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Gladys Rose
5 years ago
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B"03&'0" Photogenerated electron concentration exp("#x) at time t = 0 666 vde A E h > E g h+ SEMICONDUCTOR OPTICS Drift velocity (m s -1 ) to reside) requires an exchange of momentum that cannot be

7 B"03&'0" Photogenerated electron concentration exp("#x) at time t = vde A E h > E g h+ SEMICONDUCTOR OPTICS Drift velocity (m s -1 ) to reside) requires an exchange of momentum that cannot be accommodated by the emitted photon. 5 Momentum may be conserved however by the participation x in10the interaction. Phonons can carry relatively large momenta but of phonons Electron ev; see Fig ) so their transitions typically have small energies appear horizontal on the E-k diagram (see Fig ). The net result is that 4 momentum is10conserved but the k-selection rule is violated. Because phononhole assisted emission involves the participation of three bodies (electron photon and phonon) the probability of its occurrence is quite low. Thus Si which is an indirect-bandgap 103 semiconductor has a substantially lower radiative recombination coefficient than does GaAs which is a direct-bandgap semiconductor (see Table ). Silicon is therefore not an efficient light emitter whereas GaAs is. B W CHAPTER 16 e E iph Figure Photon emission in an Electric fieldindirect-bandgap (V m -1 ) semiconductor. The recom- R bination of an electron near the bottom of Drift velocity vs. electric field for and band electrons Si.near the top the holes conduction with ain hole of the valence band requires the exchange 1999 S.O. Kasap Optoelectronics (Prentice Hall) of energy and momentum. The energy may be carried off by a photon but one or more phonons are also required to conserve is absorbed throughout the momentum. This type of multiparticle interaction is therefore unlikely. concentration that decays Vr An infinitesimally short light pulse depletion layer and creates an EHP exponentially 1999 S.O. Kasap Optoelectronics (Prentice Hall) -.&*&("*"%*&+0:#2*"+$2>("0$E' I'*+$'0$%#2*"+$2>0 R621)L28*1*LH21))<*+( '<1)<*+(T21)L28S JK*+$'0$%L2*"+$2>0 R)'8<1LS Photon Absorption Is Not Unlikely in an Indirect-Bandgap Semiconductor. Although photon absorption also requires energy and momentum conservation in an indirect-bandgap semiconductor this is readily achieved by means of a two-step process (Fig ). The electron is first excited to a high energy level within the conduction band by a k-conserving vertical transition. It then quickly relaxes to the bottom of the conduction band by a process called thermalization in which its momentum is transferred to phonons. The generated hole behaves similarly. Since the process occurs sequentially it does not require the simultaneous presence of three bodies and is thus not unlikely. Silicon is therefore an efficient photon detector as is GaAs. Figure Photon absorption in an indirect-bandgap semiconductor via a vertical (k-conserving) transition. The photon generates an excited electron in the conduction band leaving behind a hole in the valence band. The electron and hole then undergo fast transitions - to the lowest and highest possible levels in the conduction and valence bands respectively releasing their energy in the form of phonons. Since the process is sequential it is not unlikely. c. Absorption Emission and Gain in Bulk Semiconductors We now proceed to determine the probability densities of a photon of energy hv emitted by a bulkm"*"+&0*+)%*)+"0 being or absorbed semiconductor material in a direct band-to-band R621)L28*1L<1**<1LS ef4%qe;21(;io*33 41E2*)8&'(')*(*+('

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10 3'3.&*&($&("0 V r (a) SiO 2 Electrode p + h" > E g h+ E e W (b) net en d Antireflection coating (c) en a E (x) n I ph R Electrode Depletion region x x V out 3:$:'3.&*&($&("0 SiO 2 Electrode p + (a) (b) (c) en d en a h" > E g (d) net E (x) E o i-si n + W E h +e Electrode x x E max I p h R V out (a) A schematic diagram of a reverse biased pn junction photodiode. (b) Net space charge across the diode in the depletion region. N d and N a are the donor and acceptor concentrations in the p and n sides. (c). The field in the depletion region S.O. Kasap Optoelectronics (Prentice Hall) The schematic structure of an idealized pin photodiode (b) The net space charge density across the photodiode. (c) The built-in field across the diode. (d) The pin photodiode in photodetection is reverse biased S.O. Kasap Optoelectronics (Prentice Hall) V r 3$'3.&*&($&("0 R(72'*2E"0C 41+*2-*)O<)(&'E)*83*9'1 *L<'1Z[ "$ 32L*D'3;I*E'3<L&(+28(;<1L :$ I'*+2<*-2*L'D*1*)TH)<cP3*--<1N;*1+*'E )<5;-<'1 ST N)*Q3'1L*(21-<(9I* SiO 2 Electrode p + (a) (b) (c) en d en a h" > E g (d) net E (x) E o i-si n + W E h +e The schematic structure of an idealized pin photodiode (b) The net space charge density across the photodiode. (c) The built-in field across the diode. (d) The pin photodiode in photodetection is reverse biased. I p h 1999 S.O. Kasap Optoelectronics (Prentice Hall) V r Electrode R x x V out

