Detector Quang Hoc Bang Ban Dan

5/25/2018 Detector Quang Hoc Bang Ban Dan

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________________________________________________________________________________NGUYN CH THNH - tctquang hc bn dn Lp hc chuyn Sn Thng 11 nm 2004

223

Institut d Alembert

tctQuang hc bng Bn dn

NGUYN Ch Thnh

Phng Th nghim Phtnic Lng tv Phn tTrng i Hc SPhm Cachan

n vNghin cu Hn hp s8537,Trung Tm Quc gia Nghin cu Khoa Hc Php

61 avenue du Prsident Wilson94235 Cachan cedex

Php

[email protected]


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NGUYN Ch Thnhsinh nm 1956 ti Vit Nam. Hon thnh lun n Tin sVt l Trngi Hc Paris 11 nm 1993 trong lnh vc Quang hc v Phtnic. Sau thi gian lm nghin cusau tin sti trng Ecole Normale Suprieure de Cachan (ENSC), ni ng tip tc nghin cucc ti trong lnh vc Quang phv ng dng, ng thi vi vic ging dy vt l, ng volm vic ti Trung tm Quc gia Nghin cu Khoa Hc Php (CNRS) nm 1996 vi tcch l KsNghin Cu. Thi gian t1996 n 2002, ng lm nghin cu Phng th nghim Vt l

Laser, thuc Vin Galile, ti Trng i hc Paris 13, trong lnh vc Vt l Laser, Quang Phlaser, o lng tn sv ng dng. Tnm 2002, ng vlm nghin cu ti Phng th nghimPhtnic Lng t v Phn t (Laboratoire de Photonique Quantique et Molculaire) thuctrng ENSC trong lnh vc microphtnic v ng dng. ng l trng nhm nghin cu"Microphtnic v linh kin phtnic lm bng polymer cho truyn thng quang hc" ca Phngth nghim LPQM. Ti y ng tham gia cc ti nghin cu trong cc lnh vc Microlaser,Quang hc phi tuyn, Microphtnic phn t, Quang hc tch phn v linh kin phtnic lmbng polymer.Tnm 1993, song song vi hot ng nghin cu khoa hc, ng ging dy ti trng ENSCvi tcch l cn bthnh ging trong lnh vc Quang in thc, Quang hc, Vt lLaser vlinh kin phtnic ttrnh Cnhn Vt l v Vt l ng dng, o to ti nng Vt l, oto Gio sThc sngnh Vt l v Vt l ng dng n Master Vt l v Vt l ng dng.


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MC LC

I. DN NHPI.1. Nhc li cc im chnh trong tng tc phtn-bn dn

I.1.1 Chuyn dch in ttrong cht bn dnI.1.2 Phn xv hp thphtn

I.2. tctquang hc bn dnI.2.1. Nguyn l vn hnh cbnI.2.2 Cc c trng chung

a)Hiu sut lng t.b) p ng c trng theo ph.c)nhy.d) p ng thi gian.

II. CC BTIP GIP BN DNII.1 Btip gip p-n

a)Btip gip p-n trng thi cn bng nhit ng.b) Btip gip p-n c phn cc.c) in dung chuyn tip v in dung khuch tn.

II.2. Tip xc kim loi-bn dnII.2.1. Btip gip Schottky

a) Btip gip Schottky trng thi cn bng nhit ng.b) Btip gip Schottky c phn cc.

II.2.2. Tip xc thun trIII. TCTQUANG HC LNG T

III.1 TCTQUANG DN INIII.1.1 Vn hnh ca mt tctquang dn inIII.1.2 khuch i ca quang dn inIII.1.3 p ng thi gian

III.3 TCTQUANG HC DNG BTIP GIPIII.3.1 it quang p-n

a)Vn hnh ca it quang p-n.b) p ng thi gian.

III.3.2 it quang p-i-nIII.3.3 it quang dng hiu ng nhn in

a) Vn hnh ca it quang dng hiu ng nhn in.b) p ng thi gian.

III.3.4 it quang SchottkyIII.3.5 it quang MSM (Mtal-Semiconducteur-Mtal)III.3.6 it quang dng cu trc dn sng

IV. TING N TRONG CC TCTQUANG HC BN DNIV.1 Gii thiu tng quanIV.2 Cc ngun tingn

IV.2.1 Tingn phtnIV.2.2 Tingn do sto cp v ti hpIV.2.3 Tingn do nhn inIV.2.4 Tingn nhitIV.2.5 Tingn 1/f

IV.3 nhy c trngIV.3.1 nhy c trng ca tctquang hcIV.3.2 nhy c trng ca tctquang dn in

IV.3.3 nhy c trng ca it quang p-i-nIV.3.4 nhy c trng ca it quang dng hiu ng nhn inIV.3.5 o tn hiu quang hc bng php o trc tip vi cc it quang

a) o tn hiu quang bng php o trc tip vi it quang p-i-nb) o tn hiu quang bng php o trc tip vi it quang dng hiu ng nhn in

V. O TN HIU QUANG HC BNG PHP O KT HP

TI LIU THAM KHO TM TT


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I. DN NHPTrong mt htruyn thng quang hc (chng hn nhtruyn tn hiu bng si quang, vn hnh ca cc linh

kin chc nng trong quang hc tch hp), tctquang hc l linh kin c chc nng chuyn i tn hiu quangthnh tn hiu in chng c thc xl bng cc linh kin in t. Cc tctquang hc dng trong truynthng quang hc l nhng tctlng tlm bng bn dn. Trong ti liu ny, chng ti trnh by hai phn chyu: phn u trnh by cc tctquang hc bng bn dn di cch nhn ca vt l cc linh kin bn dn vphn sau trnh by hiu nng ca cc linh kin ny trong vic o cc tn hiu quang.

Trong phn u, trc ht chng ti nhc li cc im chnh ca tng tc phtn vi cht bn dn trong cch

o sng quang v cc b tip gip bn dn-bn dn v bn dn-kim loi. Kn, chng ti trnh by vt l cctctquang hc bn dn trn csca cchquang dn in trong mt cht bn dn (tctquang dn in),trong cc btip gip bn dn (it quang p-n, it quang p-i-n, it quang dng hiu ng nhn in) v trong ccbchuiyn tip kim loi-bn dn (it quang dng hiu ng Schottky, it quang kim loi-bn dn-kim loi).

Trong phn sau, chng ti strnh by, di quan im vt l cc linh kin, cc dng tingn (ting Php: bruit/ting Anh: noise) khc nhau trong qu trnh o cc tn hiu quangbng php o trc tip cng nhcc tham sctrng cho hiu nng ca cc tctlng t. Chng ti trnh by sau php o kt hp (dtection cohrente/coherent detection) cc tn hiu quang, php o c sdng nhiu trong cc htruyn thng quang hc nhm cithin hiu nng ca qu trnh o tn hiu quang.

I.1. Nhc li cc im chnh trong tng tc phtn-bn dn

I.1.1 Chuyn dch in ttrong cht bn dn

Cc chuyn dch quang hc gia cc trng thi in ttrong mt cht bn dn gm: hp th, bc xtpht v bc xcng bc. Qu trnh hp thmt phtn trong cht bn dn l kt quca vic chuyn dch mt

in ttrong vng ho trsang vng dn ca cht bn dn. Qu trnh ny tng ng vi vi vic to ra mt cp htmang in tdo in t-ltrng xy ra ngay sau tng tc gia mt phtn vi cht bn dn. Cchny c sdng trong phng php o thng lng phtn bng cht bn dn. Chc nhng phtn m nng lng lnhn rng ca vng cm mi c cht bn dn hp th: hEg.

Tng tc phtn-in t(hay phtn-l trng) ny phi tun theo cc nh lut bo ton, nh lut phn bthng k lng tca cc ht mang in c khnng tng tc vi cc phtn. Cc chuyn dch quang hc trongbn dn tun theo cc nh lut bo ton sau y:

(i) Bo ton nng lng :Eil nng lng ban u (trc tng tc) v Efl nng lng sau cng (sau tng tc) ca in tv Epl

nng lng ca phtn ; theo nh lut bo ton nng lng, ta c: Ef- Ei= Ep (I.1)Du+ tng ng vi trng hp hp thmt phtn v du tng ng vi trng hp bc xmt phtn.

(ii) Bo ton ng lng ( kp r

hr = ):

fkr

v ikr

l cc vectsng trc v sau tng tc ca in t, pkr

l vectsng ca phtn ; theo nh lut

bo ton ng lng, ta c :pif

kkk rrr

= (I.2)

Vi cp ht tng tc ny, trong phm vi cc cht bn dn thng dng v cc phtn trong di phhng ngoi,ngi ta chng minh rng : klectron>>kphoton. iu ny dn n kt qul: kfki (I.3)

absorption

(a) (b) (c)

missiontransitions

non-radiatives absorption thermalisation

Hnh I.1 : Cc chuyn dch gia cc vng trong cht bn dn: (a) chuyn dch trc tip; (b,c) chuyn dch gin tip

Ngha l vectsng in tc bo ton trong tng tc vi phtn. Ngi ta biu din sbo ton vectsngin tbng mt vectsng c vtr thng ng trong khng gian k. Nhvy, v l do bo ton vectsng in ttrong tng tc vi phtn, chc nhng chuyn dch in tc vtr thng ng trong khng gian k mi c xeml chuyn dch quang hc. iu kin ny chc ththc hin c trong cc cht bn dn c vng cm trc tip(tc l nh ca vng ho trngay hng thng ng vi y ca vng dn) (hnh I.1.a). Thc vy, trong mt chtbn dn c vng cm trc tip, cc chuyn dch quang hc theo vtr thng ng tun theo ng cc nh lut boton nng lng (EC - EV= h) v ng lng (kf= ki). Hin tng bc xv hp thdc thc hin trong cccht bn dn kiu ny.


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Trong cc cht bn dn c vng cm gin tip (tc l nh ca vng ho trkhng xp hng thng ng vi yca vng dn), chuyn dch quang hc chc ththc hin c vi iu kin c thm mt cchchuyn dch phthch hp. Trong cchny, sng gp ca mt ht tng tc thba (nhphnn quang hc chng hn) l cnthit tun thcc nh lut bo ton. Trong trng hp ny, cc chuyn dch in tgia y ca vng dn vnh ca vng ho tr(hnh I.1.b) khng cn vtr thng ng trong khng gian k ; chng khng cn l chuyn dchquang hc na. Trong loi vt liu ny, chuyn dch quang hc kh c khnng thc hin c, l do l v cc nhlut bo ton khng cn c tn trng.

