Three dimensional modeling and simulation of a nano MISFET photodetector

Transcription

1 JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 9, No. 9, Septeber 007, p Three denonal odelng and ulaton of a nano MISFET photodetector M. KABEER a, K. GOWRI, V. RAJAMANI * Departent of Electronc and Councaton Engneerng, P.S.N.A. College of Engneerng and Technolog Dndgul 64 6, Tal Nadu, Inda a Departent of Inforaton Technolog B.S. Abdur Rahuan Crecent College of Engg., Chenna, Talnadu, Inda Th paper deal wth three denonal nuercal odelng of an cally controlled nano MISFET photodetector for OEIC photorecever degn. The feld dependent oblty of the carrer n the urface potental condered for odelng. The exact potental profle of the devce under llunated condton ha been coputed nuercally by olvng 3D Poon equaton ung boundary value proble. The electrc feld profle along the length, wdth and thckne of the channel and oblty of the carrer have alo been tuded extenvely under llunated condton to have an n depth analy. The photocurrent of the nano MISFET under llunated condton ha alo been calculated and preented. The tranfer charactertc of the MISFET ha been tuded nuercally and preented n th paper. The reult how that the devce charactertc are trongly nfluenced by ncdent cal radaton. The wtchng peed ha alo ncreaed under llunated condton becaue of the hgher electron oblty and horter trant te. The odel enable one to etate varou paraeter whch deterne the potental ue of th devce for photo detecton n OEIC photorecever. (Receved July 3, 007; accepted Augut, 007) Keyword: Nuercal 3D odelng, MISFET, Photodetector, Photocurrent, OEIC recever 1. Introducton 1.1 InP MISFET a photodetector The econductor of choce for hgh-peed electronc and oelectronc applcaton are copred of group III-V copound uch a ndu phophde (InP) and gallu arende (GaA). The bulk phycal properte of III-V econductor are, n any cae, far uperor for electronc and photonc applcaton than thoe of the ot coonly ued econductor, lcon. A large nuber of work ha been reported over pat decade concernng the ue of Feld Effect Trantor n cally controlled applcaton ncludng aplfcaton, ocllaton and cal detecton. Coercally avalable FET do not have the dered photo entvty for the propoed applcaton. The cally gated FET tructure have been found to have better entvty a copared to ther conventonal counter part. In vew of th, MISFET condered for t potental ue a hgh peed photodetector. The bac requreent of oelectronc ntegrated crcut (OEIC) are copatblty of the ateral for the conttuent electronc and cal devce and planarty. Invetgaton of ntegrated crcut currently beng carred out by any worker anly ung GaA. However, ot of IC reported o far have eployed MESFET, beng n contrat to S-MOS IC, nce etalnulator-gaa MIS tructure cannot be ued to for n- channel at the econductor urface at preent due to large nuber of nterface tate [1]. In pte of the ucceful operaton of MESFET IC, IC wth MISFET are tll favorable a havng large tolerance n applcable logc wng, threhold voltage control, and degn of crcut. Epecall enhanceent-ode MISFET are hghly derable for low power logc applcaton. The photoentvty of etal-nulatoreconductor (MIS) tructure ha opened up the poblty of ther ue a nfrared photodetector, cally varable reactor (OVR) and cal CCD for old-tate agng [-5]. The bac MIS confguraton can alo be ued to realze cally gated MISFET photodetector [6-8]. A nuber of tude have been carred out to exane the potental of the MIS confguraton for uch oelectronc applcaton. The ntal nvetgaton n th area were confned to conventonal MOS devce. Succeful developent of heterogate MIS tructure tulated further reearch n th area wth pre epha on photoentvty. A nuber of novel MISbaed devce were propoed, tuded and teted ubequently [3-8].The reult of prelnary nvetgaton are qute encouragng. It ha been found that the bac MIS coponent can be ued a a potental photoenng eleent for developng any oelectronc yte. Hgh electron oblty wth hgh carrer peak and aturaton velocte ake InP a derable ateral for hgh peed nulated-gate feld-effect trantor. Hgh breakdown feld, good theral conductvt and a low onzaton coeffcent ake InP equally attractve for old-tate crowave power applcaton [9, 10]. Inulated gate are preferable to Schottky gate nce larger operatng voltage and hgher output power are ore readly obtaned.

