrrs th dffus ad ar swt furthr b th jucto lctrc fld, th lctros toward th actv chal ad th hols toward th substrat. A hotovoltag s lod. Ths jucto s forward bas whch ooss th trfac barrr ottal causg a dcras th chal-substrat dlto 6

3 tratoal Joural of Coutr Alcatos ( Volu 3 No.4, Octobr rgo wdth rsultg a gratr og of th coductg chal. Ths s th tral hotovoltac ffct. Th dlto rgo st u b a tral rvrs bas at th gat of th c rovds a addtoal ara for hoto absorto. Wh lluatd, ths rgo acts a alost dtcal ar to that of a Schottk hotodod. Th hgh tral lctrc fld assocatd wth th Schottk gat jucto causd lctro-hol ars gratd b th hoto absorto to sarat. addto, lctro-hol ars gratd b hoto absorto awa fro th dlto rgo (wth th -t taal lar ca cotrbut to th hoto ffct b dffusg to th dg of th dlto rgo bfor rcobato taks lac. Ths s th tral hotovoltac ffct. Wh th hoto rg s qual to, or gratr tha, th badga of th scoductor, th hoto-ctato of fr carrrs (lctro-hol ars ca tak lac. th utral rgo, th trasort chas of carrrs s du to dffuso ad rcobato. So th cotut quato s rrstd b a scod ordr dffrtal quato. Th frquc ddt ac quato s gv b [] ( d D D ( Frst w cosdr utral chal rgo. ths ar, w ar ot cosdrg t so th coot jωt quato ( s ot thr. Also, w ar dog ac aalss. So, ol Φ coot s thr. Th ffct of surfac rcobato s ot rst cas of lctros sc ol th rsc of gatv tras has b assud at or clos to th surfac. For /, w substtut quato (4 ad w gt = (3 whr, + ad - dcat th osto ad s th sac st. Th tr -α(d- usg ft dffrc thod ca b wrtt as ( ( d d / / whr s th sac st. (4 Substtutg th ft dffrc aroato for th scod ordr artal drvatv ad for th tr -α(d- quato (, th followg quato s obtad: d ( D / D / (5 Th abov quato ca b wrtt as D [ d ( ] / [ ] / (6 For ubr of ft dffrc ots, = to - s substtutd quato (6, so that - quatos wth ukows ar obtad. So th boudar codtos ar usd to kow out of th ukows. Th boudar codtos ar At = dg, ( d dg (7 ad at = ds, ( d ds (8 whr d s th surfac to substrat thckss, τ ω s th lft of th lctros udr ac codto, dg s th tso of th Schottk jucto dlto rgo th chal asurd fro th surfac ad ds s th tso of th - jucto dlto rgo th chal asurd fro th surfac. τ ω s gv b [] (9 j whr τ s th lft of th lctros ad ω s th frquc. dg s gv as [] dg ( B v( qn dr v gs V OP ( assug th abrut jucto aroato whr N dr s th quvalt costat dog coctrato, Φ B s th Schottk barrr hght, s th osto of Fr lvl blow th coducto bad, v( s th voltag dro bath th gat whch vars fro at th sourc d ad at th dra d. s th dra to sourc voltag ad v gs s th gat to sourc voltag. V OP s th hotovoltag across th Schottk jucto. ds s gv as [] N A ds a ( vb v( vbs VOP N dr qn A ( assug abrut jucto aroato whr N A s th substrat dog coctrato, v b s th bult- ottal, v bs s th substrat ottal ad V OP s th hotovoltag across th chal-substrat jucto. 7

