Photodetectors Read: Kasip, Chapter 5 Yariv, Chapter 11...
Transcript of Photodetectors Read: Kasip, Chapter 5 Yariv, Chapter 11...
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Photodetectors Read: Kasip, Chapter 5
Yariv, Chapter 11 Class Handout
ECE 162C Lecture #13
Prof. John Bowers
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Definitions
• Quantum efficiency η: Ratio of the number of electrons collected to the number of photons incident.
• Responsivity: current out divided by optical power incident
AWhc
eheRd /24.1
ηληλ
νη ===
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External Quantum Efficiency and Responsivity
Different contributions to the photocurrent Iph. Photogeneration profiles corresponding to short, medium and long wavelengths are also shown.
Schematic photogeneration profiles
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photonsincident ofNumber collected and generated EHPfree ofNumber
=eη
External quantum efficiency (QE) ηe of the detector
υη
hPeI
o
phe /
/=
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Responsivity R
o
ph
PI
==(W) Power OpticalIncident
(A)nt PhotocurreR
hce
he
eeλ
ηυ
η ==R
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Responsivity R
Responsivity (R) vs. wavelength (λ) for an ideal photodiode with QE = 100% (ηe = 1) and for a typical inexpensive commercial Si photodiode. The exact shape of the responsivity curve depends on the device structure.
The line through the origin that is a tangent to the responsivity curve at X, identifies operation at λ1 with
maximum QE
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pin Photodiode
The responsivity of Si, InGaAs and Ge pin type photodiodes. The pn junction GaP detector is used for UV detection. GaP (Thorlabs, FGAP71), Si(E), IR enhanced Si (Hamamatsu S11499), Si(C),
conventional Si with UV enhancement, InGaAs (Hamamatsu, G8376), and Ge (Thorlabs, FDG03). The dashed lines represent the responsivity due to QE = 100 %, 75% and 50 %.
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pin Photodiode
WV
WV rr
o ≈+= EE
WAC roεε=dep
d
Wtv
=drift
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pin Photodiode
Drift velocity vs. electric field for holes and electrons in Si.
d
Wtv
=drift
Width of i-region (Depletion region)
Drift velocity Transit time (Drift time)
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pin Photodiode Speed
A reverse biased pin photodiode is illuminated with a short wavelength light pulse that is absorbed very near the surface. The photogenerated electron has to diffuse to the depletion region where it is swept into the i-layer and drifted across.
In time t, an electron, on average, diffuses a distance given by
= (2Det)1/2
Electron diffusion coefficient
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PIN Impulse Response?
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PIN Impulse Response?
Levevj hhee σσ +=
Displacement current flows. That is what is measured in an external circuit, not conduction current.
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Avalanche Photodiodes (APDs) • α Rate at which electrons multiply • β Rate at which holes multiply
• A large ratio of α/β or β/α results in a large gain bandwidth product and low noise amplification. True for Si
• Most III-Vs have a small ratio, and limited gain bandwidth product. The noise is larger, but still lower than a PIN receiver.
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E l e c t r i c F i e l d [ k V / c m ]
I n 0 . 5 3 G a 0 . 4 7 A s e l e c t r o n s h o l e s
G a A s e l e c t r o n s h o l e s
I n P h o l e s
e l e c t r o n s
F r o m : S i : P . P . W e b b , " M e a s u r e m e n t s o f I o n i z a t i o n C o e f f i c i e n t s i n S i l i c o n a t L o w E l e c t r i c F i e l d s " , G E C a n a d a I n c .
I n P : L . W . C o o k , e t . a l . , A p p l . P h y s . L e t t . 4 0 ( 7 ) , 1 A p r i l 1 9 8 2
I n G a A s : T . P . P e a r s a l l , A p p l . P h y s . L e t t . 3 6 ( 3 ) , 1 F e b r u a r y 1 9 8 0
G a A s : H . D . L a w a n d C . A . L e e , S o l i d - S t a t e E l e c t r o n i c s , 2 1 , 1 9 7 8
2 0 0 3 0 0 4 0 0 5 0 0 0 . 1
1
1 0
1 0 0
1 0 0 0
1 0 e 4
1 0 e 5
1 0 e 6
Si (electron
s)
Ioni
zatio
n C
oeffi
cien
ts[c
m -1
]
Ionization Coefficients for Semiconductors
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- - - -
+ - - -
- -
- - +
+
- -
- -
+ + + +
P+ N+
Si Multiplication Layer
- - - + +
- +
- - + +
- +
- - + +
- +
- - +
InP Multiplication Layer
P+ N+
The Avalanche Multiplication Process
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Avalanche Photodiode Gain or Multiplication M
Ionization coefficient ratio
αe = Aexp(-B/E) Chyoweth's law
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Avalanche Photodiode Gain or Multiplication M
M = exp(αew) Ionization coefficient
kwkkM
e −−−−
=])1(exp[
1α
Electrons only
Electrons and holes
k = αh / αe
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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.
Heterojunction Photodiodes: SAM
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Heterojunction Photodiodes: SAM
(a) Energy band diagrams for a SAM detector with a step junction between InP and InGaAs. There is a valence band step ΔEv from InGaAs to InP that slows hole entry into the InP layer.
(b) An interposing grading layer (InGaAsP) with an intermediate bandgap breaks ΔEv and makes it easier for the hole to pass to the InP layer for a detector with a graded junction between InP and InGaAs. This is the SAGM structure.
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Heterojunction Photodiodes: SAM
Simplified schematic diagram of a more practical mesa-etched SAGM layered APD
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SAM APDS: Need for Separate Absorption and Multiplication Regions
Small bandgap avalanche regions tend to have large dark current.
Absorption Region
Multiplication Region
e- e- e-
Electric Field
position
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Electric Field Simulation for SHIP
-600
-500
-400
-300
-200
-100
0
100
0.4 0.8 1.2 1.6 2
Elec
tric
Fiel
d [k
V/c
m]
Distance into Detector [µm]
Si InGaAs
5 V 10 V 23 V
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Gain and Dark Current vs. Bias. 23 µm diameter SHIP
0.01
0.1
1
10
100
0.1
1
10
100
1000
10000
0 5 10 15 20 25
Gai
n
Dar
k C
urre
nt [µ
A]
Reverse Bias [V]
Theoretical Gain Measured Photocurrent Gain
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0.1
1
10
1 10 100 1000
3 dB
Ban
dwid
th [G
Hz]
Gain
SHIP Detector 3-dB Bandwidth versus gain
315 GHz GB
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Comparison of Acheivable GB Product for SHIP, SL, and InP APDs
0
100
200
300
400
500
600
700
800
0.1 1
Gai
n-B
andw
idth
-Pro
duct
(GH
z)
Multiplication Layer Thickness [µm] 5
SHIP APD SL APD
InP APD
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Staircase APD: Use of bandgap engineering to increase the ratio of ionization coefficients.