2 880 M. Kabeer, K. Gowr, V. Rajaan Photo detector the frt eleent of the photo recever crcut whch nterpret the nforaton contaned n the cal gnal. In general the cal gnal generally weakened and dtorted when t eerge fro the end of the fber. To overcoe th proble, the photo detector ut eet very hgh perforance requreent lke, a hgh repone or entvty n the eon wave length range of the cal ource beng ued, a nu addton of noe to the yte, a fat repone peed to handle the uffcent data rate, nentve to varaton n teperature and to be copatble wth the phycal denon of the cal fber. Optcal recever ue conventonal photodetector n dcrete for n conjuncton wth other gnal proceng unt realzed wth dcrete actve coponent MISFET. The peed of uch photodetector erouly lted by the paratc nductance and capactance ntroduced by the nterconnecton of varou tage. The overall perforance of the cal recever can be greatly proved by onolthcally ntegratng the electronc and other oelectronc coponent n the ae chp known a OEIC. The cally gated MISFET tructure ha been found to have better entvty and act a a hgh peed photodetector for ther poble ue n cal recever of future generaton cal councaton yte. The theoretcal odel developed by any reearcher howed that the devce charactertc were trongly nfluenced by ncdent cal radaton [11-13]. Yaaguch et al [8] deontrated the operaton of an enhanceent ode MISFET a a new hgh gan photodetector. The lght wa focued on the etranparent etal gate to ncreae the photoentvty. It wa hown that the photocurrent gan n the order of 10 3 can be obtaned under applcaton of gate ba. It wa explaned that the aplfcaton due to the ncreae n the urface channel conductvty va the developent of an electron qua-fer level hft toward the conductveband edge under llunaton. Chakrabart et al [11] developed a e-nuercal odel of a novel hetero-gate MISFET to deterne the cally controlled charactertc of the devce under teady llunaton. It wa found that at a contant dran voltage and the gate voltage the aturaton dran current, tranconductance and dran conductance of the devce can be controlled by ncdent cal llunaton. It wa alo reported that the photocurrent decreae wth ncreae n the gate voltage above the threhold voltage. Th becaue, below the trong nveron regon, the ncreae n the dran current due to the photo generaton neglgble. A three denonal nuercal odelng of nano MISFET wa reported and tuded the electrcal charactertc by olvng the 3D Poon equaton nuercally recently [14]. The potental profle, electrc feld profle and oblty of the nano MISFET were alo tuded. The dran current and the tranconductance were alo calculated by applyng electrc feld and oblty of the carrer. In th paper, we have developed 3D nuercal odel of an InP MISFET a photodetector for the OEIC applcaton. The 3D Poon equaton ha been olved nuercally to obtan varou devce paraeter uch a feld profle, oblty of the carrer, potental profle photocurrent, trancondutance and reponvty under the llunated condton. 1. Theoretcal three denonal nuercal odelng of nano MISFET photodetector Fg. 1. Scheatc dagra of an InP- Al O 3 NANO MISFET Photodetector under conderaton. The effect of llunaton on the bac MIS (Metal- Inulator-Seconductor) tructure a well a MISFET (Metal Inulator Seconductor Feld Effect Trantor) ha drawn conderable attenton becaue of ther enorou potental for ther ue n cal agng and detecton. The photoentvty of MISFET largely depend on the cal repone of the bac MIS confguraton. When cal gnal appled on a econductor, carrer are generated ether by band-to-band tranton (ntrnc) or by tranton nvolvng forbdden gab energy level (extrnc), reultng n an ncreae n channel conductvty. For the ntrnc econductor, the ncreae of conductvty under llunaton anly due to the ncreae n the nuber of carrer. For the extrnc econductor, photo exctaton ay occur between a band edge and an energy level n the energy gab. Photo conductvty can take place by the aborpton of photo of energy equal to or greater than the energy eparaton of the band gap level and the conducton or valence band. Fg. 1 how the Scheatc dagra of an InP- Al O 3 MISFET tructure under conderaton. Applcaton of a large potve voltage on the etal gate caue trong nveron to occur n the econductor at the nterface. When an cal radaton of utable wavelength ncdent on the urface of the e tranparent etal gate, a part of the total ncdent cal power trantted through the nulaton layer nto the econductor where t aborbed. Aborpton of th radaton reult n the generaton of exce electron-hole par. The goal of the odelng proce to ake ure that all the paraeter that are needed for the characterzaton of the devce are copletely and accurately repreented. It

