Gain Optimization of SI 0.53 GE 0.47 Heterojunction Bipolar Phototransistor

Size: px

Start display at page:

Download "Gain Optimization of SI 0.53 GE 0.47 Heterojunction Bipolar Phototransistor"

Stephany Wright
5 years ago
Views:

1 Intrnational Journal of Photonis. ISSN Volum 6, Numbr 1 (2014), pp. 1-8 Intrnational Rsarh Publiation Hous Gain Optimization of SI 0.53 GE 0.47 Htrojuntion Bipolar Phototransistor Dr. R. P. Mishra #1, Dr. Satyajit Sn Purkayastha #2, Mr. Satyajit Das #3 #1 Dirtor, #2 Dpartmnt of Eltrial and Eltronis, #3 Dpartmnt of Eltronis and Communiation, #1,#2,#3 Amrapali Institut of Thnology and Sins, Haldwani, Uttarakhand, India. #1 dirtor.rpm@gmail.om, #2 satyajitsnpurkayastha@gmail.om, #3 satyajitdas20@gmail.om, ABSTRACT: Dmand for high spd opto-ltroni intgratd iruits has indud th intgration of p-i-n photodiod in HBT itslf. Combinly th dvi is alld Htrojuntion Phototransistor. In this papr Si 0.53 G 0.47 HPT is onsidrd and gain analysis in trms of photourrnt dnsity is don. Opration of transistor is dsribd. Mathmatial drivation of gain formula and subsqunt analysis of paramtrs spak that dvi undr study is apabl of dttion as wll as amplifiation by β tims of th rivd signal. Th us of singl HPT to play th dual rol will gratly simplify th fabriation of th OEIC rivr in th monolithi form. On th basis of th rsults it is xptd that proposd HPT with drivd optimum paramtrs will mrg as an xllnt altrnativ for high spd widband optial rivrs. 1. INTRODUCTION In optial ommuniation, rivr dsignrs hav mployd PIN photodiod as th dttor du to its ompatibility with that of HBT [1]. Howvr this adds to olltor apaitan and sris rsistan thus dgrading th high frquny apabilitis of HBT [2]. An altrnat to PIN that has not bn yt mployd xtnsivly, is th bipolar Htrojuntion phototransistor HPT. With this, In addition to its pitaxial and fabriation pross ompatibility with th HBT, th HPT offrs th advantag of optial gain whih an ontribut to improvd rivr snsitivity and signal to nois ratio [3]. In this papr th matrial ombination Si 0.53 G 0.47 is takn as it givs bttr optial rspons in 1550 nanomtr wavlngth rgion and gain analysis of Si 0.53 G 0.47 typ Papr Cod: IJP

2 2 Dr. R. P. Mishra t al HPT is don. Th dvi undr onsidration has an n-typ Si mittr, p-typ Si 0.53 G 0.47 bas and n-typ Si 0.53 G 0.47 Colltor n-typ Si Emittr N P N p-typ Si 0.53 G 0.47 Bas n-typ Si 0.53 G 0.47 Colltor Fig1. Shm of Si 0.53 G 0.47 HBT as shown in fig(1). Th HBT improvs on BJT in trms of spd and frquny [3]. In SiG gradd htrostrutur transistors, th amount of grmanium in th bas is gradd, making th band gap narrowr at th olltor than at th mittr, thus ahiving th taprd band gap to improv frquny rspons [4]. Fig (2) HBT nrgy band diagram 2. DEVICE CONFIGURATION: In NPN HBT diffring smiondutor matrials ar usd for mittr and bas rgion to rat htrojuntion. Th fft is to limit th injtion of hols from th bas into th mittr rgion, baus th potntial barrir in th valn band is highr than in th ondution band. This allows a high doping dnsity to b usd in th bas, thus rduing th bas rsistan whil maintaining th gain. Du to th nd to manufatur HBT dvis with xtrmly high dopd thin bas layrs, molular bam pitaxy is prinipally mployd [5]. In addition to bas, mittr and olltor layrs highly dopd layrs ar dpositd on ithr sid of olltor and mittr to failitat an ohmi ontat, whih ar plad on th ontat layrs aftr xposur by photolithography and thing [6]. Th ontat layr undrnath th olltor namd subolltor, is an ativ part of th transistor. PIN typ photo dttor is formd by bas-olltor-subolltor layrs.