Ngc li, trong loi vt liu c vng cm gin tip, hp thmt phtn l iu c ththc hin c, nhvomt cchtrung gian nhc trnh by trn hnh I.1.c. Trong trng hp ny, mt phtn c nng lng ln hn

rng ca vng cm (h> EC - EV) c thc hp thbng mt chuyn dch quang hc c vtr thng ng gianh ca vng ho trv y thhai ca vng dn (vectsng k c bo ton), nng lng tha trong qu trnhhp thny sc tiu tn di dng nhit trong vt liu.

Chc nhng ht ti in m nng lng, ng lng v mt trng thi tha mn cc iu kin bo ton mic khnng tham gia vo cc tng tc vi phtn. Cc ht ti in ny c gi tn l cc ht ti in quang hc(gi tt l ht quang ti in).

I.1.2 Phn xv hp thphtn

Hnh I.2

Gi 0(E) l thng lng phtn ti vi nng lng E = h. Thng lng ny c o bng sphtn c nnglng E p ln mt n vbmt ca cht bn dn trong mt n vthi gian. R(E) l hsphn xca cht bndn i vi bc xc nng lng E (phn ln cc cht bn dn c hsphn xl R 30%). t(E) l thng lngtruyn qua, ngha l thng lng cc phtn xm nhp vo bn trong thtch ca cht bn dn v r(E) l thnglng ca cc phtn nng lng E phn x trn b mt ca cht bn dn (hnh I.2): t(E) = [1-R(E)] 0(E).

Hnh I.3 : Gin phn bthng lng phtn trong bn dn

c trng ca shp thcc phtn trong qu trnh truyn bn trong cht bn dn l hshp th(E,x),

c nh ngha nhsau :)xE,()xE,(d

dx1

x)(E, (I.4)Nhvy ta c: (E,x) = 0(E)[1-R(E)] exp[-(E)x] (I.5)

Hsphn xR(E) phthuc vo bn cht ca cht bn dn, nhng ni chung gi trca n t thay i theonng lng phtn khi m nng lng ny rt gn vi nng lng vng cm ca cht bn dn ; nhvy chng ta c

thvit: R(E) = R. Tri li, gi trca hsny thay i rt nhiu theo gc chiu ca chm tia ti. Gi trny t

cc tiu khi chm tia ti thng gc vi bmt bn dnv khi hsphn xbng :2

S

S

1n1n

R

= (I.6)trong nSl chit sut ca mi trng bn dn. Nu chm tia ti gm cc phtn n sc, hsquang hc tocp in t-ltrng sbng ng tsut bin mt ca cc phtn :

G(E,x) =dx

)xE,(d

= (1-R) 0(E)(E) exp[-(E)x]Nu chm tia kch thch l a sc th hsquang hc to cp l : G(E,x) =

E

x)dE',G(E' (I.7)

i vi mt cht bn dn, nu hshp th(E) = 0 i vi tt cphtn m nng lng E < Eg(vt liu trong


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sut i vi cc phtn ny) v (E) hng si vi tt ccc phtn m nng lng E > Eg, chng ta c thvit : G(E,x) = (1-R) exp(-x) ')dE(E'0 Eg (I.8)Nu 0l thng lng ca tt ccc phtn m nng lng E > Eg th ta c:

G(E,x) = 0(1-R) exp(-x) (I.9)

Hnh I.4 : Bin i ca hshp thv bdy hp ththeo di sng ca 4 cht bn dn (theo[11])

Hshp thca vt liu bn dn thay i theo bc sng (hay theo nng lng phtn) ca bc xkch thch.Hnh I.4 biu din ng cong h s hp th theo bc sng ca bc x kch thch ca 4 loi vt liu bn dnthng dng trong cng nghchto cc tctquang hc.

Cc phtn c hp thto sinh cc cp in t-ltrng (l cc ht quang ti in) thng dtrong vt liu bndn. C hai loi qu trnh xy ra ngay sau khi to cp i vi cc ht quang ti in: hoc l chng tti hp trongqu trnh khuch tn bn trong bn dn vi thi gian sng (thi gian sng nca in tv thi gian sng pcaltrng), hoc l chng bin trng qut ngay vcc tip im thun tr(ohmique - ohmic) vi mch ngoi. Ccphng trnh m tcc cchny c suy ra tcc phng trnh tin ho sau y:

xJe1nt)(x,Gtnn

noptique (I.10)

x

J

e1

p

t)(x,Gtp p

poptique

(I.11)trong mt dng in tJnv mt dng ltrng Jpc xc nh bng cc phng trnh:

Enxn

DeJ

nnn = (I.12)

Epxp

De

Jpp

p (I.13)e l in tch ca in t; n, Dn, nv p, Dp, pln lt l mt , hskhuch tn v linh ng ca in tvca ltrng trong bn dn.

I.2. tctquang hc bn dnI.2.1. Nguyn l vn hnh cbn

Hnh I.5 : Ba cchca o lng tsng quang hc: (a) php o tng ng vi chuyn dch quang hc gia hai vng km vivi to cp in t-l trng ; (b) vi c chvt ro thbng hin tng quang pht xbn trong vt liu ; (c) dch chuynquang hc tmt mc lin kt n mt mc tdo (theo [1])


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Nguyn l vn hnh cbn ca cc tctquang hc bn dn l : trong trng thi khng c kch thch quanghc, cc ht ti in trong cc vt liu bn dn khng c khnng to ra trng thi dn in ; bi v, hoc l chngcngtrong mt vng khng c khnng tham gia dn in (chng hn trong vng ho trblp kn, nhtctquang hc bn dn ni bm) ; hoc l chng bchn bi mt ro thnng (chng hn nhro thSchottky), hocl chng tn ti trn mt mc lng t lin kt (nh l tctquang hc bn dn ngoi lai, tctquang hcdng ging lng t). Nhvy chnh dch chuyn quang hc gia hai tp hp cc mc nng lng lng t(tp hpny ng gp vo trng thi dn in, tp hp kia ng gp vo trng thi khng dn in) l ngun gc ca ccho sng quang hc (hay o thng lng phtn). Nguyn l gii thch v sao ngi ta dng tn gi chng loichung cho cc tctquang hc bn dn l tctlng t.

I.2.2 Cc c trng chung

Cc c trng ca mt tctquang hc l:

a)Hiu sut lng t.

Hiu sut lng ttrong : i= (Thng lng cc ht quang ti in)/(Thng lng cc phtn ti)= 1- exp(-d) (voir figure I.4). Hiu sut quang hc : optique= 1 R Hiu sut ton b(hiu sut lng t) : = i .optique= (1-R)[ 1- exp(-d)] (I.14)(n vm-1) l hshp thca vt liu v d (n vm) l di (hay su) ca vng hp thtrong tct.

Hiu sut lng tl mt hm sca bc sng v hshp thv hsphn xthay i theo bc sng.

b) p ng c trng theo ph.

Hnh I.6 : Biu din ng p ng c trng theo phca cc tctbn dn

p ng c trngca mt tctc nh ngha bng tssau :i= (Cng dng quang in)/(Cng sut chiu sng ca sng quang ti)1,24

(m)g

hge

h

ge

P

I

inc

phi

=== (A/W) (I.15)vi : hiu sut lng t; g : hskhuch i ca tct; e : in tch cbn ; v : tn sv bc sng casng quang ti. p ng c trng ca mt tctlng tphthuc vo bc sng ti. Cc ng biu din png c trng ca tct lng t c mt gii hn trn theo bc sng (bc sng ngng) tng ng vi rng ca vng cm ca cht bn dn (hnh I.6). Hnh I.7 biu din cc ng p ng c trng tiu biu ca vitctquang hc bn dn.

Hnh I.7 : p ng c trng theo phca vi tctquang hc bn dn [theo [6])


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p ng ca mt tctc thbxung cp tutheo iu kin chiu sng trn n. Mt p ng ng phi lp ng phthuc tuyn tnh vo cng sut chiu sng ca sng ti. Thnhng, p ng ca mt tcttrnnbo ho khi m n c chiu sng qu mc. Do , cn thit phi bit di ng tuyn tnh (dynamique linaire/linear dynamic range) p ng ca mt tctquang hc sdng ng thit bny.

c)nhy.

i lng biu din khnng o cng sut chiu sng ti thiu ca sng quang ti m vn khng bln vi tingn ca ttc gi l nhy ca tct. Tham sbiu din nhy ny l nhy c trng(dtectivit/detectivity) ca tct. Chng ta stho lun vnhy c trng ca tcttrong chng III*.

d) p ng thi gian.

Cc hng sthi gian ng gp vo vic gii hn p ng thi gian (di y gi tt l p thi) ca tctquang hc l:Thi gian cc ht quang ti in vt qua vng hot tnh ca bn dn (vi shin din ca in trng);Thi gian sng ca cc ht quang ti in trong vng khuch tn;Hng sthi gian RC, l p thi ca mch in hp thnh tin dung v in trca tctvi mch c tnhiu in.Hng sthi gian thit lp hskhuch i trong cc tctquang hc dng hiu ng nhn in.

Di truyn qua ca cc tctlng trt rng (di truyn qua ca cc tctquang hc cc nhanh c tht n hng trm GHz).

II. CC BTIP GIP BN DNCc tctquang hc bn dn c thc hin chyu trn cscc btip gip (jonctions junctions): hoc

l mt btip gip cu to thai loi bn dn khc nhau (loi n v loi p), chai c chto tmt vt liu bndn duy nht (btip gip ng th), hoc l mt btip gip cu to thai loi bn dn khc nhau c chtothai vt liu bn dn khc nhau (btip gip dth), hoc l mt btip gip kim loi-bn dn (tip gip Schottky)hoc l mt btip gip kim loi-in mi-bn dn (cu trc MIS). Trong tp bi ging ny chng ti trnh by hailoi tip gip dng trong vic chto cc tctquang hc: tip gip bn dn-bn dn vtip gip kim loi-bndn.