11 B"03&'0$67$*8 A&*W;21(;I*`+<*1+H<- O2D*3*1L(&)*8*1)*1($?*1+*G<- 23-')*1'(*)-8*+(23*-8'1-<D<(H 21)83'Y*)2-2E;1+9'1'E O2D*3*1L(&$ Responsivity (A/W) Ideal Photodiode QE = 100% ( " = 1) Si Photodiode g Wavelength (nm) Responsivity (R) vs. wavelength () for an ideal photodiode with QE = 100% (" = 1) and for a typica commercial Si photodiode S.O. Kasap Optoelectronics (Prentice Hall) ]'(*QK<<1)<*+(T21)L Photon energy (ev) Ge In 0.7 Ga 0.3 As 0.64 P " (m -1 ) Si GaAs InP In 0.53 Ga 0.47 As a-si:h Wavelength (µm) Absorption coefficient (") vs. wavelength (#) for various semiconductors (Data selectively collected and combined from various sources.) Figure 5.3

12 B"03&'0" Photogenerated electron concentration exp("#x) at time t = 0 v de Drift velocity (m s -1 ) A W B x Electron Hole h > E g E 10 3 h + e i ph R Electric field (V m -1 ) Drift velocity vs. electric field for holes and electrons in Si S.O. Kasap Optoelectronics (Prentice Hall) V r An infinitesimally short light pulse is absorbed throughout the depletion layer and creates an EHP concentration that decays exponentially 1999 S.O. Kasap Optoelectronics (Prentice Hall)

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(b) Impact of an energetic conduction electron with crystal vibrations transfers the electron's kinetic energy to a valence electron

17 R72>2'%."-.&*&/$&("9R-/< F8*2(*-TH+'1D*91L*2+&)*(*+(*)8&'('1(' )*'EI'D<1L+2<*82<-$A&<-<-2+&<*D*)TH ;-<1L20*+&'E>8+"7"+0":Q$20"(3.&*&($&("$A&* 32L**3*+(<+U*3)(&*1<1);+*-$#32%*$&'$W26&'$ E E e h+ e Ec Ev h+ n+ p s Avalanche region (a) (b) (a) A pictorial view of impact ionization processes releasing EHPs and the resulting avalanche multiplication. (b) Impact of an energetic conduction electron with crystal vibrations transfers the electron's kinetic energy to a valence electron and thereby excites it to the conduction band S.O. Kasap Optoelectronics (Prentice Hall) R72>2'%."-.&*&/$&("9R-/< F8*2(*-TH+'1D*91L*2+&)*(*+(*)8&'('1(' )*'EI'D<1L+2<*82<-$A&<-<-2+&<*D*)TH ;-<1L20*+&'E>8+"7"+0":Q$20"(3.&*&($&("$A&* 32L**3*+(<+U*3)(&*1<1);+*-$#32%*$&'$W26&'$ A&*I;3983<+29'18'+*--