3 Gain Optimization of SI 0.53 GE 0.47 Htrojuntion Bipolar Phototransistor 3 Fig (3) Dvi Configuration of HPT 3. OPERATION: Th inidnt optial signal is rivd by th HPT through th transparnt mittr as shown in figur [3] of shmati onfiguration. Th radiation passs through transparnt window of mittr and gts absorbd in th bas and olltor rgion, rating ltron-hol pairs. Thus photo-gnratd hols aumulat in th bas and altr th bas mittr potntial. This in turn alls for injtion of ltron from th mittr into th bas rgion to r-stablish th harg nutrality in this rgion. Thus urrnt gain is ahivd by normal transistor ation, whn th bas width W b is lss than th diffusion lngth of th injtd ltrons L nb. Th funtion of th wid gap mittr is to inras th mittr injtion ffiiny by prvnting rvrs injtions of hols from th bas into th mittr. Th arrir injtion aross th mittr an b writtn as [7] (1) In this xprssion, th ratio of ltron urrnt I n to hol urrnt I p rossing th mittr juntion is proportional to th ratio of doping in th mittr N d and th bas N db, and varis also xponntially with nrgy band gap diffrn E g btwn th two smiondutors. Sin E g appars in th xponntial fator, vn small valu an afft th injtion ffiiny signifiantly. Now th primary objtiv in this prsnt work is to omput photo urrnts and stablish th gain formula for th photo transistor in trms of photo urrnt dnsity. Fig 4-Currnt omponnts for HPT photo transistor

4 4 Dr. R. P. Mishra t al 4. COMPONENTS OF PHOTOCURRENT: For omputation of various omponnts of urrnts, Lt us look at th fig 4. Hr optial illumination is through th mittr sid. Th mittr mtal and mittr N + ontat layrs ar partially rmovd to allow light pntration. Th ffts of optial absorption in th mittr ar ngligibl du to its wid band gap. Th optial absorption in th bas portion of th mittr-bas spa harg rgion is also ngligibl. Sin th bas is muh mor havily dopd than th mittr, th pntration of th spa-harg rgion into th bas is vry small and muh lss than th optial absorption lngth so thr is ngligibl absorption in spa harg rgion. Optial absorption in th bas produs a photo urrnt I p that ontributs to th bas urrnt. This photo urrnt dnsity ompriss thr omponnts. 1. J op = du to optial absorption in th quasi nutral bas. 2. J dpl = du to absorption in bas olltor spa harg rgion within a diffusion lngth from th dg of th bas olltor spa harg rgion. 3. J p = du to absorption in th nutral olltor. Now Th total photo urrnt an b xprssd as I ph = A E (J ph + J dpl + J op ), (2) Whr A E = mittr ara, now to find out photo urrnt dnsity J ph, lt us dfin D nb = th ltron diffusion onstant n ltron minority arrir lif tim in th bas. V jp = th band bnding on th bas sid of th mittr-bas Htrojuntion [8] E V jp = V bi VBE (3) Whr V = built in potntial of th mittr bas juntion E bi E Nd and E Nd B N ab (4) whr N d = doping in th mittr N db = doping in th bas absorption offiint in th olltor olltor optial quantum ffiiny W BC width of th olltor spa harg rgion. Gnration ( g ) in th olltor rgion [8] Lp L W W E p S W ---- (5) g q 2 2 L p 1 b B BC 2 Lp E1 E k D p Whr W LpS W E 1 Cosh Sinh Lp Dp Lp (6) W LpS W E 2 Sinh Sinh Lp Dp Lp (7) Whr

5 Gain Optimization of SI 0.53 GE 0.47 Htrojuntion Bipolar Phototransistor 5 S = Surfa rombination vloity Photo urrnt dnsity J ph is givn by th diffrn of th olltor and mittr urrnts dnsity as givn by J ph = (J J ) opt (8) Aftr substituting th valus w gt th final rsult for th photo urrnt dnsity W W Jno b F b B b BC o Jph Fo g q 1 n ( x )[ ] o p n n n W b WB f n n Cosh 1 1 fn n Cosh 1 n -- (9) Lnb Lnb In this quation th first trm ariss from optial absorption in th nutral olltor and diffusion of th photo gnratd hols to th bas olltor spa harg rgion whr thy ar swpt aross by th ltri fild and olltd. Basd on th abov analysis w an find th optial gain G for th thr trminal onfiguration for th photo transistor in trms of th small signal urrnt gain and photo urrnt dnsity. 1J ph G (10) qf o 5. ANALYSIS: Looking at quation (10) of small signal urrnt gain and photourrnt dnsity (9) w find that small signal urrnt gain is strong funtion of th baswidth (w B ). Plotting optial gain G vrsus W B (varying from 0 to 0.25 µm) Fig (5): plot of optial gain vrsus bas width From this plot of fig(5) W find that at vry small bas width, th small signal urrnt gain is drasing rapidly kping th bas doping fixd. Ovrall th small signal urrnt gain rdus with th inras in bas width or ara. Thrfor it is nssary to kp bas ara or width as small as possibl.