II.1 Btip gip p-n

Btip gip p-n c cu to tmt vt liu bn dn c pha tp loi p mt bn v pha tp loi n bn cnli (hnh II.1.a). ng c trng I(V) ca dng in chy qua btip ni p-n biu thhiu ng chnh lu.

a)Btip gip p-n trng thi cn bng nhit ng

kho st cc b tip ni p-n, chng ti dng m hnh n gin ca b tip ni gin on (jonctionabrupte/ abrupt junction) mt chiu: vi phn gi trx > 0, cht bn dn c pha tp loi n vi mt khng i

NDcc nguyn tcho (donneurs/ donors); vi phn gi trx < 0, cht bn dn c pha tp loi p vi mt khngi NAcc nguyn tnhn (accepteurs/ acceptors). ngay st lp tip gip (xung quanh vtr x = 0 ): p(x) < pp=NAv n(x) < nn= ND. Vng in tch khng gian (ZCE) c trng bng in tch cnh eNAbn pha p v in tchcnh eNDbn pha n (hnh II.1.a et c). Sphn bin tch lng cc ny to nn mt in trng v do mthiu thVd(hiu thkhuch tn). Hiu thny, trong trng thi cn bng cho php xp ngang hng cc mc nng

lng Fermi; vi kBl hng s Boltzmann, ta c:

=2i

ADBd n

NNln

e

TkV (II.1)

Tphng trnh Poisson, ta c thsuy ra hm shiu th:

p2

pS

An

2n

S

D V)d-(x2eN

V)d-(x2eN

-V(x) (II.2)trong biu thc ny S l hng sin mi ca cht bn dn. in trng hng theo trc x v c biu din

bng: )d-(x

eN-E(x) p

S

A= cho trng hp dp


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vi

2/1

D2

BSDn

N2e

TkL

= v

2/1

A2

BSDp

N2e

TkL

= ln lt l di Debyetrong vng n v trong vng p. di Debye

l i lng o chiu su xm nhp ca cc ht in tdo trong vng ZCE.Nu btip gip c cu to theo cch khng i xng, vng ZCE strin khai chyu trong vng t pha tp.

Chng hn trong btip gip p+n : NA>> ND, ta c : W

2/1

2i

ADDnn

n

NNln2Ld

(II.5)

Hnh II.1. Btip gip gin on p-n trng thi cn bng nhit ng

b) Btip gip p-n c phn cc

Khi btip gip p-n c p in th, ro thnng ca n bbin i v kt qul cc ht ti in tdo skhuch tn tvng c mt cao sang vng c mt thp. Nu hiu thphn cc Vapp> 0, ta c trng thiphncc thng(polarisation en direct/ forward bias) (hnh II.2) v nu Vapp< 0, ta cphn cc ngc (polarisation eninverse/ reverse bias) (hnh II.3).

(i) Phn cc thng.

Khi ta p mt hiu thVappthp phn cc thng btip gip, hiu thny ltch xa hai mc nng lngFermi nm hai bn ca vng ZCE : EFp= EFn- eVapp. Trong hai vng trung ho in thvn khng i, ni duy nhttrong btip gip m in thc thgim l vng in tch khng gian ZCE. rng ca vng ZCE do gimthiu.


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Hnh II.2. Btip gip p-n c phn cc thng

Phng trnh truyn ti in tc vit :n

pn n-n(x)-x

nDx

-tn (II.8)

Trong Dnl hskhuch tn v n l thi gian sng ca in ttrong vng p. Nu gi nnn DL ldikhuch tnca in ttrong vng p th li gii ca phng trnh (II.8) c vit l:

n

pp

n

p

TeVapp/kpp

L

Wdsinh

L

Wxsinh

1)(enn-n(x) B (II.9)

trong Wpl chiu rng ca vng p. Dng in ton phn to bi vic phun in tvo vng p cng l dng inkhuch tn im x = -dp(quy c chiu dng l chiu ca dng in chy tvng p sang vng n):

=

= 1-

Tk

eVexpj1)(e

L

Wdcoth

L

neDj

B

appns

TeVapp/k

n

pp

n

pnn

B (II.10)

Trong biu thc ny jnsl dng in khuch tn gii hn ca in t:

=n

pp

n

pnns L

Wdcoth

L

neDj (II.11)

p dng cng lp lun nhtrn, ta c dng in to bi vic phun ltrng trong vng n, vi ppp DL l di khuch tnca ltrng trong vng ny:

=

= 1-

Tk

eVexpj1)(e

LdW

cothL

peDj

B

appps

TeVapp/k

p

nn

p

npp

B (II.12)

Trong jpsl dng in khuch tn gii hn ca ltrng:

=p

nn

p

npps L

dWcoth

L

peDj (II.13)


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233

Dng in ton phn chy qua btip gip di phn cc thng l tng sca hai dng dn to bi cc ht tiin thiu sc phun vo hai pha ca vng in tch khng gian ZCE :

= 1

Tk

eVexpjjj)(Vj

B

appsatpnapp (II.14)

Trong jsatl dng in bo ho ca btip gip p-n v c vit:

=

p

nn

p

np

n

pp

n

pnpsnssat L

dWcoth

L

peD

L

Wdcoth

L

neDjjj (II.15)

Hnh II.3. (a,b) Btip gip p-n c phn cc ngc v (c) ng c trng I(V) ca btip gip p-n

(ii) Phn cc ngc.

Hthc (II.14) c thit lp trong trng hp phn cc thng vi hiu thVapp> 0. Trong trng hpphn cc ngc (vi hiu thVapp< 0) cc phng trnh thit lp trn vn ginguyn dng nhng trong trnghp ny ro thnng c nng cao (hnh II.3.a,b). Trong iu kin , hai vng trung ho ca btip gip sbthiu ht cc ht ti in thiu s. Ngay khi gi trca hiu th phn cc-Vapp>> (kBT/e), ta c : j(-Vapp) - jsat.

ng c trng I(V) ca mt it tip gip p-n c biu din trn hnh II.3.c.c) in dung tip gip v in dung khuch tn

Khiphn ccngc,vi phn bin tch trong vng ZCE, btip gip p-n to nn in dung vi phn:

WS

dVdQ

C St = (II.16)Trong dQ l in tch to ra trong vng ZCE khi hiu thphn cc bin i mt lng dV. W l rng ca vngZCE v c xc nh bng hthc (II.4), S l tit din thng ca btip gip. Ctc gi l in dung tip gip(capacit de transition) ca btip gip p-n. Nu btip gip rt t i xng, chng hn trong btip gip p+n : NA

>> ND, ta c : W dn. Nhvy:appd

1/2DS

n

St

VV

12eN

Sd

S

dVdQ

C

= (II.17)


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234

Khi btip gip c p mt hiu thxoay chiu: Vapp(t) = V0+v.exp(jt), vi v L).Tphng trnh Poisson:

S

D2

2

eN

-dx

V(x)d = , ta c thsuy ra phn bca in trng :- E(x) = L)-(x

eN

-S

D (II.19)


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235

V phn bca in th(hnh I.2.5.b) :

= Lx-2x

eN

-V(x)2

S

D (II.20)

Hnh II.5. Btip gip Schottky trng thi cn bng nhit ng

Hiu thkhuch tn, i lng xc nh chiu cao ca ro thnng trong cht bn dn c vit:2

S

DSmd L2

eN0)V(x-L)V(x

e-e

V = (II.21)T suy ra brng ca vng in tch khng gian ZCE trng thi cn bng:

1/2

SmD

S1/2

D

dS )(eN2

eN

V2L

= (II.22)

b) Btip gip Schottky c phn cc

(i) c trng in thdng in.Kho st trng hp btip gip c p mt in thdng tnh theo chiu tkim koi sang bn dn (Vm

VS> 0) (hnh II.6.b). Vng dn ca bn dn trong trng hp ny c nng cao mt mc bng eVappv congca n gim bt. Kt qul ro thtbn dn kim loi bgim chiu cao trong khi chiu cao ro thtkim loibn dn vn khng i. Trng hp ny khin cc in ttdo trong bn dn khuch tn sang kim loi v tothnh dng in khuch tn chy tkim loi sang bn dn. Btip gip nhvy cphn cc thng. Dng inhp bi cc ht ti in thiu s(trong trng hp ny l cc ltrng) l khng ng k, do dng in chytrong btip gip chyu l dng cc ht ti in a s.

mt tip gip hai mi trng ca btip gip c tn ti ng thi mt dng in to bi pht xnhit in t(mission thermolectronique/ thermoelectronic emission). Ngun gc vt l ca ca dng in ny l hin tngpht xnhit in t; l cc nhit in tc ng nng vt qua ro thbmt ca mt vt liu. Dngin ny nhthc ng gp bi pht xnhit in tca cc ht ti in tdo theo hai chiu : tkim loi


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236

sang bn dn v ngc li tbn dn sang kim loi. Phn ng gp ca mi pha (kim loi bn dn hay bn dnkim loi) phthuc vo chiu cao ca mi ro th. Cn trong vng in tch khng gian ZCE ca cht bn dn,dng in di chuyn tun theo cc quy lut ca hin tng khuch tn v c hin din ca graien mt cc ht tiin v quy lut ca in trng hin din trong vng ny. Dng cc ht ti in a sl dng in duy nht tnti, do dng in ny phi c bo ton ; nhvy ta c thtnh ton dng in ny trong vng ZCE hay l mt tip gip hai mi trung.