4 tratoal Joural of Coutr Alcatos ( Volu 3 No.4, Octobr V OP ad V OP ar calculatd as follows: Th tral hotovoltag V OP across th Schottk jucto s calculatd usg th rlato [] kt J kt qv ( V OP l l ( q J s q J s whr J s s th rvrs saturato currt dst across th Schottk jucto. ( s th ubr of hols crossg th jucto at =. k s th Boltza costat, T s th absolut tratur ad q s th lctroc charg. Th tral hotovoltag V OP across th chal-substrat jucto s obtad usg th rlato [] kt J ( a kt qv ( a V OP l l (3 q J s q J s whr J s s th rvrs saturato currt dst for th - jucto. (a s th ubr of hols crossg th jucto at =a, a s th actv lar thckss. ( ad (a s obtad b solvg th cotut quato for hols th Schottk jucto dlto rgo ad th chal-substrat dlto rgo rsctvl. Now b substtutg th boudar codtos, - quatos wth - ukows ar obtad. Ths quatos ar solvd b wrtg th coffcts of th ukows a atr. t wll b a (-(- atr. Ths atr wll b ultld b th (- atr of th ukows to -. Ths wll b th lft had sd of th quato. Th rght had sd wll b th (- atr cosstg of th valus obtad fro th rag trs. Now usg atr vrso w wll gt th valus of. Th th boudar valus ar add to th valus to -. Ths s do for all th boudars obtad b takg ach valu of dg ad ds. Now, ths valus of ar usd to calculat th aout of charg th utral chal rgo whch s th tgrato or suato of ubr of lctros ach sac st ultld b th lctroc charg. t s gv as ch [ 3 Ths s do for ach ar of boudars. q... ] (4 Th currt th utral chal rgo s gv b [] ch Z ch (5 whr s th dra to sourc voltag, µ s th oblt of lctros, Z s th chal wdth ad s th chal lgth. Hr th lts of tgrato ar: as th lowr lt ad as th ur lt. W us suato to calculat th currt sc th dscrt quvalt of tgrato s suato. Th lts ad s th voltag v( bath th gat whch vars as at th sourc d ad at th dra d. For ft dffrc ots thr wll b valus of th voltag v(. Th valu obtad b takg th dffrc btw a two coscutv valus s. Ths s th voltag dro bath th gat as w go fro sourc to dra. Th w start fro th sourc d ad calculat th valu of charg btw th frst ar of boudars fro quato (. Th w ultl ths valu of charg wth th frst voltag dro at th sourc d. Th w calculat th valu of charg btw th scod ar of boudars fro quato (. Th w ultl ths valu of charg wth th scod voltag dro. Slarl w rat ths rocdur for th othr ar of boudars tll w rach th th ar of boudars. Th w add all th valus ad ultl th rsultg valu wth µz/ to obta th valu of chal currt for th corrsodg valu of. Sc w var btw to V volts takg ubr of ots, th valu of chal currt s calculatd for ach valu of. So, w gt valus of chal currt for valus of. Now w cosdr th utral substrat rgo. Th boudar codtos ar At = w, ( d w (6 ad at =d, (7 w s gv as [] qn w ds ( vb Vds vbs whr N N A N dr (9 (8 Now th quato (6 s solvd usg th boudar codtos gv b quatos (6 ad (7 th slar ar as dscrbd for th utral chal rgo. Now th aout of charg th utral substrat rgo s calculatd as sub [ 3 Ths s do for ach ar of boudars. q... ] (3 Th currt th utral substrat rgo s gv b [] 8

5 tratoal Joural of Coutr Alcatos ( Volu 3 No.4, Octobr sub Z sub (3 th dlto rgo, th trasort chas of carrrs s du to drft ad rcobato. So th cotut quato s rrstd b a frst ordr dffrtal quato. Th frquc ddt ac quato s gv b [] v v ( d (3 Frst w cosdr th Schottk jucto dlto rgo. For / w substtut quato (3 ad w gt (33 whr + ad - dcat th osto ad s th sac st. Substtutg th ft dffrc aroato for th artal drvatv ad for th tr -α (d- quato (3, th followg quato s obtad: v d v ( / / Th abov quato ca b wrtt as v [ ] [ ] ( d / Th boudar codtos ar At =, (36 ad at = dg, d ( d dg (37 whr dg s gv b quato (. / (34 (35 Now th quato (35 s solvd usg th boudar codtos gv b quatos (36 ad (37 th slar ar as dscrbd for th utral chal rgo. Now th aout of charg th Schottk jucto dlto rgo s calculatd as d [ 3 q... ] (38 Ths