18 .D2321+&*8&'(')<')*-R.%J-S2*;-*)O<)*3H<1 '89+23+'II;1<+29'1-21)E'-8*+('-+'8H -<1+*(&*H&2D*&<L&-8**)21)<1(*123L2<1$ A&**2*-*D*23.%J)*-<L1)*8*1)<1L'1 2883<+29'1$?**O*+'1-<)*2+"2%.:*.+&)E. R-/C ia&*1j-<)*<-(&<121)<33;i<12(*))<*+(3hth(&* 3<L&(<1+<)*1( ia&**8b(h8*32h*-2*t;<*)o<(&<1(&*(&* -;T-(2(*$.(&<18B32H*<-E2T<+2(*);1)*(&*1j *L<'1$A&*I<))3*32H*<-3<L&(3H)'8*)R23I'-( <1(<1-<+S$.U1238j*L<'1<-3'+2(*)2((&* +';1(**3*+(')*$ ia&*)<')*<-*d*-*)t<2-*)('<1+*2-*u*3)- O<(&<1(&*)*83*9'1*L<'1O&**2T-'89'1 '++;-$ Electrode h" > E g n et E (x) SiO 2 p Avalanche region E I p h e h + s p + Absorption region (a) A schematic illustration of the structure of an avalanche photodiode (APD) biased for avalanche gain. (b) The net space charge density across the photodiode. (c) The field across the diode and the identification of absorption and multiplication regions S.O. Kasap Optoelectronics (Prentice Hall) n + R Electrode x x (a) (b) (c) E h + e E e E c E v n + p s h + Avalanche region (a) (b) (a) A pictorial view of impact ionization processes releasing EHPs and the resulting avalanche multiplication. (b) Impact of an energetic conduction electron with crystal vibrations transfers the electron's kinetic energy to a valence electron and thereby excites it to the conduction band S.O. Kasap Optoelectronics (Prentice Hall)

19 =2$' $'E>"%2++$"+#)>63>$%26&' f<(&<123*1l(&.67*$"*+;*1(<-<1+*i*1(*)th2e2+(' Z[ L)>63>"%2++$"+#)>63>$%26&' f<(&<123*1l(&.67*$"*+;*1(<-<1+*i*1(*)th2e2+(' V r "32+2*"RQ0&+36&'2'(L)>63>$%26&'9RL< R-/?*(*'g;1+9'1'E(O')<5**1(I2(*<23-.T-'89'1'++;-<1(&*3<L&(3H)'8*)1(H8* *L<'1 =;3983<+29'1'++;-<1(&*)'8*)](H8**L<'1 621)L28)<5**1+*-T*(O** )41% &*38('I'D**3*+('1-2+'--(&*)*D<+*?'3*-<1(&*)*D<+*(*1)('T*(288*)TH 8'(*1923k8'+V*(-lD2321+*T21)Z[-3'O* *-8'1-*9I*-$.)D21(2L*QF1*(H8*'E+2<*<1(&* I;3983<+29'1O&<+&<I8'D*-(&*k<'1<\29'1 29'l21)(&**TH(&**);+*-(&*L2<121) <I8'D*-(&**-8'1-*9I* (a) (b) Electrode h E (x) InP EE I p h P + N n n + Avalanche region InP e h + InGaAs x R Absorption region Simplified schematic diagram of a separate absorption and multiplication (SAM) APD using a heterostructure based on InGaAs-InP. P and N refer to p and n -type wider-bandgap semiconductor S.O. Kasap Optoelectronics (Prentice Hall) E c E E v InP InP E v E v e h + InGaAsP grading layer h + E c InGaAs E v InGaAs E v V out (a) Energy band diagram for a SAM heterojunction APD where there is a valence band step E v from InGaAs to InP that slows hole entry into the InP layer. (b) An interposing grading layer (InGaAsP) with an intermediate bandgap breaks E v and makes it easier for the hole to pass to the InP layer

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23 M&D"7"+X*.$0$0E&&(*&V'&DY

24 R++280&O("*"%*&+0 =21H8&'(')*(*+('*3*I*1(-R8<0*3-S+21T*8;(('L*(&*('E'I22H-$AO'8<1+<823E'I- 'E*2)B';(+<+;<(H'E(&*)*(*+('-<L123-+&2L*B+';83*))*D<+*R>>JS21)+'I83*I*1(2H I*(23B'0<)B-*I<+'1);+('R>=FK(*+&1'3'LHS M&D"7"+X*.$0$0E&&(*&V'&DY Current I d + I p h Illuminated P o I d + I ph + i n I d Dark n p Time R A V ou In pn junction and pin devices the main source of noise is shot noise due to the dark current and photocurrent S.O. Kasap Optoelectronics (Prentice Hall) V r ]/%

Lecture 12. Semiconductor Detectors - Photodetectors

Lecture 12. Semiconductor Detectors - Photodetectors Lecture 12 Semiconductor Detectors - Photodetectors Principle of the pn junction photodiode Absorption coefficient and photodiode materials Properties of semiconductor detectors The pin photodiodes Avalanche