Hnh II.6. Btip gip Schottky c phn cc

) Dng nhit in t: Githit rng tt ccc nguyn tcho v nguyn tnhn trong cht bn dn ubion ho, dng nhit in ttrng thi cn bng nhit ng c vit nhsau:

jmsc= jscm=

Tk

eV-exp

m2Tk

eNTk

eV-exp

NN

TAB

d1/2

C

BD

B

d

C

D2* (II.23)

trong A* l hng shiu dng Richardsonnh ngha bng:3

2B

**

h

kme4A = ; m* l khi lng hiu dng ca

in t; kBl hng sBoltzmann v h l hng sPlanck.Khi btip gip c phn cc thng (Vm VS> 0) : chiu cao ca ro thkim loi bn dn khng i : dng

nhit in tjmscc ginhc. Ro thbn dn kim loi, ngc li trthnh e(Vd Vapp), chiu cao ro thnhvy bhthp. Dng nhit in tton phn c vit thnh:

je= jscm- jmsc=

TkeV-exp-Tk

)V-e(V-exp

m2Tk

eNB

d

B

appd2/1

C

BD (II.24)

Nhvy:

1ejj Tk

eV

seeB

app

(II.25)


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237

vi jsel dng nhit in tbo ho:Tk

eV-1/2

C

BDse

B

d

em2Tk

eNj

(II.26)) Dng in khuch tn: Dng in ny l kt quca skhuch tn cc in ttdo trong vng ZCE ca

cht bn dn. Tnh ton [2] cho thy dng in khuch tn c thc vit di dng :

1ejj Tk

eV

sddB

app

(II.27)

vi jsdl dng in khuch tn bo ho:Tk

eV-1/2

appd

1/2

S

DnDsd

B

d

eVV

2eNeNj

= (II.28)

) Thp thai dng in: Cc biu thc thu c thai cch tnh ton (dng nhit in tv dng khuchtn in t) l rt gn nhau v chng cng thay i theo quy lut (eeVapp/kB

T-1), l hm sca in p Vapp, vy th ta

c thvit:

= 1-ejj Tk

eV

satB

app

(II.29)

Trong dng in bo ho thay i vi quy lut hm mtheo nhit v theo hiu scng thot ca kim loiv ca bn dn: jsatexp(- eVd/kBT). c trng I(V) ca btip gip c trnh by trn hnh II.6.d. Bi v dngin c bo ton, hai biu thc tm c ca hai loi dng in phi ng nht. Ta c thxem nhhai loi dngin ny c mc ni tipv dng in chy qua btip gip bchi phi bi mi vng m n chy qua(ngha ltrong vng ZCE dng in ny l dng khuch tn, trong khi mt tip gip hai mi trng dng in ny l dng

nhit in).(ii) in dung vi phn.

Nu btip gip c phn cc di hiu thVappm theo chiu thng (Vm-VS> 0), chiu cao ro thtrthnh

e(Vd-Vapp) v rng ca vng in tch khng gian ZCE c vit :1/2

appdD

Sapp )V-(VeN

2)L(V

= . Cng nhtrong

trng hp ca btip gip p-n, mt thay i vi phn dVappsko theo mt thay i brng L(Vapp) ca vng ZCEv kt qul mt in tch vi phn dQ c pht trin trong vng ny. Nhvy cu trc btip gip Schottky cmt in dung vi phn. in tch khng gian c vit: QS= - Qm= eNDL = [2SeND(Vd-Vapp)]1/2. in dung vi phn

ca btip gip:L

V-V2eN

dVdQ

)C(V S1/2appd

1/2DS

app =

= (II.30)I.2.2.2.Tip xc thun tr

Tip xc thun tr(contact ohmique/ ohmic contact) l tip ni cn thit c thni mch ngoi vi vnghot ng ca mt linh kin bn dn. Tip xc thun trc nh ngha nhl tip xc kim loi-bn dn c intrtip xc rt nhso vi in trca khi bn dn hay in trni tip ca cc linh kin bn dn. Nu btip gipSchottky c c trng bng hiu ng chnh lu th ngc li, tip xc thun trc c trng I(V) tuyn tnh theonh lut Ohm, theo chai chiu phn cc. in trca tip xc thun trphi c gi trthp nht c ththc hinc, ngha l sgim hiu thtrn tip xc thun trphi nhhn rt nhiu so vi sgim hiu thbn trongkhi bn dn.

Nu nhi vi bc huyn tip Schottky dng bn dn loi n, iu kin cn thit l : m> SC(n), th iu kincn thit cho tip xc thun trcng loi, ngc li, phi l : m< SC(n).

iu kin cn thit i vi tip xc kim loi v bn dn loi p cng tng t nh trn : i vi b tip gipSchottky ta c: m< SC(p) ; trong khi i vi tip xc thun trth: m> SC(p) .III. TCTQUANG HC LNG T

III.1 TCTQUANG DN IN

III.1.1 Vn hnh ca mt tctquang dn inKho st mt linh kin quang dn in c dng hnh hc nhtrnh by trn hnh II.1. Khi khng c chiu

sng, in dn (conductivit/ conductivity) ca linh kin biu din theo mt cc ht ti in tdo (n0v p0) v linh ng ca chng (net p) l: 0= n0ne + p0pe (III.1)

Khi linh kin c chiu sng, gi n v p l mt in tv mt ltrng do kch thch quang hc tonn, in dn ca linh kin, trong trng hp ny, trthnh: = 0+ ; vi l bin thin ca in dn khilinh kin c chisng v c nh ngha bng: = e(nn + pp). V rng n v p l mt cc ht ti insinh ra do sto cp bng hp thcc phtn, cho nn squang in tng bng squang ltrng : n = p.Do ta c thvit: = enn(1+p/n)


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238

Hnh III.1 : Svn hnh ca mt linh kin quang dn in

Hai in cc (l tip xc thun tr) c dng p mt in trng E song song vi bmt ca linh kin vthu thp cc ht quang ti in. Githit rng di khuch tn ca cc ht quang ti in l ln (chngkhng bti hp trc khi n c in cc), th mt cc ht ti in thng dsinh ra do chiu sng c thbiu

thbng : n =wld0 (III.2)

Trong l hiu sut lng tton phn; 0l thng lng phtn ti v l thi gian sng ca cc ht quangti in.

Bin thin tng i ca in dn biu din theo thng lng ca tn hiu quang ti (0) l :

wld

)e(

d 0pn = (III.3)

Khi linh kin quang dn in c dng trong mt mch in, i lng thng dng biu din l in trca linh kin ; in trca tctquang dn in l : Rd= l/(wd). Vy th bin thin in trca tctquangdn in theo thng lng ca tn hiu quang ti c biu din bng:

d

Rwdd

ldR d2d = wld

)e(R 0pnd

(III.4)

Hnh III.2 : Sdmch in dng phn cc tctquang dn in

Tn hiu quang hc c biu din bng cng sut ca sng quang ti (o bng Watt) vi githit rng sng

quang ti l n sc :

hcP 00= (Watt). c tho tn hiu in ca tct, ta c thphn cc tcttheo s

mch in trnh by trn hnh III.2. Hiu tho c trn in trRLca phti l :Ld

L0

RRRV

V . Bin thin ca

hiu thdo bin thin in trca tctquang dn in (hay ni cch khc tn hiu in biu ththeo tn hiu

quang ti) c vit : 0pn

2dL

dL02

dL

dL0 Pwldhc

)(e

)R(R

RRV

)R(R

dRRV-dV

== (III.5)

p ng c trng theo hiu thca tctnhvy l : V () = dAhc)(

)R(RRieR

PdV

d

pn

dL

dL

0

(III.6)Trong Ad= wl l din tch bmt ca tct; i l cng dng in chy qua linh kin.

Ta cng c thbiu din p ng c trng ca tcttheo dng in :

i () = dhcA)(e

)R(RiR

PdV

R1

Pdi

d

pn

dL

d

0L0

(III.7)Tbiu thc ny ta thy, tng gi trp ng ca tctcn phi lm tng hiu sut lng t, thi gian sngca cc ht quang ti in, linh ng ca cc ht ti in ny v dng in phn cc trong linh kin (bng cchtng hiu thphn cc).


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239

II.1.2 khuch i ca quang dn in

Ta c thvit p ng c trng theo dng in ca mt tctquang dn in di dng :

i() = Ghce

(III.8)

Trong G =dA

)(i

d

pn

. Cte = g. Cte ; vi Cte =Ld

d

RRR . Hsg c gi l khuch ica quang dn

in. lm n gin biu thc, chng ta kho st trng hp mt cht bn dn ngoi lai (chc mt loi ht ti

in t do). Trong trng hp ny, ta thay i lng (n+p) bng : g = wldi = 2l

wdl

i = 2d l

iR =l

lVd =

lE

; trong Vdl hiu thgia hai in cc ; E l in trng gia hai in cc. ThnhngEl = t

biu din thi gian di chuyn ca cc ht ti in gia hai in cc (di tc dng ca in trng). Nhvy khuch i ca quang dn in c biu thbng tsgia thi gian sng v thi gain di chuyn gia hai in cc

ca cc ht quang ti in: g =t

(III.9)

II.1.3 p ng thi gian

Kho st trng hp mt linh kin quang dn in ngoi lai loi p. Phng trnh tin ho ca mt cc ht

quang ti in chiu theo phng kch thch quang hc (phng x trong hnh III.1) c vit:tp =

x)(t,Goptique p

p

xJ

e

1

p

; trong tsut to cp quang hc trong cht bn dn l .Goptique(t) ; p l mt cc ht quang ti in thng dv p l thi gian sng ca cc ht quang ti in. Githit rng cchiu sngln vt liu l ng nht, bng cch tnh tch phn phng trnh ny theo bdy ca linh kin quang dn in (tx

= 0 n x = d), ta c :dt

pd= (t)Goptique -

p

p Ch rng dng quang in lu thng theo phng thng gc (phng y trong hnh v) vi phng kch thch

quang hc (phng x), do : 0x

Jp =

.

Nu cng sut chiu sng l hng sta c thvit : Goptique(t) = G0 ; V nu chiu sng bt u thi im t =0 vi: p(0) = 0, th p ng thi gian ca linh kin c vit : p(t) = pG0(1-e- t/p).

Nu cng sut chiu sng thay i theo thi gian theo quy lut hnh sin th ta c : Goptique(t) = G0 sint, vi p(0)= 0, p ng thi gian ca tcttrong trng hp ny l: p(t) = pG0(1-e- t/p)sin(t+). Bin p ng thigian trong chn nh biu din theo tn sc vit: p() = 2

p2

p0

1

G

(III.10)

Nhvy tctquang dn in phn ng nhmt blc ly htn (filtre passe-bas low-pass filter) c tn sct

(frquence de coupure cut-off frequency) l : fc=p2

1 (III.11)

Hnh III.3 : ng p ng tn sca mt tctquang dn in tng ng vi hai thi gian sng khc nhau (theo [1])Khi tng gi trthi gian sng ca cc ht quang ti in, ta lm tng di thng v cng ng thi lm gim

khuch i g ca tct. iu phn nh c trng ca mt tctquang dn in : tch skhuch i dithng l mt hng s(hnh III.3).