6 6 Dr. R. P. Mishra t al Now plotting th optial gain for bas doping N db kping th bas width fixd through quations (10),(9) and (4), w gt th following graph Fig (6): Plot of optial gain vrsus bas doping From this plot of fig(6) w find that urrnt gain is drasing rapidly with inras in bas doping. Hr w must rmmbr that to ahiv improvd frquny rspons w should hav havily dopd bas so as to rdu bas rsistan. Thrfor a tradoff is ndd btwn bas doping and urrnt gain. Now plotting th photourrnt dnsity vrsus bas width through quation (9) w gt th following urv Fig(7): Photourrnt dnsity vrsus Bas width From this fig(7) w an obsrv that photourrnt dnsity drass with th inras in bas width. From fig(6) and (7) w an optimiz that whil manufaturing Si 0.53 G 0.47 HPT th bas width must b takn as 50nm and bas doping as /m 3 or /m 3 for th optimum prforman. Now plotting quation (9) and (7) for photourrnt dnsity vrsus olltor width w kping olltor doping onstant, w gt following graph

7 Gain Optimization of SI 0.53 GE 0.47 Htrojuntion Bipolar Phototransistor 7 Fig (8): photourrnt dnsity vrsus olltor width From this fig(8) its vidnt that as th olltor thiknss or width inrass, th photourrnt dnsity also inrass linarly, thrfor olltor width may b sltd nar to 2.0 µm. Now plotting quations (10),(9) and (7) for optial gain vrsus olltor width, w find following urv. Fig(9):optial gain vrsus olltor width From fig(9) w find that optial gain inrass linarly and for olltor layr thiknss 0.7µm th optial gain starts diminishing as if olltor is ompltly dpltd, whras if w inras th olltor width, w s that olltor omponnt of photourrnt dnsity inrass almost linarly. Thrfor olltor width may b sltd as 2.0µm. 6. CONCLUSION: Proposd Htrojuntion phototransistor with optimum paramtrs has gain quation

8 8 Dr. R. P. Mishra t al in whih photourrnt dnsity givs th bnfit of β tims amplifiation of optial urrnt at th front nd of rivrs, furthr havily dopd bas and lightly dopd mittr rdu th bas rsistan and juntion apaitan whih is a nssity for improvd frquny rspons. Th disussd onfiguration of Si 0.53 G 0.47 HPT has apability to dtt as wll as amplify th optial signal ovr a larg band of frqunis. Th us of a singl HPT to play th dual rol of dttor and amplifir will gratly simplify th fabriation of OEIC rivr in th monolithi form. On th basis of th rsults it is xptd that th proposd HPT with optimum paramtrs will mrg as an xllnt altrnativ in th futur optial ommuniation systms. 7. REFERENCES: [1] P. Ashburn, SiG Htrojuntion bipolar transistors Hobokn Nw jrsy; john wily & sons [2] Rosa Lon, Radio frquny thnologis in spa appliations, jt propulsion laboratory, California Institut of thnology, California, JPL Publiations 2005, pp 7-10, http\npp.nasa.gov. [3] ChandraShkhar S, Lunardi LM t all; High spd monolithi p-i-n / HBT and HPT/HBT photo rivr implmntd with simpl photo transistor strutur, IEEE Photonis thnology lttrs 1993;5:pp [4] Ashish gupta, Prof. P hakrabarti ; Projt on Dsign and analysis of singl Si / SiGi HBT basd front nd optial rivr ; IT BHU [5] SM Friml & KP Ronkar, Gumml-Poon modl for NPN Htrojuntion bipolar phototransistor, journal of applid physis, 82(7), ot. 1997; pp [6] J.C. Campbll, S.G. Thomas t all; Intr digitd G p-i-n photodttors fabriatd on a Si substrat using gradd SiG buffr layrs ; IEEE Photonis Thnology Lttrs, vol.38, Sptmbr 2002 pp [7] E. A. Fitzgrald, ML Grn t all; Totally rlaxd G x Si (1-x) layrs with low thrading disloation dnsitis grown on Si substrats ; Applid Physis lttrs 1991, vol.59, pp [8] J.L. Liu, S.Tong t all; High quality G films on Si substrats using Sb surfatant-mdiatd gradd SiG buffrs ; Applid Physis Lttrs ; 2001, vol.79, pp [9] G. Luo, T. yang t all; Growth of high quality G pitaxial layrs on Si(100), Jpn. J. of Applid physis; 2003, vol.42, pp(l517-l519 ). [10] John D. Crsslr; SiG HBT thnology a nw ontndr for Si basd RF and mirowav iruit appliations ; IEEE Transations on mirowav thory and thniqus, vol. 46,no.5, may 1998 pp

Lecture 16: Bipolar Junction Transistors. Large Signal Models.

Lecture 16: Bipolar Junction Transistors. Large Signal Models. Whits, EE 322 Ltur 16 Pag 1 of 8 Ltur 16: Bipolar Juntion Transistors. Larg Signal Modls. Transistors prform ky funtions in most ltroni iruits. This is rtainly tru in RF iruits, inluding th NorCal 40A.