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Chapter 4
Photodetectors
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Types of photodetectors:
Photoconductos
Photovoltaic
Photodiodes
Avalanche photodiodes (APDs)
Resonant-cavity photodiodes
MSM detectors
In telecom we mainly use PINs and APDs.
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Basic operation of a PN photodetector:
Absorption takes place in 3 places: (1) within depletion region, (2) away
from it, and (3) near it.
Those from category (1) generate photocurrent (as done in class)
Those from category (2) recombine and are not useful
Those from category (3) diffuse (if they don’t recombine) to the depletion
region and then generate photocurrent. Diffusion is a slow process,
however.
Bottom line: We want to confine the absorption to the depletion region
A PIN diode has this advantage
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p+
SiO2
Electrode
ρ net
– eN a
eN d x
x
E ( x)
R
E max
e – h+
I ph
hυ > E g
W
E
n
Depletion region
(a)
(b)
(c )
Antireflection
coating
V r
(a) A schematic diagram of a reverse biased pn junction photodiode. (b) Net space charge across the diode in thedepletion region. N d and N a are the donor and acceptor
concentrations in the p and n sides. (c). The field in thedepletion region.
Electrode
V out
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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e – h+
i ph(t )
Semiconductor
(a )
V
x
(b)
(a) An EHP is photogenerated at x = l. The electron and the hole drift in opposite
directions with drift velocities vh and ve. (b) The electron arrives at time t e = ( L − l)/ve an
the hole arrives at time t h = l/vh. (c) As the electron and hole drift, each generates an
external photocurrent shown as ie(t ) and ih(t ). (d) The total photocurrent is the sum of hole
and electron photocurrents each lasting a duration t h and t e respectively.
E
l L − l
t
v evhvh
0 Ll
t
e – h+
t h
t e
t
0
t h
t e
i ph(t )
i(t )
t
0
t h
t e
evh /L + eve /Levh /L
ie(t )
ih(t )
(c )
(d)
Charge = e
evh /L ev e /L
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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0.2 0.4 0.6 0.8 1.2 1.4 1.6 1.8
Wavelength (µm)
In0.53Ga0.47As
Ge
Si
In0.7Ga0.3As0.64P0.36
InP
GaAs
a-Si:H
12
345 0.9 0.8 0.7
1×103
1×104
1×105
1×106
1×107
1×108
Photon energy (eV)
Absorption coefficient (α ) vs. wavelength (λ ) for various semiconductors(Data selectively collected and combined from various sources.)
α (m-1)
1.0
Figure 5.3
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E
CB
VB
k– k
Direct Bandgap E g Photon
E c
E v
(a) GaAs (Direct bandgap)
E
k– k
(b) Si (Indirect bandgap)
VB
CB
E c
E v
Indirect Bandgap, E g
Photon
Phonon
(a) Photon absorption in a direct bandgap semiconductor. (b) Photon absorption
in an indirect bandgap semiconductor (VB, valence band; CB, conduction band)© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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Responsivity ( R) vs. wavelength (λ ) for an ideal photodiode with QE = 100% (η = 1) and for a typiccommercial Si photodiode.
0 200 400 600 800 1000 12000
0.1
0.2
0.3
0.4
0.5
0.6
0.70.8
0.9
1
Wavelength (nm)
Si Photodiode
λ g
Responsivity (A/W)
Ideal Photodiode
QE = 100% ( η = 1)
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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Quantum efficiency η, definition.
Examples of QE for different detectors
Responsivity: definition: R=η q P/hν = ηλ (in microns) /1.24 units of
Amps per Watt
Speed: carrier transport time τtr ; RC-time constant τRC ∆f = 1/2π(τtr +τRC)
If the photodetector has gain, there is also a buildup time, τb, associated
with the gain that affects the bandwidth. τb
Tradeoff between responsivity and bandwidth
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p+
i-Si n+
SiO2
Electrode
ρ n et
– eN a
eN d
x
x
E ( x)
R
E o
E
e – h+
I p h
hυ > E g
W
(a)
(b)
(c )
(d)
V r
The schematic structure of an idealized pin photodiode (b) The netspace charge density across the photodiode. (c) The built-in field across the diode. (d) The pin photodiode in photodetection isreverse biased.