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240

III.3 TCTQUANG HC DNG BTIP GIP

Mt trong nhng kh khn ca vic sdng tctquang dn in l trkhng ca n thp. khng phi ltrng hp ca it quang hc. tct quang hc dng b tip gip (dtecteur photovoltaque/ photovoltaicdetector) sdng c trng chnh lu ca mt it p-n hoc mt it Schottky c c mt trkhng cao v tch cp in t-ltrng, sinh ra trong qu trnh tng tc vi phtn. Nhvy, chnh in trng bn trong itgivai tr lm lu thng dng in.

III.3.1 it quang dng tip gip p-n

it quang c cu to tmt btip gip p-n (tip gip ng thhay tip gip dth) vn hnh di chphn cc ngc hoc khng phn cc.

a)Vn hnh ca it quang p-n

Hnh III.4 : Nguyn l vn hnh ca mt it quang : (a) gin cc vng nng lng, (b) cu to ca btipgip p-n v hp thcc phtn, (c) ng biu din sto cp quang hc (d) cu hnh tiu biu ca mt itquang.

Khi it c chiu sng, cc phtn ti c hp th trong ba min khc nhau ca it quang (hnhIII.4.b) : cc phtn c nng lng cao (tng ng vi bc sng ngn) c hp thtrong vng p+, cc phtnc nng lng thp hn (tng ng vi bc sng di hn) c thxm nhp n vng in tch khng gian (ZCE)v n vng khuch tn ca it. Sto cp in t-ltrng bng kch thch quang hc trong cc vng ny nggp vo vic to thnh dng quang in trong it quang. Cc cp in t-ltrng sinh ra bng kch thch quanghc trong vng ZEC lp tc bqut ngay bi in trng rt mnh trong vng ny vpha cc vng p+ v n. Cc htquang ti in ny ng gp vo vic to thnh dng quang in sinh ra do to cp quang hcJG(sau y gi tt ldng quang in to cp) chy qua it quang. Cc cp in t-ltrng sinh ra bng kch thch quang hc trong cc


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241

vng trung ho (vng p+ v vng n) tkhuch tn v mt sht sbti hp trong vng ny. i vi mt sccht khc, c thkhuch tn n bin gii ca vng ZCE, chng lin bqut ngay bi in trng ca vng ZCE. Ccht quang ti in khuch tn ny ng gp vo vic to thnh dng quang in khuch tn Jndiffv Jpdiff. Dngquang in ton phn Jphl hp thnh ca hai loi dng quang in ktrn, c tnh ton ti cng mt im tronglinh kin, chng hn ti im W, ta c: W)(xJW)(xJW)(xJJ pdiffGndiffph =

(i) Dng quang in to cp:

Phng trnh tin ho ca cc ht quang ti in c vit:x

Je1

n(x)G

tn Gn

noptique , vi hsut to

cp quang hc c xc nh bi (I.9): x-0optique R)e-(1(x)G . Di chthng trc, phng trnh ny trthnh: x0Gn R)e-(1dxdJe1 ; n = 0 bi v tt ccc ht ti in trong vng ZCE bqut ngay bi in trngtrong vng ny. Dng in JGnsthu c bng cch ly tch phn phng trnh sau cng tx = 0 n x = W (b

rng ca vng ZCE), vi lu l JGn(0) = 0. Kt qul : JGn(W) = JG= )eR)(1-(1eW

0 (III.12)

chnh l dng in tchy qua bin gii gia vng ZCE v vng trung ho (vi gi trx > W).

(ii) Dng quang in khuch tn:

V rng tt ccc ht ti in sinh ra trong vng ZCE lp tc b qut ngay bi in trng, cho nn ta c:0)(xJW)(xJ pdiffpdiff = v 0)(xJW)(xJ ndiffndiff = . Mt dng quang in khuch tn Jpdiffhp thnh bi

cc quang ltrng khuch tn trong vng n (ngoi vng ZCE) sc xc nh tli gii ca phng trnh tin

ho sau: x-02

2

pp

R)e(1p

dx

dD

p . T :

x

2p

W0px/Lx/Lp e

)L(1

R)e-(1BeAep(x) p

, vi Lp= ppD l di khuch tn ca ltrng ; Dpl hskhuch tn ca ltrng.

Githit rng kch thc ca linh kin bn dn l ln c thxem rng kch thc ca vng khuch tn , t c thp dng iu kin bin v cc vo li gii tng qut ca phng trnh v suy ra l : B = 0. Hnna, ti im x = W ta c p(W) = 0, bi v tt ccc ltrng n c vng bin gii ny lin bqut ngay biin trng. T ta suy ra : ]e[e

)L(1

R)e-(1p(x) W)/Lp(xW)(x

2p

W0p

= . Mt dng quang in khuchtn ti im x = W c vit l:

Wxppdiff dx

pdeDJ =

=

Thth, ta c : W0p

ppdiff R)e-(1L1

LeJ

(III.13)Sphn bcc quang in ttrong vng trung ho p

+

c cng dng vi sphn bcc quang ltrng trongvng n. Tuy nhin, trong thc t, bdy ca vng tuyn u ny rt nhso vi 1/cho rt nhiu phtn c thxm nhp n vng ZCE, t c thto ra ti a cc ht quang ti in trong vng ny. Mt quang in ttrong vng ny do rt thp. Hn na, ta chc thxem nhkhng ng kmt in ttrong vng trung ho

p+, v rng ta c : 2innp = , vi p+>> n. Sng gp vo dng quang in khuch tn trong trng hp ny, nhvy, chyu l do ng gp ca quang ltrng trong vng trung ho n : Jdiff= Jpdiff.

Dng quang in ton phn do c vit : Jph= JG+ Jdiff= )L1

eR)(1-(1e

p

W

0

(III.14)

Dng quang in Jphcung cp bi it quang nhvy tlvi thng lng cc phtn ti 0. tctquanghc do c p ng tuyn tnh. Mt khc, tng nhy ca dng quang in ny, biu thc ca Jphchra rngcn phi tng gi trca hshp th(tc l tng hiu sut lng t) v brng W ca vng in tch khnggian ZCE v di khuch tn Lp. Dng quang in ny hp vi dng in trong ti (courant d'obscurit/ darkcurrent) Jobs" to thnh dng in ton phn u ra ca it quang : J = Jobsc+ Jph.

(iii) Dng in trong ti:Trn hnh III.5 trnh by 4 dng in (Jgn, Jndiff, Jgp, Jpdiff) cu thnh dng in trong ti ca it quang. Khi it

quang khng c chiu sng, dng in chy qua btip gip l tng sca 4 dng in khc nhau ny : Jobsc=Jndiff+Jpdiff Jgn-Jgp; trong Jgnl dng in to cp (sinh ra do chuyn ng nhit) ca in t; Jndiffl dng inkhuch tn ca in t; Jgpl dng in to cp (sinh ra do chuyn ng nhit) ca ltrng v Jpdiffl dng inkhuch tn ca ltrng.


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242

Hnh III.5 : Phn bca dng in trong ti

Dng in to cp in tJgnv dng in to cp ltrng Jgpsinh ra tchuyn ng nhit ca cc ht cuthnh vt liu bn dn trong vng ZCE. Cc ht ti in ny bqut ngay bi in trng E trong vng in tchkhng gian ny. Cc dng in ny khng phthuc vo hiu thphn cc Vapp; chng tn ti mi gi trVapp.Dng in khuch tn in tJndiffcu to tcc in ttdo trong vng trung ho n v khuch tn vpha vng

ZCE. Chiu ca dng in ny ngc vi chiu ca dng in to cp in tJgn. Dng in khuch tn ltrng Jpdiffcu to tcc ltrng tdo trong vng trung ho p v khuch tn vphi vng ZCE. Chiu ca dng in ny cngngc vi chiu ca dng in to cp ltrng Jgp. Sht ti din tdo ng gp vo hai dng in khuch tn nyphthuc vo chiu cao ro thnng ca btip gip p-n. Thm chiu cao ca ro thnng ny li tlthunvi hiu thphn cc Vapp. Cho nn cc dng in khuch tn ny phthuc vo hiu thphn cc ca it quang.

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4

-0,004

-0,003

-0,002

-0,001

0,000

0,001

0,002

0,003

0,004

0,005

0,006

Iph

Iobsc

COURANT(mA)

Tension de polarisation (V)

Courant d'obscuritPhotocourant

Hnh III.6 : c trng I(V) ca mt it quang

Ttnh ton trnh by trong mc II.1.b ca chng II ta c dng in trong ti ton phn c biu din bng

biu thc (II.14), ngha l : 1)(eJJ T/keVsatobscBapp (III.15)

Nhvy, dng in ton phn u ra ca it quang, khi n c chiu sng vi thng lng quang hc 0, c

vit l: J = Jobsc+ Jph= )L1

eR)(1-(1e1)(eJ

p

W

0T/keV

satBapp

(III.16)

Hnh III.6 trnh by cc c trng I(V) ca dng in trong ti v dng quang in ca it quang.


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Hnh III.7 : Hai cch vn hnh ca mt it quang : (a) o bng dng quang in v (b) o bng hiu thquang in

Khi sdng mt it quang o tn hiu quang hc, ta c thp dng hai cch o khc nhau: o bng dngquang in (hnh III.7.a) v o bng hiu thquang in (hnh III.7.b).

Trong so bng dng quang in, it quang c ni vi mt mch in c trkhng rt nh(RL) vdng quang in o c, c xc nh bng biu thc (III.16) trn. Ta cng c tho hiu thV hai u cain trphti RL.

Trong so bng hiu thquang in, it quang c ni vi mt mch in c trkhng rt ln (I = 0).Trong trng hp ny, phng trnh ca dng quang in trnh by trn cho thy c xut hin mt hiu thVphhai cc ca it quang : linh kin nhvy vn hnh theo cch to ra hiu thquang in. Biu thc ca hiu th

quang in Vphc vit l:

p

W

sat

0B

sat

phBph L1

e1

JR)-(1e

1lneTk

J

J1ln

eTk

V (III.17)

Chng ta thy rng, trong cch o ny, it quang khng cung cp mt p ng tuyn tnh theo thng lngphtn ti 0.

p ng c trng theo phca mt it quang c thc vit di dng (nhtrong trng hp ca tct

quang dn in): () = /eh1 = 1,24(m) (III.18)

vi hiu sut lng tton phn xc nh bi ht thc :

p

W

L1e

1R)(1 (III.19)

Tbiu thc ca p ng c trng theo phny ta thy khng c hskhuch i trong p ng ca mtit quang (so snh vi p ng ca mt tctquang dn in c hskhuch i g, ta c thxem nhhskhguch i ca it quang l 1).

b) p ng thi gian ca it quang.