3 Three denonal odelng and ulaton of a nano MISFET photodetector 881 nvolve the proper dentfcaton of paraeter and accurately defnng a ethod to fnd the. Modelng probably the ot ntenve and te conung part of the developent proce. A three denonal odelng of InP MISFET photodetector decrbed for the calculaton of varou paraeter lke potental profle, electrc feld profle, oblty profle, dran charactertc, tranfer charactertc and dran retance under dark and llunated condton. The odelng procedure tart wth olvng the three denonal Poon equaton. The 3-D Poon equaton [14] n the pace charge regon of the devce at the dran end n the n-channel MISFET under llunated and dark condton are wrtten a, z) z) z) q( Na Δn) (1a) ψ x y z ( x, z) z) z) (1b) x y z ε ε qna where, Ψ(x, z) the urface potental at a partcular pont (x, z), N a the dopng concentraton, n the exce carrer generated per unt volue n the econductor due to the aborpton of ncdent cal power denty and ξ the perttvty of the econductor. The boundary condton to olve the Poon equaton are gven below. Boundary condton wth repect to the thckne of the oxde layer are gven by ψ ( 0, z) V gf V b fb (a) ψ ( t, z) V gb V b fb (b) Boundary condton wth repect to the length of the channel are ψ ( x,0, z ) V b V op Leff, z) V b V d V op (3a) (3b) Boundary condton wth repect to the wdth of the channel are wrtten a b 0) b (4a) V gf V fb b W ) (4b) V gf V fb The exce carrer generated per unt volue ( n) n the econductor due to the aborpton of ncdent cal power denty gven b Δn 4τP 1 P (1 R )(1 R )(1 R)(1 e (1 R )(1 R )(1 R)(1 e W hγn W hγn αw αw ) ) 1 where, W the axu wdth of the depleton layer and gven by W 1/ [4ε ln(n a / n ) / qβn a ] (5) (6) where, N a the acceptor concentraton. G op (y) the exce carrer generaton rate at ant pont y n the econductor and gven by G op ( y ) p α y ( 1 R )( 1 R )( 1 R ) α e (7) hγ where p the ncdent cal power dent h the Planck contant, γ the operatng frequenc α the aborpton coeffcent of the econductor at the operatng wavelength, R, R, and R are the reflecton coeffcent at the etal gate entrance, gate-nulator nterface and the nulator-econductor nterface repectvely. The ean lfete of the norty carrer n the llunated condton, τ l can be wrtten a τ n / n Δn)τ l ( (8) where, τ the lfete of the carrer for the ntrnc econductor.. Electrc feld profle In order to obtan electrc feld profle equaton under dark and llunated condton (11a), (11b) and (11c) are olved repectvely for the correpondng profle n x, y and z drecton. ψ ( 1, j,l) ψ ( 1, j,l) Ex L x (10a) Where, V g f appled potental to the front gate V fb f flat band voltage to the front gate V fb b flat band voltage to the back gate V b bult n potental V d dran to Source voltage V op photo nduced voltage ψ (, j 1,l) ψ (, j 1,l) Ey W y (10b)

4 88 M. Kabeer, K. Gowr, V. Rajaan ψ (, j,l 1) ψ (, j,l 1) Ez t z (10c) where λ the operatng wavelength, I ph the photo repone current (the dfference between the dran current n the llunated condton to that n the dark condton). where x, y and z are the eparaton of the grd lne along the x and y drecton, ψ (, j,l) the urface potental at a partcular pont and l, w and t are the gate length, devce wdth and oxde thckne repectvely. The feld dependent oblty of charge carrer gven b µ x E x ξ ox ξ (11a) µ y E y ξ ox ξ (11b) µ z E z ξ ox ξ (11c) where, µ x, µ y and µ z denote the oblty of charge carrer n x, y and z drecton repectvely and E x, E y and E z are the correpondng Electrc feld. The dran to ource current I dop can be wrtten conderng t contnuty equaton a, ε ( V g V th V op ( x ) E x ox I dop Zμ x ) d ox (1) where, ξ ox and d ox are perttvty and thckne of oxde layer repectvely. µ x and E x denote the oblty and electrc feld along the length of the channel repectvely. Here, V g denote the gate to ource voltage and V th denote the threhold voltage of the devce. V op (x) the urface potental along the length of the channel under llunated condton and z the wdth of the channel. The tranconductance g op gven b ( 1) I dop ( 1) ( 1) V ( 1) I dop { V () } V, (13) g op g d cont V A defned by (13), the tranconductance the rato between the dran current under llunated condton and the gate voltage keepng the dran voltage contant. Moreover, the tranconductance nverely proportonal to the gate voltage. Therefore, when the gate voltage ncreae, the