s do for ach ar of boudars. Th currt th Schottk jucto dlto rgo s gv b [] d Z d (39 Now w cosdr th actv lar-substrat dlto rgo. Th boudar codtos ar At = ds, ( d ds (4 ad at = w, (4 ( d w whr ds s gv b quato ( ad w s gv b quato (8. Now th quato (35 s solvd usg th boudar codtos gv b quatos (4 ad (4 th slar ar as dscrbd for th utral chal rgo. Now th aout of charg th actv lar-substrat dlto rgo s calculatd as d [ 3 Ths s do for ach ar of boudars. q... ] (4 Th currt th actv lar-substrat dlto rgo s gv b [] d Z d (43 Th back lluatd c cossts of a -t uforl dod s-sulatg substrat followd b a taall grow o-latd actv lar of -t dog. Th o latd rofl s rrstd b th Gaussa dstrbuto gv b [], [] R ( N (44 whr s th latd dos, σ s th straggl aratr ad R s th rojctd rag. 9

6 ds(a tratoal Joural of Coutr Alcatos ( Volu 3 No.4, Octobr Tabl. Valus of dffrt aratrs usd for calculato [] Paratr Na Valu Ut σ Straggl Paratr ( R Projctd Rag.86-7 ( µ Elctro oblt.45 ( /V.s Z Chal Wdth -6 ( α Absorto. 6 ( - Coffct τ Elctro ft. -6 (s τ Hol ft. -8 (s v Carrr Vloct. 5 (/s drcto d Thckss of th. -6 ( c cludg substrat Chal gth ( a Actv ar.5-6 ( Thckss Posto of Fr. (V vl blow th coducto bad Φ B Schottk Barrr.9 (V Hght N T Tra Dst. 5 ( - k Catur factor for ( 3 /s hols k Catur factor for ( 3 /s lctros v bs Substrat ottal. (V N A Substrat dog. ( -3 coctrato N dr Equvalt costat ( -3 dog coctrato Th ac dra-sourc currt du to o-latato s obtad usg th rlato [] o Z o (45 whr o s th ac chal charg du to o-latato ad s calculatd fro [] ds q N( d (46 o dg Z s th chal wdth ad s th chal lgth. Th ac coot of th total dra-sourc currt s cotrbutd b th carrrs du to o-latato ad otcal grato th chal ad substrat rgos. t ca b rrstd as [] ds ( total (47 o ch d sub whr d s th su of currts d ad d. Th swtchg t of th back lluatd OPFET dds o th actv chal thckss. Th swtchg t s coutd for dffrt actv lar thckss rssd b th followg quato [] ( B VOP vgs / qn davga V s ( B V / (48 OP v / gs whr q s th lctroc charg, s th chal lgth, N davg s th avrag chal dog coctrato, a s th actv lar thckss, v gs s th gat to sourc voltag, Φ B s th Schottk barrr hght, V OP s th hotovoltag across th Schottk jucto, V s s th saturato vloct. Tabl shows th valus of dffrt aratrs usd calculato. 3. RESUTS AND DSCUSSONS = 3 / -s = 4 / -s = 5 / -s d=.* -6 a=.* Vds(V Fg 3: ds-vds charactrstcs for dffrt radato flu dsts. Fgur 3 shows th currt-voltag charactrstcs of th back lluatd c for dffrt radato flu dsts. t s s that wth th cras th radato flu dst, th currt crass. Ths s bcaus wth th cras th radato flu dst, th volu grato rat of carrrs crass. That s wh or carrrs ar gratd ad hc th currt crass. Thr ar svral chass b whch th dra currt crass du to lluato whch las th hoto ffcts th c. cdt hotos caus a chag th bult- voltag of th gat jucto ad b ths chas, th dra currt crass. Both th hotocoductvt ffct th sourc-gat ad th dra-gat rgos ad th chag of th gat dlto wdth ar rsosbl for th cras of th dra currt. Wth lluato, th ottal barrr s lowrd ad th osto of th dlto dg o th chal sd of th