Hnh III.8 : Mch in tng ng ca mt it quang vn hnh theo cch o dng quang in

p ng thi gian ca mt it quang c xc nh t li gii cc phng trnh ng lc hc (I.10) v(I.11). Cc phng trnh ny m tsphn btheo thi gian cc ht quang ti in trong cc vng c tng tc viphtn, vi mt dng in in tJnv mt dng in ltrng Jpc xc nh bng (I.12) v (I.13). Victnh ton thu c li gii cc phng trnh ny kh nhiu kh, trong khi ta c thdon c nhng kt quvt l chnh yu ca cc li gii . Do vy, y chng ta, thay v i tm li gii tm hnh ton hc ca p ngthi gian, i tm gii thch vt l ca hin tng ny. Chng ta quan tm n cc hng sthi gian ng gp vo

vic to nn p ng thi gian ca mt it quang. C ba hng sthi gian v mi hng stng ng vi mt hiung vt l khc nhau.

Hng sthi gian tng ng vi dng in khuch tn (td). Hng sny chnh l thi gian sngca cc htquang ti in (vy th td= ). Gi tr ca hng sny thay i t10-4s trong Si n 10-8s trong GaAs, ngha ltng ng vi cc di truyn qua (bande passante passband) c rng t10 KHz n 100 MHz. Di truyn quavi rng ny khng p ng c vi yu cu o cc tn hiu quang thay i vi tc nhanh.

Hng sthi gian mch RC (tRC). Vng in tch khng gian ZCE ca it quang to ra mt in dung chuyntip Cdca tct, xc nh bi hthc (II.23) [xem mc II.1.c]. Khi dng it quang o cc tn hiu bin inhanh, in dung ny hp vi in trca tct(Rp) v in trphti (RL) (nhc m ttrong mch in

tng ng trnh by trn hnh III.8) mt mch in m tn sct l :d

c RC21

f (III.20)


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Trong R l in trtng ng ca hai in trmc song song Rpv RL.

Th d: i vi mt it quang c tit din ca vng ZCE l: A = 1 mm2, c pha tp vi mt 1015cm-3v c p vihiu thVD V = 10V ; ta c brng ca vng ZCE l: W = 3,6 m v in dung chuyn tip l 30pF. Ni vi mt in trphti c gi tr50, it quang ny c tn sct bng: 1/2RC = 110 MHz.

Hng sthi gian tng ng vi di chuyn ca cc ht ti in trong vng in tch khng gian ZCE (tt). Hng

sthi gian ny c c trng bng: tt=appVw ; trong w l brng ca vng ZCE, l linh ng ca cc

ht quang ti in v Vappl hiu thphn cc. Hng sny c gi trnhnht trong ba hng slin hn p

thi ca linh kin. Thc vy, brng vng in tch khng gian ZCE tri di khong 1 m khi it quang c pmt in thphn cc chng Vapp=1V, ngha l mt in trng trung bnh bng 10

4V/cm. Ngay i vi mt vt liubn dn m cc ht ti din c linh ng thp, chng hn = 100 cm2/V/s (gi trny tng ng vi vn tc dichuyn chng 106cm/s), ngha l tng ng vi mt thi gian di chuyn ca ht ti in trong vng ZCE bng 100ps, hay mt tn sct chng 10 GHz.

p ng thi gian ca linh kin l thp ca ba hng sthi gian m biu thc gn ng c thvit l [10]:2/12

t2RC

2d tttt (III.21)

Hai hng sthi gian u tin to ra cc gii hn khng chp nhn c vi vic o cc tn hiu quang bin iunhanh, nhtrong trrng hp ca truyn thng quang hc tc cao m di truyn qua cn thit phi ln hn GHz.i vi cc loi ng dng ny ta cn phi dng it quang p-i-n.

III.3.2 it quang p-i-n

Hnh III.9 : Sca mt it quang p-i-n : (a) cu trc ca it quang p-i-n ; (b) gin vng nng lng ;(c) ng biu din sto cp bng kch thch quang hc v (d) cu hnh tiu biu ca mt it quang p-i-n


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Chng ta thy rng trong mt it quang dng btip gip p-n, sgii hn brng ca vng in tch khnggian ZCE l mt yu tng gp vo hn chca dng quang in v ca di truyn qua ca linh kin.lm tngbrng ca vng ZCE, trong cu trc ca mt it quang p-i-n, ngi ta chen vo gia hai vng pha tp p v n cabtip gip mt cht bn dn ni bm (chi trong tp-i-n l vit tt ca chintrinsque/ ni bm) rt t pha tp.

Brng ca vng ZCE, di chphn cc ngc, tri rng trn sut vng i, v sto cp bng kch thchquang hc xy ra chyu trong (hnh III.9.a v b). Cu trc ny c nhng u im sau:- N c mt in dung chuyn tip Ct= A/W thp v do to nn mt tn sct kh cao.- V rng vng in tch khng gian ZCE rt rng, tri di n kh su trong vng trung ho n (W >> 1, vngkhuch tn ht ti in c gim thiu rt nhiu), do sng gp ca dng in khuch tn Jdiffcng nhthi

gian sng ca cc ht quang ti in l khng ng k(thi gian sng ny lin quan n sti hp cc ht tiin trong cc vng khuch tn). Nhvy ta c thbqua sng gp ca hng sthi gian tng ng vi dngin khuch tn vo p ng thi gian ca tct. Phn ng nng ng ca linh kin do chyu c quytnh bi dng in to cp JG.

Nguyn l vn hnh ca it quang p-i-n tng tvi nguyn l vn hnh ca it quang dng btip gip p-n ;iu khc bit l rng ca di truyn trong it quang p-i-n rt ln.

Th d: Trong mt it quang p-i-n c tit din tip gip A = 0,01 mm2, c brng vng ZCE (l vng i) W=10 m scho mtin dung chuyn tip l Cd= 0,1 pF. Ni linh kin vi mt in trphti R = 50 , it quang ny sc mt tn sct fc=1/2RCdbng 30 GHz. in thphn cc c ln chng (VD Vapp) = 100 V , in thny to ra trong vng ZCE mt intrng E = 102V/ 10m = 105V/cm. in trng ny y cc ht ti in c linh ng chng = 100 cm2/V/s chuynng vi vn tc 107 cms-1. Vn tc ny tng ng vi thi gian chuyn ng trong vng ZCE l t= 100 ps, ngha l itquang c mt tn sct chng 10 GHz.

Chng ta thy rng trong th dny thi gian chuyn ng ca cc ht quang ti in trong vng in tch khnggian l yu thn chca di truyn qua. Nhvy, ta khng thtng brng W ca vng ZCE m khng lm hnchrng di truyn qua ca tct. Thth cn phi chn mt gii php tha hip tt gia in dung chuyntip Cdv thi gian chuyn ng ca cc ht quang ti in ttrong vng in tch khng gian c thtng rng di truyn qua ca tct.

III.3.3 it quang dng hiu ng nhn in (photodiode avalanche/ avalanche photodiode)

Trong mt it quang dng hiu ng nhn in (sau y gi tt l it quang nhn in) ngi ta sdnghin tng nhn in i vi cc cp in t-l trng khuch i sht quang ti in c to ra v do khuch i p ng ca tctquang hc.

a) Vn hnh ca it quang nhn in

Hnh III.10 : Cchnhn in trong btip gip p-n (theo [5])

Scchca hiu ng nhn in trong mt it quang p-i-n c trnh by trn hnh III.10. Btip gip p-n(hay p-i-n) ca it quang c phn cc ngc vi mt hiu th cao (nhng thp hn hiu thnh thng tension de claquage/ breakdown voltage - ca btip gip). Hiu thcao ny to ra mt brng kh ln ca vngZCE v ng thi cng to ra mt in trng rt mnh trong vng ny. Mt photon hp thim A to ra mt

cp in t-ltrng bng chuyn dch in tA B (in tB trong vng dn v ltrng A trong vng hotr). Hai ht quang ti in ny c t trong in trng rt mnh v do chng lp tc c gia tc. Vi sgiatc ny, in tB thu c mt ng nng quan trng trong ltrnh ca n, v nhvy nng lng ton phn cain tny ln hn nng lng y ca vng dn. Thnhng, ltrnh ca ht in tbgin on do va chmngu nhin vi mng tinh th, chng hn im C. im ny, in tchia bt mt phn nng lng ca n chomng tinh th. Nng lng chuyn giao ny ln hn rng ca vng cm v sc ion ho nguyn t, to nnchuyn dch in tF E. Mt cp in t-ltrng thcp nhvy c sinh ra, v n lt chng c gia tcbi in trng, ng thi vi in tscp. in tscp ny, sau khi chuyn giao bt nng lng im C,ri xung im D v tip tc c gia tc trn mt ltrnh mi. Ltrng sinh ra im F c gia tc theo chiungc li v bng va chm vi mng tinh thim G, to nn chuyn dch in tH I, dn n kt qul lnny mt cp in t-ltrng mi khc c sinh ra. Ln na, cp ht ti in ny li c gia tc di in trngv tham gia vo qu trnh ion ho cc nguyn tbng va chm. Nhvy, mt hiu ng dy chuyn to cp in t-


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ltrng xy ra trong vng nhn in ca linh kin. Kt qul dng quang in scp c nhn ln bng hiu ngnhn in ny v to thnh u ra ca linh kin mt dng quang in c khuch i.

Dng quang in chy qua it quang nhn in (c tit din tip gip A) nhvy bt ngun tba dng in:Dng in to cp scp sinh ra do hp thmt phtn hoc do chuyn ng nhit. Trong mt lp c bdy

dx ca vng nhn in, dng in to cp bng : AeGdx ; trong G l hsto cp ton phn (to cp quang hcv to cp nhit hc).

Dng in to cp thcp do va chm ca in tvi mng tinh th, dng in in tthcp c vit :nIndx, trong nl hsnhn in ca in tv c nh ngha bng s ndx cc cp in t-ltrng thcpsinh ra bi chuyn ng ca mt in txuyn qua mt khong cch dx trong vt liu di mt in trng mnh.

