Photodetectors; Receivers

Transcription

1 Photoetectors; Receivers They covert a otical sigal to a electrical sigal through absortio of hotos a creatio of HP. Their esig is more comlicate tha the otical trasmitters because the receivers must first etect weak istorte sigals a the make ecisio o what tye of ata was set base o a amlifie versio of this istorte sigal. Tyes of hotoetectors; hoto-mutiliers, hotoioes, hotocouctors,

2 q( ) Photocouctors q[ ( ) ( )] q )( ) ( For -tye semicouctor Light = o + = o + I hoto w A semicouctor slab of legth, with w a eth is illumiate with light of wavelegth. G & G G G G L 1999 S.O. Kasa, Otoelectroics (Pretice Hall) After alyig a voltage J ( J J L ) ( ) J L qgl ( ) Dark curret esity Photocurret esity

3 Photocouctors IL qgl ( ) A I L qgl ( )(1 ) A t t Trasitio time Gai is the ratio of the rate at which charge is collecte by the cotacts to the rate at which charge is geerate So, we efie gai as: Gai I L (1 qgl ( A) t )

4 Juctio The etire semicouctor is a siglecrystal material: -- regio oe with accetor imurity atoms -- regio oe with oor atoms --the a regios are searate by the metallurgical juctio.

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6 ρ(c/cm³) e _ + e s e s a ( ( ), ), Poisso' s quatio ( ) e a = ( ) 1 ( ) ( ) s s ( ) ma For Si Charge eutrality: a 14 s r (11.7)(8.851 ) F / cm The eak electric fiel Is at = ma e s ea s

7 1/ ma 1/ ma 1/ ) ( ) ( ) ( a a s R bi R R bi a a s R bi a a R bi s e W e e W 1 a a bi s e W For zero bias For reverse biase W bi ma For reverse biase For zero bias

8 1999 S.O. Kasa, Otoelectroics (Pretice Hall) Photoioes (a) lectroe SiO + r I h R out h> g h+ e Atireflectio coatig W lectroe Deletio regio (b) et e (c) e a () ma (a) A schematic iagram of a reverse biase juctio hotoioe. (b) et sace charge across the ioe i the eletio regio. a a are the oor a accetor cocetratios i the a sies. (c). The fiel i the eletio regio.

9 Power absortio P( X ) e ( ) Power geeratio P( X ) (1 e ( ) )

10 Power Geeratio a Absortio Recall: m 1.4 g ( ) g ( e ) is the uer cut-off wavelegth g amles: λ g for Si= 1.1 µm,, Ge = 1.87 µm, GaAs=.87 µm P( X ) e ( ) Power absortio i semicouctors P( X ) (1 e ( ) ) Power geeratio

11 Quatum fficiecy & Resosivity et # ofhpgeer ate # oficiet hotos I P / q / h teral Q. fficiecy Q is always < 1 sice ot all hotos geerate HP a also geerate HP may recombie without cotributig to the hotocurret. Furthermore, if the semicouctor legth is comarable with the eetratio eth (1/α) the ot all the hotos will be absorbe. η it = # of free HPs / absorbe hotos (which ormally is large)

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13 Photoioes r (a) lectroe SiO + I h R out If a otical ower, P is iciet o a hotoioe, the HP geeratio rate will be: h> g Atireflectio coatig (b) e et h+ e W lectroe Deletio regio G( ) s e s (c) e a () ma I teral Q. fficiecy is: et qa w G( ) qa # ofhpgeer ate # oficiet hotos I P / q / h (a) A schematic iagram of a reverse biase juctio hotoioe. (b) et sace charge across the ioe i the eletio regio. a a are the oor a accetor cocetratios i the a sies. (c). The fiel i the eletio regio S.O. Kasa, Otoelectroics (Pretice Hall) P (1 R w s sw f sw s e qa[1 e ] qa [1 e ] ha Ah ) P (1 R ) f Iciet hoto flu/uit area R f is reflectivity of the light etrace

14 Resosivity (A/W) Ieal Photoioe Q = 1% ( = 1) Si Photoioe Wavelegth (m) Resosivity (R) vs. wavelegth ( ) for a ieal hotoioe with Q = 1% ( = 1) a for a tyical commercial Si hotoioe. g Resosivity secifies the hotocurret geerate er uit otical ower. It is eeet o materials a the wavelegth. Tyical values A/W R 1999 S.O. Kasa, Otoelectroics (Pretice Hall) hotocurret( A) icieto ticalower ( W ) I P et q hc