V out
Electrode
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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hυ > E g
p+ i-Si
e – E
h+
W l
Drift
Diffusion
A reverse biased pin photodiode is illuminated with a shortwavelength photon that is absorbed very near the surface.
The photogenerated electron has to diffuse to the depletionregion where it is swept into the i-layer and drifted across.
V r
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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Operation of PIN diodes
Properties;
1. Controllable increase in width of the depletion region (good)
2. Reduced junction capacitance (reduced RC constant, good for speed)
3. Diffusion current is reduced due the increase of the depletion region
width
4. Higher carrier transport time (bad for speed).
5. Next step: reduce absorption in doped layers in a PIN by using higher
bandgap material such as InP (in the case of InGaAs absorber)
6. This is what PIN heterostructures do
7. Next thing is to have gain: impact ionization
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š p+
SiO2Electrode
ρ net
x
x
E ( x)
R
E
hυ > E g
p
I ph
e – h+
Absorptionregion
Avalanche
region
(a)
(b)
(c )
(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) Thefield across the diode and the identification of absorption and multiplication regions.
Electrode
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
n+
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h+
E
š n+ p
e –
Avalanche region
e –
h
+
E c
E v
(a ) (b)
E
(a) A pictorial view of impact ionization processes releasing EHPs and the resulting avalanche multiplication. (b) Impact of an energeticconduction electron with crystal vibrations transfers the electron's
kinetic energy to a valence electron and thereby excites it to theconduction band.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
V
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E
N n
Electrode
x
E ( x)
R
hυ
I p h
Absorption
region
Avalanche
region
InP InGaAs
h+
e – E
InP
P+ n+
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.
V r
V out
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
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Modes of Operation of an APD
• Recall that APDs are photodetectors that haveoptoelectronic gain
• Linear mode (sub-breakdown):
photocurrent ∝ optical power e.g., optical communication
• Geiger mode (post-breakdown): Ideally, eachdetected photon results in breakdown
e.g., photon counting, coincidencecounting
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Current Photodetectors Needs for Long-haul
Optical Communication
• High quantum efficiency at 1.55 µm (beyond
80%).
• High speed: 10 Gbps (OC 192) and beyond.
• Internal gain: ~10-50 (preferred over externalEDFA amplification).
• Wavelength selectivity (WDM).
• Low noise (excess noise, dark current).
• Compactness, reduced cost, OEIC (solid-state).
Thi M l i li i & Ab i R i
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Thin Multiplication- & Absorption-Region
APDs• Benefits:
– High speed (up to 40 GHz +)
– Low multiplication noise (factor of ~2)
• similar mechanism as noise suppression in superlattice MQW
APDs).
– Higher optimal gain values: better SNR and BER
– Breakdown characteristics (Geiger mode)— “not so good”
• Challenges:
– Quantum efficiency must be enhanced by employing new structures:
• Waveguide structures (lateral absorption)• Resonant-cavity structures (vertical absorption)
• Detection efficiency is low 1.55µm and beyond
• Dark current is always a problem
Ed l d id APD’
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• Edge-coupled waveguide APD’s:
– Idea: Reduce absorption-region (~0.8 µm or less) width
without killing quantum efficiency.
– High gain-bandwidth (> 12 GHz at gain of 10)
– Reduce charge-space effects
– Challenge remains: coupling efficiency (QE ~25%)
InGaAsabsorption
InAlAs
multiplicationInP buffer n: InAlAs
InGaAs cap
p: InAlAs
800nm
p: charge
InP Substrate
[Kinsey et al, 00]
l i g h t
400 nm
1 0 µ m
800 nm
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• Resonant cavity photodiodes
n- contact
}
InP/InGaAsP
Bragg reflector
InGaAs
absorption layer n
p p: InP
n: InP
Input light
.
.
.
i
•Increase: Quantum efficiency
•Increase: Bandwidth
•Wavelength selectivity
•Drawbacks:•Increased fab. complexity
•Selectivity may not
be desirable in someapplications
[Unlu et al, 1995]