7 Swtchg t(s tratoal Joural of Coutr Alcatos ( Volu 3 No.4, Octobr chal/substrat trfac ovs toward th trfac. Th rsultg ffctv chal wdth, thrfor, crass. Ths crass th dra currt. Aothr raso for th cras of th dra currt s th rducto of th sourc to gat ad gat to dra rsstacs srs wth th actv chal du to lluato [] Vs=.* 5 /s B=.9V Ndavg=.658* 3 / Actv lar thckss( -7 Fg 4: Swtchg t v/s actv lar thckss. Fgur 4 shows th lot of swtchg t vrsus actv lar thckss of th back lluatd c. t s s that wth th cras actv lar thckss, th swtchg t dcrass. 4. CONCUSON Th cotut quato for lctros has b solvd usg ft dffrc thods to lot th -V charactrstcs of th back lluatd c for dffrt radato flu dsts. Sc th currt crass wth th cras th radato flu dst th c wth back lluato ca b usd for th dsg of hgh sd otcal dtctor coucato b crasg th radato flu dst ad for th dsg of otcal swtch b crasg th actv lar thckss. Th c ca b usd as a dtctor sc wth sall chag th radato flu dst, thr s larg chag th dra to sourc currt. Th c ca b usd as a hgh sd otcal dtctor bcaus of th hgh currt obtad du to rducto th wdth of both th dlto rgos du to hotovoltac ffct ad th cras th coductvt of th chal ad substrat rgos du to hotocoductv ffct. Ths s bcaus th rsos sd of th dtctor s dtrd b th wdth of th dlto rgo. ssr th wdth, hghr th sd. Also, th swtchg t dcrass wth th cras th actv lar thckss ad th valu of swtchg t s low. So, th c ca b usd as a swtch. 5. ACKNOWEDGEMENT Authors thaks Dr. B.B. Pal, TBHU ad Dr. R. P. R. C. Aar, T, Muba for rovdg costat hl ad couragt ad cssar gudac. 6. REFERENCES [] K. Balasubadra, A. Arular, V. Rajaa, K. Sakaraaraaa, Two Dsoal Nurcal Modlg ad Sulato of a Uforl dod GaAs MESFET Photodtctor, Joural of Otcal Coucatos, 9(84, [] Nadta Saha Ro ad B. B. Pal, Frquc-Ddt OPFET Charactrstcs wth rovd Absorto udr Back lluato, Joural of ghtwav Tcholog, Vol. 8, No. 4, , Arl. [3] Youssf Zbda ad S. Abu_Hlwh, AC Charactrstcs of Otcall Cotrolld MESFET (OPFET, Joural of ghtwav Tcholog, Vol. 5, No. 7,. 5-, Jul 997. [4] Nadta Saha Ro, B. B. Pal, ad R. U. Kha, Aalss of GaAs OPFET wth rovd Otcal Absorto udr Back lluato, EEE Trasactos o Elctro Dvcs, Vol. 46, No., , Dcbr 999. [5] Nadta Saha Ro, B. B. Pal, ad R. U. Kha, Frquc- Ddt Charactrstcs of a o-latd GaAs MESFET wth Oaqu Gat Udr lluato, Joural of ghtwav Tcholog, Vol. 8, No.,. -9, Fbruar. [6] M. K. Vra ad B. B. Pal, Aalss of Burd Gat MESFET Udr Dark ad lluato, EEE Trasactos o Elctro Dvcs, Vol. 48, No. 9,. 38-4, Stbr. [7] B. B. Pal, Shubha, K. Ho Kuar ad R. U. Kha, Frquc Ddt Bhavour of a o latd GaAs OPFET cosdrg th Photovoltac Effct ad Gat Dlto Wdth Modulato, Sold Stat Elctrocs, Vol.38, No. 5,. 97-, 995. [8] Jaa V. Gatod ad Rajsh B. oha, O Dsoal Ft Dffrc Sulato of Back lluatd OPFET, Procdgs of tratoal Cofrc & Worksho o Ergg Trds Tcholog, Muba, Volu-,. 58-6, Fbruar 5th & 6th. [9] Suta Mshra, V. K. Sgh ad B. B. Pal, Effct of Radato ad Surfac Rcobato o th Charactrstcs of a o-latd GaAs MESFET, EEE Trasactos o Elctro Dvcs, Vol. 37, No.,. -, Jauar 99. [] Shubha R. Saa, R. B. oha, R. U. Kha, B. B. Pal, Gralzd dc odl of GaAs otcal fld ffct trasstor cosdrg o-latd rofl, Otcal Egrg, 37(4, Arl 998, [] S. N. Chattoadha, N. Motoaa, A. Rudra, A. Shara, S. Srra, C. B. Ovrto, ad P. Pad, Otcall Cotrolld Slco MESFET Modlg Cosdrg Dffuso Procss, Joural of Scoductor Tcholog ad Scc, Vol. 7, No. 3,. 96-8, Stbr 7. [] Shh-Hs o, ad Ch-Pg, Nurcal Aalss of th Photoffcts GaAs MESFETs, EEE Trasactos o Elctro Dvcs, vol. 39. o. 7, , Jul 99.