15 i hotoioe SiO lectroe + lectroe (a) i-si + Drawback of hotoioes: et Large eletio caacitace C = εa / w arrow eletio withs; eetratio eth (b) (c) e e a () There is always a traeoff betwee the trasit time (ue to the iuce electric fiel) a the absortio rate h> g () o W h + e r The schematic structure of a iealize i hotoioe (b) The et sace charge esity across the hotoioe. (c) The built-i fiel across the ioe. () The i hotoioe i hotoetectio is reverse biase S.O. Kasa, Otoelectroics (Pretice Hall) I h R out

16 h> g + Diffusio e i-si h + Drift W A reverse biase i hotoioe is illumiate with a short wavelegth hoto that is absorbe very ear the surface. The hotogeerate electro has to iffuse to the eletio regio where it is swet ito the i-layer a rifte across. r 1999 S.O. Kasa, Otoelectroics (Pretice Hall)

17 Avalache hotoioe (APD) lectroe h > g SiO + I h e h + š + R (a) g i (1 P P P 3...) et lectroe M g i 1 P P P P (b) M 1 1 ( is ee o the tye of materials a the temerature r br ) () Avalache regio Absortio regio (a) A schematic illustratio of the structure of a avalache hotoioe (APD) biase for avalache gai. (b) The et sace charge esity across the hotoioe. (c) The fiel across the ioe a the ietificatio of absortio a multilicatio regios S.O. Kasa, Otoelectroics (Pretice Hall) (c)

18 h + e c e v h + + š Avalache regio (a) (b) (a) A ictorial view of imact ioizatio rocesses releasig HPs a the resultig avalache multilicatio. (b) Imact of a eergetic couctio electro with crystal vibratios trasfers the electro's kietic eergy to a valece electro a thereby ecites it to the couctio ba S.O. Kasa, Otoelectroics (Pretice Hall)

19 Atireflectio coatig lectroe SiO + Guar rig + (a) š + Avalache breakow (b) š + Substrate lectroe Substrate lectroe (a) A Si APD structure without a guar rig. (b) A schematic illustratio of the structure of a more ractical Si APD 1999 S.O. Kasa, Otoelectroics (Pretice Hall)

20 r I h lectroe IP IP IGaAs R out Photo h e lectroe P + IP Substrate lectroe I.53 Ga.47 As (5-1m) Absortio lay Grae IGaAsP (<1 m) IP (-3 m) Multilicatio layer. P + IP (-3 m) Buffer eitaial layer Simlifie schematic iagram of a more ractical mesa-etche SAGM layere APD. () P + + Avalache regio h + Absortio regio 1999 S.O. Kasa, Otoelectroics (Pretice Hall) Simlifie schematic iagram of a searate absortio a multilicatio (SAM) APD usig a heterostructure base o IGaAs-IP. P a refe a -tye wier-baga semicouctor S.O. Kasa, Otoelectroics (Pretice Hall)

21 Photo lectroe lectroe I.53 Ga.47 As (5-1m) Absortio lay Grae IGaAsP (<1 m) IP (-3 m) Multilicatio layer. P + IP (-3 m) Buffer eitaial layer P + IP Substrate Simlifie schematic iagram of a more ractical mesa-etche SAGM layere APD S.O. Kasa, Otoelectroics (Pretice Hall)

22 1 m h e c g c + g 1 + h + v (a) (b) ergy ba iagram of a staircase suerlattice APD (a) o bias. (b) With a alie bias S.O. Kasa, Otoelectroics (Pretice Hall)

23 h mitter Base Collector e + e h + The ricile of oeratio of the hotoioe. SCL is the sace charge layer or the eletio regio. The rimary hotocurret acts as a base curret a gives rise to a large hotocurret i the emitter-collector circuit. SCL SCL I h B BC CC 1999 S.O. Kasa, Otoelectroics (Pretice Hall)

24 oise i Photoetectors Due to the thermal geeratio I I + I h Curret Illumiate P o I + I h + i rms of the fluctuatios i the ark curret: I Dark Time R A ou i ark [ qi B] 1/ I juctio a i evices the mai source of oise is shot oise ue to the ark curret a hotocurret. r B is the frequecy bawith of the hotoetector 1999 S.O. Kasa, Otoelectroics (Pretice Hall) i quatum [ qi B] 1/ SR sigalower oiseower I i i iark iquat q( I I ) B

25 oise i Photoetectors oise equivalet ower (P): a hotoetector arameter, is the otical sigal ower require to geerate a hotocurret sigal that is equal to the total oise curret i the etector at a give wavelegth withi a bawith of 1 Hz. R SR =1 at B=1 Hz