Dng in to cp thcp do va chm ca lvi mng tinh th, dng in ltrng thcp c vit : pIpdx,trong pl hsnhn in ca ltrngv c nh ngha bng s pdx cc cp in t-ltrng thcp sinh rabi chuyn ng ca mt ltrng xuyn qua mt khong cch dx trong vt liu di mt in trng mnh.

Ch rng cc hsnhn in ca in tv ca ltrng (nv p) phthuc rt mnh vo in trng v gitrca chng khng ng kdi in trng yu.

Trong chtnh (ngha l G l hng s), ta c : AeGIIdxdI

- ppnnn ;

v AeGIIdx

dIppnn

p .Tnh lin tc ca dng in ton phn i hi phi c : I = In+ Ip. Bng cch thay thIntrong phng trnh thhai

ththc ny, ta c : AeGI)I(dx

dInpnp

p Phng trnh vi phn ny c li gii tng qut :

pn

n)xnp(

pAeGI

Ce(x)I

. Hng s tch phn C c xc

nh tcc iu kin bin tip xc ca vng n ti x = 0 v tip xc ca vng p ti x = L. Githit rng (trongtrng hp l tng) tip xc p khng thphun c mt in tno vo vng nhn in (do : In(L) = 0), vtip xc n khng thphun c mt ltrng no vo vng nhn in (do : Ip(0) = 0). Sbo ton dng in

ton phn i hi : I = Ip(L) = In(0). T suy ra : )Lpn(pn

)Lpn(

e

1eAeGI

= (III.22)

Tsgia dng quang in scp v dng in sau cng u ra, sau khi chy qua vng nhn in c di

L, c vit l :

=

)Lpn(

pn

)Lpn(

primairee

1e

L1

AeGL

I

II

(III.23)

Biu thc (III.23) cho thy rng thng lng ca dng in chy qua vng nhn in c khuch i so vithng lng ca dng in scp bi mt hsnhn in Mxc nh bng:

M =

)L(

pn

)L(

pn

pn

e

1eL1

(III.24)

Hnh III.11 : ng biu din hsnhn in M theo i lng nLvi cc gi trkhc nhau ca tham s k = p/n(theo [1])


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Trng hp c bit khi m n= p = , hsnhn in trthnh : M =L1

1 .Tphng trnh ca hsnhn in M ta thy, vi mt gi trnht nh ca in trng, phi c mt gii hn

bt buc cho di L ca vng nhn in. Thc vy, nu di L tng n mt gi trlm cho mu sca phngtrnh trn trthnh khng, th hsnhn in stin ti v cc, v trong trng hp , mi to sinh v cng nhca dng in scp cng c thdn n sto thnh dng in lm on mch (court-circuit/ short-circuit) itquang hc. Dng in ny c thto nn snh thng lm ph hutctquang hc nu dng din khngc hn chbng cch khc.

p ng ca it quang dng hiu ng nhn in c mt hskhuch i M tng tnhtrong trng hp

ca tctquang dn in (vi hskhuch i g), nhng vi mt bn cht vt l cbn hon ton khc.b) p ng thi gian

Tng tnhtrong trng hp ca it quang p-n hay p-i-n cp trn, thi gian p ng ca mt itquang nhn in phthuc vo cc hng sthi gian tng ng vi thi gian chuyn ng ca cc ht quang tidin trong vng hot tnh, vi thi gian sng ca chng trong vng khuch tn v vi hng sthi gian ca mch RCca linh kin. Skhuch i bn trong cc it quang nhn in, da trn cchion ho cc nguyn tbng vachm ca cc ht ti in vi mng tinh th, cung cp thm mt gii hn khc cho p ng thi gian ca linh kin,bi lsthit lp hskhuch i ca it khng thxy ra tc thi. i vi cc linh kin m sion ho chlinquan n mt loi ht ti in, th thi gian thit lp hskhuch i bng thi gian di chuyn cuht ti xuynqua vng nhn in. Nhng i vi cc linh kin m trong chai loi ht ti in cng tham gia vo cchionho cc nguyn t, th vic thc hin nhiu "ltrnh khhi" l cn thit c ththu c mt hskhuch inht nh v s"ltrnh khhi" cn thit ny cng nhiu khi gi trcc hsnv pcng gn nhau. Thi gianthit lp hskhuch inhvy tng ln. Hng sthi gian ny ng gp vo vic gii hn rng di truynqua ca it quang nhn in. Tng tnhtrong trng hp ca tctquang dn in, ngi ta thy rng,trong mt siu kin vn hnh, tch s(hskhuch i rng di truyn qua) ca it quang nhn in lhng s; iu c ngha l ngi ta khng thlm tng hskhuch i m khng lm suy gim di truyn quaca linh kin.

III.3.4 it quang Schottky

Hnh III.12 : Svn hnh ca mt it quang Schottky : (a) Vn hnh bng cchpht xbn trong vt liu ca mt quangin tkhi hp thmt phtn c nng llng h> (m-SC) (ro thSchottky) ; (b) To cp in t-l trng bng kch thchquang hc trong cht bn dn vi phtn c nng lng h> Eg; (c) Vi hiu thphn cc ngc ln, it quang Schottky c thvn hnh nhmt it quang nhn in (theo [4]).


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it quang dng ro thSchottky c cu to tmt btip gip dthkim loi-bn dn. Btip gip ny lmxut hin mt phn cch ca hai vt liu mt ro thnng emsbng khong cch gia mc nng lng Fermica kim loi v y vng dn ca cht bn dn: ems= es- eSC; esl cng thot ca kim loi v eSCl nnglng cn thit giin tbn trong cht bn dn. Nguyn l vn hnh ca mt it quang Schottky da trn cchpht xbn trong vt liu (mission interne/ internal emission)ca mt quang in ttkim loi sang vng dnca cht bn dn khi c kch thch quang hc. Qu trnh ny c trnh by bng strn hnh III.12.a.

it quang Schottky c cu to tmt mng mng kim loi (hoc mt mng mng hp kim kim loi-bn dnc tnh cht vt l nhmt kim loi)) c phtrn mt bn dn. Mt chm tia sng ti gm ccphtn cnng lng h> msc hp thtrong mng mng kim loi ; chm tia ny chuyn giao nng lng ca ccphtn cho cc in tm nng lng di mc Fermi ca kim loi. Cc quang in tdo c pht xvobn trong vt liu bn dn bng cch vt qua ro thnng ems= es- eSC. Cc quang in tny xm nhpvo vng dn ca cht bn dn v ng gp vo vic to thnh dng quang in chy qua it quang. Loi pht xbn trong vt liu ny i hi mt bc sng ct (longueur d'onde de coupure/ cut-off wavelength) bt buc, xc

nh bng:(eV)

1,24(m)

msc = .

Shp thcc phtn trong mng mng kim loi l rt thp, bi v trong kim loi cc quang in tbmtnng lng trn mt on ng di chuyn rt ngn do bva chm: qung ng tdo trung bnh LBca cc quangin tcht50 n 100 trong phn ln cc kim loi. Hshp thca cc kim loi c o bng tlnghchca bsu hiu ng da (mt vt liu kim loi c in trsut 10.cm sc mt bdy hiu ng da trong vnghng ngoi chng 200 v mt hshp th= 1/= 5.105cm-1). Nhvy, ngi ta sc mt hschuyn iphtn thnh quang in ttrong kim loi c ln bng [1- exp(-LB)]. Ngoi ra, khng phi tt ccc quang intc kch thch nng lng vt ro thnng shon ton xm nhp c vo trong cht bn dn, bi vphng chuyn ng ca cc quang in tny khng nht thit l phong xm nhp vo bn trong bn dn. Cc

tnh ton [1] cho thy rng hiu sut lng tca loi tctquang hc ny l rt thp v c thc vit didng sau y:

2msmF

L hh8E

1)e(1 B (III.25)

Trong EFml mc nng lng Fermi ca kim loi.

Hnh III.13 : (a) Cu trc ca mt it quang Schottky ; (b) ng biu din truyn sut ca mng mng kim loi (y lmng mng bng vng) theo dy trong trng hp khng c v c lp chng phn x nhm ci thin truyn sut (theo[10]).

Mc d c hiu sut lng trt km, it quang Schottky vn c sdng v nhiu l do:Trong cng nghbn dn, ngi ta khng thto ra c vi tt ccc vt liu bn dn hai loi pha tp kiu p

v kiu n. B tip gip Schottky c bit c li trong trng hp ny. Trong c ch vn hnh ca it quangSchottky, bc sng ct ca ttkhng bp t bi rng vng cm ca cht bn dn nhng bi chiu caoca ro thnng do tip xc kim loi-bn dn to ra. Nhvy, ngi ta c thsdng mt vt liu bn dn dchto lm (substrat) cho linh kin (chng hn nhsilicium) v chn la mt kim loi thch hp to ra bcsng ct mong mun (th dnhcp vt liu bn dn Si loi p v kim loi PtSi cho mt bc sng ct bng 5,6 m).

Trong mt btip gip Schottky, ngi ta cng c ththc hin c cchto cp cc ht quang ti innhtrong trng hp ca it quang dng btip gip p-n (hnh III.12.b) hay nhtrong trng hp ca it quangnhn in (hnh III.12.c) o cc sng quang ti m bc sng c thxuyn qua c mng mng kim loi xm nhp vo trong cht bn dn.

p ng thi gian ca cc it quang p-n v p-i-n mt mt bgii hn bi thi gian sng ca cc ht ti introng cc vng khuch tn nm bn cnh vng ZCE v mt khc bi hng sthi gian RC do in dung v in trca tctp t. gim thiu hng sthi gian u tin, ngi ta lm gim brng ca vng khuch tn trnmt phn ca btip gip. Nhng vic gim bt brng ca vng khuch tn ny khng thtrnh c vic kotheo s tng in tr ni tip ca tct, m iu li ko theo vic tng hng s thi gian RC. it quangSchottky, ngc li, cho php thc hin vic gim thiu ny, v lin trca kim loi l rt thp. Hn na, cchquang dn in trong it quang Schottky chlin quan n mt loi ht ti in a sduy nht (l in t) m linh ng thng kh ln ; do loi tctquang hc ny vbn cht c p thi nhanh hn cc linh kin dng


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ng thi hai loi ht ti in (nhit quang p-n hay p-i-n). Ngi ta thc hin c cc it quang Schottkym di truyn qua c tht n 100 GHz [1].