tranconductance decreae. The dran retance r dop and the reponvty of the devce have been obtaned fro (14) a gven below. r dop { V () } d V g, cont Vdop I op d g R ( ) g ( 1) Vdop( 1) ( 1) I op( 1) I d (14) ph λ (15) P 3. Coputatonal technque The 3D Poon equaton (1a, 1b) are olved ung Lebann teratve ethod to deterne the urface potental at llunated and dark condton for a fxed value of gate voltage and aued value of the dran voltage. The nuercally etated value of the urface potental utlzed to deterne the electrc feld and oblty n all the three drecton. The dran current ha been calculated fro equaton (1). The voltage profle and the electrc feld dtrbuton n the channel have been etated for accurate calculaton of the dran to ource current. The voltage profle n the channel obtaned by calculatng the dfferental change n the voltage acro the channel and aued to be very all (1V) at the ource end and low feld oblty ha been ued to begn the nuercal coputaton fro the ource end. In order to obtan the channel voltage profle, the 3D Poon equaton (9a and 9b) are nuercally olved by conderng the entre channel to be dvded nto a large nuber of eleentary trp wthn the devce boundary. A outcoe of th proce, we obtan the potental at every pont n the channel and t varaton along the length and wdth of the channel a well. ψ (, j, l) ψ (, j, l ψ ( 1, j, l) ψ ( 1, j, l) ψ (, j 1, l) ψ (, j 1, l) (9a) 1) ψ (, j, l 1) ((q(na Δn) / ε) / 6) ψ (, j,l) ψ ( 1, j,l) ψ ( 1, j,l) ψ (, j 1,l) ψ (, j 1,l) (9b) ψ (, j,l 1) ψ (, j,l 1) ((qna / ε) / 6) The potental at the urface end of the gate the u of bult-n voltage and appled dran voltage. The effectve gate voltage at the econductor-nulator nterface the dfference between V G and V FB, where V FB the flat-band voltage of the devce and an electrc-feld at the edge of the depleton regon zero. 4. Reult and dcuon Fg. how the dtrbuton of potental along the length of the channel that etated ung (9a) and (9b). It een fro the fgure that the urface potental decreae lnearly near the ource end and ncreae lnearly near the dran end. The potental ncreae near the dran becaue of the appled potve ba at the dran de. It oberved that the urface potental under llunated condton lghtly hgher than that of the potental under dark condton due to the exce generated electron hole par.

5 Three denonal odelng and ulaton of a nano MISFET photodetector 883 een that the electrc feld near the dran end n the llunated condton le copared to the value of the dark. A a reult a hgh dran voltage needed to attan aturaton n the llunated condton. It well known that, when an cal gnal llunated on the devce ore and ore electron hole par are generated. A the charge carrer are ore crowded under llunated condton, ther oblty get reduced n all three drecton. It can be een fro (11a, 11b and 11c) that the electrc feld drectly proportonal to oblty. Therefore the feld n all three drecton get reduced due to the decreae n oblty. Fg.. Potental dtrbuton along the length of the channel under dark and llunated condton. The potental profle along the channel and wdth of the devce under both dark and llunated condton n a three denonal fgure hown n Fg.3. The devce exhbt the lar knd of charactertc along the length and wdth of the channel. In both cae t can be een that the potental reache abruptly hgher near the dran end due to the hgh electrc feld near the dran end whch caue a rapd conducton. The effect of potental under llunated condton alo oberved. Fg. 4. Varaton of electrc feld profle along the length and wdth of devce. Fg.5 how the two denonal vew of electrc feld along the thckne of oxde layer. It can be een that Ez rapdly decreae near the dran end a oppote to that of Ex. Th due to the fact a Vg appled, the electron acqure ore energy to ove along the length of the channel.e., n x drecton than along the other two drecton. The electrc feld n Y- drecton and Z- drecton wll follow the ae charactertc. Fg.3. Varaton of potental dtrbuton along the length and wdth of the channel. The dtrbuton of electrc feld along the length and wdth of the devce under conderaton under dark and llunated condton depcted n the Fg. 4. It een that the electrc feld E x ore donant than feld E y. The electrc feld ncreae rapdly near the dran end a een n Fg 4. Th due to the fact that the carrer denty near the dran end experence a rapd decreae n urface concentraton whch call for a rapd ncreae n the electrc feld to antan the contant dran current. It Fg.5. Varaton of electrc feld along the thckne of the oxde layer.

