Semiconductor Optical Communication Components and Devices Lecture 15: Light Emitting Diode (LED)

Transcription

1 Semicoducto Otical Commuicatio Comoets ad Devices Lectue 15: Light mittig Diode (LD) Pof. Utal Das Pofesso, Deatmet of lectical gieeig, Lase Techology Pogam, Idia Istitute of Techology, Kau htt://

2 Doig fo P-N juctio LD hece F c m kt e 3 e ~ m C 3 kt lectos N f O C C F V Holes V 3 h ~ m V F V V V F V

3 LD Mateial Poeties: Fo efficiet light emissio, diect bad-ga semicoductos ae efeed (e.g. GaAs). As caies ecombie acoss the active egio of the P-N juctio device, the active egio caot be eithe - o -tye as thee ae a lot of electo as well as a lot of holes. The Femi eegy, theefoe, has to slit ad have seaate quasi-femi eegy fo electos ( F ) ad quasi- Femi eegy fo holes ( f ). The wavelegth of the light emitted is the ootioal to this bad-ga ( g = h). X valley gi1 G valley gd egy L valley gi Light Hole Heavy Hole Slit Bad K

4 Diect ad Idiect tasitio i Semicoductos - LD s - (k ) 1 Diect Bad Ga c Absotio Recombiatio missio egy + + Sotaeous missio g h (k) h v k g elec. khole 0 Reduced Plak s Costat Fequecy of the Photo 0 h k 0 Idiect Bad Ga Recombiatio emissio k c OP g k k h g AP v a k k 0 h k

5 (a) quilibium (V=0) - juctio LD I O alied V (b) Fowad Bias (V=V f ) V f V V V O I I e qv k T B (c) Revese Bias (V=-V ) V 1 R s () ( C )... kt 1.5 Itegatio of eegies fom the bad ga to all electo ad hole eegies g lecto h egy v F V V O Diffusio e Diffusio h Dift e qv o Dift h v F V V v O C F V V f Diffusio e Fh F Diffusio h Dift e v Q(V o -V f ) Dift h V C F O V v F V V Dift h Dift e Q(V o +V f ) g egy Bad tails due to heavy doig of - ad - tye mateial v V V ( ).. kt Caie Cocetatio R ( ) A(, ) f ( )[1 f ( )] d s 1 1 C V 1 CV C 3 ( m ) 1 CV ( ) 3 g F ( kbt) ( ) u s 4 u du c h V e 1 0

6 Light Outut (Powe) Light Outut (mw) Light Outut (mw) LD Powe as a fuctio of Dive Cuet The excess ijected caie P Ohmic Cotact cocetatio deceases t 0e as Dielectic Isulato tot (Oxide/Nitide) + GaAsP egio The ate of chage fo a cuet d J + GaAsP P = 40% desity J ad deletio width d is dt qd J tot m qd I R tot, q P it + GaAs substate N Ohmic Cotact it hci q Cuet (I) 3 R it R tot Ideal: Liea tot Whee the 1s tem o the ight is the ate of ijected chage caie coc. ad the d is the ate of sotaeous decay. J At steady state the: tot qd Assumig each e-h ai oduces oe hoto P it = R tot.h O P it =(I/q)h(c/)=(hc/q)I=[1.396/(mm)]I 1 10 Not all ecombiatios oduce hotos. Hece P it = it (hc/q)i High Sloe but Satuatio effects at high cuets Due to oadiative ecombiatio effects aisig out of Auge Recombiatio Cuet (ma) Low Sloe due to oadiative ecombiatio ate lage i comaiso to adiative ecombiatio ate Cuet (ma)

7 LD xtactio fficiecy 1 F q c I I 1 cosq c q q q I Cost. siqdqd siqdqd F q c 1 F = 1- =η 1 1 si 1 1 q q c q Poit Light Souce 1 - juctio e(xtactio) Note that 1 is the semicoducto efactive idex ad is usually ai o lastic ecasulatio Plastic ecasulatio lectical Cotacts 3

8 ij 1 m m LD xteal fficiecy ve eglectig citical agle, still e = ½ due to loss i the substate h L o e L o e h 1 (mobility, majoity caie cocetatio, ad diffusio legth) The fial fficiecy is the give by tot ij e lectical Cotacts To icease e oe could use a hemisheical dome, but it is exesive. So a cheae alteative is to use lastic ecasulatio whee o =1, of the semicoducto is ~3.5-4, ad 1 is usually ~1.5

9 Stuctues of LD (Imotat oes) - I: Light Outut Light Outut Dome LD tye eitaxial laye tye substate Plaa LD Diffused -tye Ohmic cotacts Ohmic cotacts Dome ad laa LD ae used i most dislay devices whee the iteest is i extactig the maximum amout of light fom the device. =>Light is emittig i all diectios ad usig a les aagemet to focus the light Buus ad edge-emittig LD ae used maily i otical fibe commuicatio systems

10 Stuctues of LD (Imotat oes) - II: A ossible device stuctue is show i Fig. Ufotuately, the ecombiatio ocess that leads to hoto oductio ivolves the Z imuity cetes, ad hoto emissio ocesses ivolvig imuity cetes ae much less efficiet tha bad to bad ocesses. i - GaN Blue Photos Ohmic Cotacts + - GaN Sahie Substate (Tasaet) Dielectic (Oxide o Nitide)

11 Review Poblems 1. Photos of wavelegth =813 m ae absobed i IP at oom temeatue (g=1.344ev, m e *=0.08m o, m h *=0.60m o ) ad excites electo-hole ais (HP). Calculate the aveage kietic eegy of the electos ad holes befoe they elax to the bottom of the bads. [Hit: they will ot be the same]. Fid the eak emissio wavelegth ( o ) fo a Al 0. Ga 0.8 As LD oeatig at 400 o K, give the bad gas AlAs G =3.03eV, AlAs X =.15eV, AlAs L =.36eV, GaAs G =1.43eV, GaAs X =1.73eV, ad GaAs L =1.89eV. (assume liea iteolatio to be valid) 3. Deive the factio of adiatio escaig fom a LD (oit souce) imbedded i a medium of efactive idex 1 ito the ue medium of efactive idex, assumig thee is o absotio i the medium. Also assume the tasmissio coefficiet is that of omal icidece at the iteface. 4. A LD at oom temeatue ude 0.8V fowad bias coducts a cuet of I =1mA ad emits light at a eak wavelegth of 1.0µm. The adiative ad the o-adiative time costats ae 0.1s ad 1s, esectively. Assumig that the uity extactio efficiecy ad the ijectio efficiecy is 0.9, calculate the owe covesio efficiecy of the diode. Is this calculated efficiecy geate tha o less tha uity? Does it suise you, exlai. What is the ew covesio efficiecy, if fo the same set of coditios, the emissio wavelegth is ow 1.3µm?