III.3.5 it quang MSM (Kim loi-Bn dn-Kim loi)

Hnh III.14 : (a) Cu trc it quang MSM ; (b) Cu hnh cc in cc.

Nhm ci thin hn na di truyn qua ca it quang v to thun li cho vic thc hin tch hp tctquang hc vo mch tch hp (in tv quang hc), ngi ta xut mt cu hnh khc ca it quang: itquang MSM. it quang hc ny cng cu to trn csca tip gip Schottky ; nhng cu trc ca n l cu trcphng (planaire/ planar): cc in cc ca it quang MSM c thc hin trn cng mt mt phng (mt phngca linh kin nhc trnh by trn hnh III.14.a) trong khi hai in cc ca it quang Schottky bnh thng ct trn hai mt phng song song khc nhau ( "nh" v "y" ca linh kin nhc trnh by trn hnhII.13.a). it quang MSM c cu to t mt lp mng cht bn dn, thc hin bng phng php epitaxi(pitaxie/ epitaxy) trn mt lm bng vt liu bn in mi (semi-isolant/ semi-insulant). Cc in cc kim loica it c sp xp theo hnh cc ngn tay an nhau v c du thay i lun phin (hnh II.14.b). Cc ht quangti in c to ra do hp thcc phtn ti trong lp bn dn nm gia cc tip xc kim loi-bn dn ni tip(vng cm quang/photosensible trn hnh II.14.b) bqut ngay bi in trng kh mnh gia cc in cc v ctp kt bi cc in cc ny. Linh kin c phn cc sao cho lp bn dn trthnh hon ton thiu ht in tch tdo nhm tng cng ti a shp thbn trong cht bn dn v ng thi gim ti thiu p thi ca it quang.Khong cch gia cc in cc phi nhhn di khuch tn ca cc ht quang ti in to nn hiu sut lntrong vic tp kt cc ht quang ti in. Nhng nu cc in cc chim nhiu din tch ca linh kin, iu ny cnguy clm gim kh mnh nhy ca it quang MSM. Nhvy, kch thc, khong cch v slng cc incc c ti u ho c ththc hin uc cc hiu nng nhmong mun nu trn.

Hnh III.15 : Mt it quang MSM dng cu trc dn sng [10].

it quang MSM c nhiu u im so vi diode quang Schottky. Trc nht, cc it quang MSM do c din tchin cc v khong cch gia cc in cc rt thp (khong t1 n vi m), thi gian di chuyn ca cc quangin tl kh ngn v in dung ca cc it ny l kh thp ; iu dn n vic tng rng di truyn quaca linh kin (ngi ta thc hin c cc it quang MSM bng Si c di truyn qua rng n 110 GHz). Mtu im khc ca linh kin ny l cu trc phng ca n, c bit thun li cho vic phkim loi to thnh incc. Vmt cng nghchto, vic ny kh dlm v thun tin cho vic tch hp it quang vi cc linh kin int tch phn khc nhm thc hin my thu tn hiu quang dng mch quang in t tch hp (OptoelectronicIntegrated Circuit - OEIC); bi v cng nghny l hon ton ging vi cng nghdng chto cc transistordng hiu ng trng (FET). Hn thna, cu trc MSM cng ng thi to thun li cho vic tch hp linh kin nyvo cc mch quang hc tch phn (optique intgre/ integrated optic) nhm thc hin cc it quang dng cu trcdn sng (xem mc III.3.6 sau y). Trn hnh III.15 biu din smt it quang MSM dng cu trc dnsng. Nhc im ca it quang MSM l p ng ni ti ca n thp. L do l thu nhp cc phtn trn bmtcm quang ca linh kin kh thp do din tch cc in cc lm gim thiu bmt cm quang. Ngi ta c thci

T. Hsiang et al., SPIE Proc. 2022, 6, 1993.


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thin nhc im ny bng cc sdng cc in cc trong sut (it quang MSM bng InGaAs dng cc in ccbng Cadmium Tin Oxide-CTO hay bng Indium Tin Oxide-ITO).

III.3.6 it quang dng cu trc dn sng

Trong cu trc ca mt it quang thng thng, thu c hiu sut lng tton phn ti u (xc nhbng hthc III.19), cn thit phi c W >> 1 v Lp>> 1, ngha l phi tng brng ca vng in tch khnggian v di khuch tn ca cc ht quang ti in. Thnhng, khi lm tng gi trhai tham sny, ta khngtrnh khi lm suy gim rng ca di truyn qua do vic tng thi gian chuyn ng trong vng in tch khnggian v tng thi gian khuch tn ca cc ht quang ti in. Nhng nu by gita ghp linh kin ny vo trong

mt cu trc dn sng quang tch hp, ta c thng thi ci thin c hiu nng ca hiu sut lng tv rng di truyn qua ca n.

Hnh III.17 : Sdca mt it quang dng cu trc dn sng.

Thc vy, mt it quang c ghp vo mt bdn sng, m cu trc c trnh by trn hnh III.17, cho thyc nhiu u im. Trc ht, v rng vng hot tnh ca tctc chiu sng theo phng song song chkhng phi theo phng thng gc vi tit din ca btip gip, dng quang in u ra ca tctc vit l:

)e1(eJ L-0ph (III.26)

Trong L l chiu di ca vng hot tnh o theo phng truyn ca sng quang. Trong cu hnh ny, v rng brng W ca vng in tch khng gian ZCE v chiu di L ca vng hot tnh l c lp vi nhau, cho nn ngi tac thchn la mt cc ht ti in ca vng hot tnh v hiu thphn cc Vappsao cho brng W ca vngin tch khng gian bng vi bdy ca bdn sng ; trong khi , chiu di L c thc thc hin vi mt di thch hp thomn iu kin L >> 1. Do m hiu sut lng tc tht gi trbng 1 vi bt kgi trno ca hshp th, bng cch iu chnh di L, vi iu kin l mt mt do khuch tn v do hp thca cc ht ti in l khng ng k.

Hnh III.18 : Sdmt it quang dng cu trc dn sng lm bng InGaAs c ghp vo bdn sng lm bng InP [12].

u im thhai ca loi tctny l rng ca di truyn qua. Thc vy,v rng tctquang hc cto nn trong mt knh dn sng rt nh, cho d c chiu di L tng i ln, linh kin ny vn c in dung cabtip gip rt b ; iu dn n gi trrt b ca hng sthi gian RC, m hng sny li l gii hn chyuca di truyn qua. Bi v tt ccc phtn n u c hp thtrc tip trong vng in tch khng gian, khngnhng hiu sut lng tc tng ln m thi gian khuch tn cc ht ti in cng c trit tiu. Kt qul rng ca di truyn qua skhng bgii hn bi hng sthi gain khuch tn na. Cc tnh ton [12] cho thy rngmt it quang dng cu trc dn sng lm bng AlGaInAs-GaInAs, vn hnh bc sng 1,55 m, c tht nhiu sut lng tbng 0,94 v di truyn qua bng 100 GHz. it quang dng cu trc dn sng cn c mt uim khc: l khnng c tch hp trong mt linh kin quang hc tch phn dng thc hin vic o tn hiuquang, mt chc nng khng ththiu c trong cc linh kin chc nng quang hc. Hnh III.19 trnh by sdmt it quang dng cu trc dn sng lm bng InGaAs c ghp vi mt bdn sng lm bng InP.


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IV. TING N TRONG CC TCTQUANG HC BN DN

IV.1 Gii thiu tng quan

Khnng o mt tn hiu quang hc rt yu lun bgii hn bi shin din ca ting n trong qu trnho. Ngi ta nh ngha y thut ng tingn (bruit/ noise) dng chcc thng ging ngu nhin trong tnhiu in do tctquang hc cung cp. Ting n ny lun c thm vo tn hiu phi o v do trong mt qu

trnh o, chng ta khng thloi bhon ton ting n, nn mc ch ca ta l tm cch ti a ho tstn hiu sovi ting n. Chng ta nhc li y vi khi nim thng k ca tingn trong cc tctquang hc:Cng sut ca tingn c biu din bng phng sai 2 cc thng ging ca i lng o c (dng quang

in hay quang hiu th), chng hn dng in u ra ca tctquang hc :

[ ] [ ] dtii(t)T1

ii(t)ii2T

0

222B

2i (IV.1)

Trong i l dng in trung bnh v T l thi gian tch phn ca qu trnh o(temps d'intgration/ integrationtime) hay l hng sthi gian ca tn hiu in cung cp bi tct.

Cn sbc hai ca phng sai hay cn c gi l lch chun (cart-type/ standard deviation) (hay gi trhiu dng- valeur efficace/ root mean square trong trng hp ca mt tn hiu quy tm hay tn hiu m gi trtrung bnh bng khng) c trng cho tingn ca i lng o :

[ ] [ ] dtii(t)T1

ii(t)i2T

0

2Bi (IV.2)

Trng hp c nhiu ngun ting n tn ti cng lc trong qu trnh o tn hiu quang, ting n ton phntrong qu trnh o c biu thbi1/2

n

1jj

2B

1/2n

1j j

2Btotal

1/2n

1j

2jtotal iii

=

=

= = (IV.3)

Vic o ting n trong cc tctquang hc thng c gn l in vi di truyn qua i vi tn hiu introng hthng o tn hiu quang hc, khi m thng lng cc phtn ti c bin iu theo thi gian. Trng hpn gin nht l trng hp m hthng o p ng mt cch ng u vi mi tn sbin iu c gi trgia f1v f2; v ngoi khong tn sny khng c p ng ca hthng (hay p ng bng khng) : di rng truyn quanhvy bng f = f2 f1. Trong thc t, p ng c trng ca mt tctquang hc thng phthuc vo tnsbin iu. Nu nhp ng ca hthng o tn hiu quang hc thay i mt cch lin tc theo tn sbin iu,ta c thkhi qut ho khi nim di truyn qua di vi tn hiu in theo nh ngha sau y:

= 0 2max df(f)f (IV.4)Trong (f) = TF[R(t)] l p ng tn sca hthng o v R(t) l p ng thi gian ca cng ho, TF l bini Fourier. maxl gi trcc i ca (f).

Trong mt hthng o m ngi ta tch phn tn hiu in trong mt khong thi gian tch phnT, vi mtxung vo thi im t = -T/2, p ng thi gian ca hthng c dng nhsau :

(t) =

Detector Quang Hoc Bang Ban Dan

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