6 884 M. Kabeer, K. Gowr, V. Rajaan The dtrbuton of oblty n all the three drecton are hown n Fg. 6 and Fg. 7 for both the dark and llunated condton. A oblty drectly proportonal to electrc feld (µξe), the hape of the oblty curve under x, y and z drecton are lar to that of E x, E y and E z. condton hgher and th due to the generaton of exce carrer under llunaton. Under Dark condton Under llunated condton Fg. 8. Varaton of the dran current wth appled dran voltage. Fg. 6. Moblty dtrbuton along the length and wdth of the channel. The tranfer charactertc of the MISFET photodetector are hown n Fg. 9 for dfferent dran voltage. A the appled gate to ource voltage ncreae, the dran current alo ncreae lnearly a hown n the fgure for a fxed dran voltage. When the dran voltage ncreae further, the dran current alo ncreae rapdly a hown n the Fg. 9. The effect of llunaton alo depcted n the ae fgure. Fg.7. Moblty dtrbuton along the thckne of oxde layer. The varaton of the current for dfferent appled gate-to-ource voltage are depcted n Fg. 8 for the dark and llunated condton. It een that the for the appled gate-to-ource voltage, the dran current ncreae gnfcantly. When the appled gate voltage further ncreaed, dran current alo gnfcantly ncreae. The channel wdth anly deterned by appled gate to ource voltage. The charge carrer pa through the channel and thereby conducton ad to take place. When dran voltage ncreae further, ore charge carrer try to pa through the channel due to whch dran current ncreae. But the charge carrer can utlze only the channel wdth that created earler. That the reaon the dran current aturate after a certan lt even f dran voltage ncreaed further. The current under llunated Fg. 9. Tranfer charactertc of the MISFET photodetector. The tranconductance wth the appled gate to ource voltage depcted n Fg. 10 for the both dark and llunated condton. Due to the appled voltage and exce carrer generaton, photo voltage nduced n the channel. The reducton n the tranconductance of devce due to the photovoltage nduced n the channel. Th can be evdent fro the fgure that the tranconductance of the devce under llunated condton lower than that of the dark condton.

7 Three denonal odelng and ulaton of a nano MISFET photodetector 885 Acknowledgent The author are grateful to the All Inda councl for Techncal Educaton (AICTE), New Delh, Inda for provdng reearch grant (003/BOR/RPS/080/06/07) n the year Reference Fg. 10. Varaton of the tranconductance wth gate voltage. 4. Concluon Photorepone charactertc of a three denonal nuercal odel of nano MISFET photodetector wa preented for utablty n the OEIC recever applcaton. The effect of varou nternal devce paraeter uch a electrc fled, oblty of the carrer n the preence of llunaton, potental dtrbuton of the carrer have alo been tuded extenvely through nuercal ulaton. The dran current and tranfer charactertc of the nano MISFET photodetector ha alo been calculated nuercally. Tranpendence of the 3D photomisfet ha alo been calculated. The crcut level paraeter of the devce under llunated condton wll be tuded extenvely. The perforance of the devce n OEIC applcaton wll alo be analyzed. [1] H. Haegawa, T. Sawada, IEEE Tran., Electron Dev. ED-7(6) 1055 (1980). [] J. Grovalet, C. Jund, IEEE Tran. Electron Devce, ED 14, 777 (1967). [3] A. Sher, Y. H. Tuo, J. A. Morart W. E. Mller, R. K. Crouch, J. Appl. Phy. 51, 137 (1980) [4] P. Chakrabart, B. R. Abraha, A. Dhngra, A. Da, B. S. Sharan, V. Mahewar, IEEE Tran. Electron Devce, ED 39, 507 (199). [5] B. B. Pal, S. S. Nelon, IEEE Tran. Electron Devce, ED 40, 1878 (1993). [6] K. Okaoto, I. Inoue, Sold-State Electron. 16, 657 (1973). [7] P. Chakrabart, S. Kuar, P. K. Rout, B. G. Rap-Pa, Proceedng of 3rd Aa Pacfc Mcrowave conference, Tokyo, Japan, 575 (1990). [8] E. Yaaguch, T. Kobayah, Jpn. J. Appl. Phy. 1, 104 (198). [9] D. L. Lle, D. A. Colln, Thn old fl 56, 5 (1979). [10] D. L. Lle, D. A. Colln, L. G. Mener, L. Meck, Electron Lett. 14, 657 (1978). [11] P. Chakrabart, B. K. Mhra, Y. Pratap Redd S. Prakah, Phy. Stat. Sol., 77 (1995). [1] M. Arand et al., Electron. Lett. 19, 433 (1983). [13] L. Meck, et al. Proc. IEDM, 767 (1986). [14] K. Gowr, P. Rajeh, V. Rajaan, Phyca E : Low denonal Syte and nanotructure 36, 5 (007). * Correpondng author: rajavan@redffal.co