pn Junction Photodetectors
Most detectors used in communications are based on the pn or pin junction
is a semiconductor.
A pn junction is formed by doping adjacent regions of a semiconductor with
excess donor and acceptor atoms, respectively.
© 2012 Henry Zmuda
Set 8 - Detectors
1
pn Junction Detectors – Quantum Efficiency
In a reverse-biased photodiode it is desired that the light be absorbed in
the depletion region so that carriers can be collected before they
recombine.
Power absorbed elsewhere does not contribute to the overall photocurrent.
Incident Light
wp
W
––
––
––
––
––
––
p – region
© 2012 Henry Zmuda
wn
++
++
++
++
++
++
The quantum efficiency
is determined by the
amount of power
absorbed in the
depletion region relative
to the total incident
power on the detector.
n – region
Set 8 - Detectors
2
pn Junction Detectors – Quantum Efficiency
Reflection from the surface reduces the incident power by R, leaving
(1 – R)Po
The transmitted light must pass through the p-region, whose absorption
coefficient αp is essentially identical to the bulk attenuation coefficient for
the semiconductor material.
The amount of power reaching the depletion edge is:
( )
(
)
P w p = Po 1− R e
−α pw p
The depletion region will absorb light depleting the power by exp[-αw]
The fraction of light deposited in the depletion region is 1 – exp[-αw]
The overall quantum efficiency is
© 2012 Henry Zmuda
η = (1− R) e
Set 8 - Detectors
−α pw p
(
1− e −α w
)
3
pn Junction Detectors – Quantum Efficiency
It is clearly desirable to reduce reflections and absorption in the bulk
region.
This can be done by making wp as thin as possible, making W as wide as
possible, and adding an anti-reflection coating (AR coating) to the detector
substrate.
Recombination of photo-generated carriers will also reduce the quantum
efficiency.
Recombination is increased by defects and impurities which act as
recombination centers in the lattice, so these must be minimized.
© 2012 Henry Zmuda
Set 8 - Detectors
4
pn Junction Detectors – Quantum Efficiency – Example
Determine the quantum efficiency of a Si photodiode operating at 700 nm
with the following parameters:
α Si = 104 cm -1
1 µm
nSi = 3.5
κ = 11.7
Incident Light
atoms
N d = 10
cm 3
atoms
N a = 1017
cm 3
ni2 = 1.96 ×1020 cm -6 at T = 300°K
p – region
n – region
18
© 2012 Henry Zmuda
Set 8 - Detectors
5
pn Junction Detectors – Quantum Efficiency – Example
Total depletion width:
⎡
2κε o
Nd
W = a+b=
Φ − Va ) ⎢
+
(
q
⎢⎣ N a ( N a + N d )
W = 0.112 µ m
⎤
Na
⎥
Nd ( Nd + Na ) ⎥
⎦
⎛
Na
Na ⎞
Naa = Ndb ⇒ b =
a ⇒ W = a + b = ⎜ 1+
a⇒a=
⎟
Nd
Nd ⎠
⎝
W
Na
1+
Nd
W
W
a=
=
= 0.909W = 0.1018 µ m
17
1.1
10
1+ 18
10
© 2012 Henry Zmuda
Set 8 - Detectors
6
pn Junction Detectors – Quantum Efficiency – Example
Power loss due to Fresnel reflection:
2
2
⎛ n − 1 ⎞ ⎛ 2.5 ⎞
R=⎜
⎟ =⎜
⎟ = 0.31
⎝ n + 1 ⎠ ⎝ 4.5 ⎠
The transmitted light must propagate through wp = 1 – 0.1 = 0.9 µm of
silicon before reaching the depletion region.
This will have an attenuation of exp[-104 x 0.9 x 10-4] = 0.4
40% of the light transmitted through the front surface makes it to the
depletion region.
Only a fraction of the light is absorbed in the depletion region:
1 - exp[-104 x 0.112 x 10 –4] = 0.106; only 10% of the light that reaches the
depletion region is actually absorbed there.
© 2012 Henry Zmuda
Set 8 - Detectors
7
pn Junction Detectors – Quantum Efficiency – Example
The quantum efficient is:
η = (1 − R ) e
−α p w p
−α w
1
−
e
(
)
= 0.69 ⋅ 0.4 ⋅ 0.1
= 0.0276
The quantum efficiency for this example is less that 3%
This illustrates a major problem with pn-junction photodetectors.
A large reverse bias can widen the depletion region width, but breakover
becomes an issue.
The preferred solution is to use a pin diode.
© 2012 Henry Zmuda
Set 8 - Detectors
8
pin Detectors
The pin diode (p-intrinsic-n) is the most widely used detector in
optoelectronic systems.
It is difficult to increase the depletion region of a typical pn junction beyond
1 or 2 µm which is what limits the quantum efficiency and the capacitance.
The width of the depletion region can be artificially increased by adding an
intrinsic semiconductor between the doped region.
© 2012 Henry Zmuda
Set 8 - Detectors
9
pin Detectors
Incident Light
metal
SiO2
p+
Intrinsic Electric Field
n+
© 2012 Henry Zmuda
Set 8 - Detectors
10
pin Detectors
http://ecee.colorado.edu/~bart/book/book/chapter4/ch4_7.htm#4_7_1_1
© 2012 Henry Zmuda
Set 8 - Detectors
11
pin Detectors
An n-doped semiconductor has a layer of intrinsic material grown on the
surface followed by a thin epilayer or diffusion of p-doped material.
The undoped intrinsic layer is made think enough to absorb most of the
incident radiation, so that most free carriers are generated in this region.
Contact is made to the p-region through an annular contact at the surface,
and to the n-doped substrate through a ground plane.
A thin (1000 Angstrom) layer of SiO2 is sometimes deposited on the
surface of the detector to reduce reflections.
Under reverse bias, a strong static field is established across the intrinsic
region to rapidly sweep out any photo generated carriers.
© 2012 Henry Zmuda
Set 8 - Detectors
12
Anti-Reflection Coatings – A Single Layer Coating
n0
Incident
Light
n film
d film
© 2012 Henry Zmuda
nsubstrate
Optical
Component
AR Coatings
13
Anti-Reflection Coatings – A Single Layer Coating
n0
Incident
Light
n film = n0 ns
d film
1 λo
=
n1 4
nsubstrate
Optical
Component
No reflected
wave at λo
© 2012 Henry Zmuda
AR Coatings
14
Anti-Reflection Coatings – A Single Layer Coating
⎛
⎛ π λo ⎞
ns ⎞
1−
cos
⎜⎝ n ⎟⎠
⎜⎝ 2 λ ⎟⎠ −
0
Γ (λ ) =
⎛
⎛ π λo ⎞
ns ⎞
⎜⎝ 1+ n ⎟⎠ cos ⎜⎝ 2 λ ⎟⎠ +
0
⎛ n1 ns ⎞ ⎛ π
j ⎜ − ⎟ sin ⎜
⎝ n0 n1 ⎠ ⎝ 2
⎛ n1 ns ⎞ ⎛ π
j ⎜ + ⎟ sin ⎜
⎝ n0 n1 ⎠ ⎝ 2
λo ⎞
λ ⎟⎠
λo ⎞
λ ⎟⎠
r1 ( λ = λo ) = 0 ⇒ if n0 ns = n12 , r1 = 0
© 2012 Henry Zmuda
AR Coatings
15
1.  If normal incidence in not assumed, the reflection with angle
varies as:
2.  Note that for non-normal incidence that the reflection
minimum does not go to zero or even occur at the central
wavelength.
© 2012 Henry Zmuda
AR Coatings
16
Common Coating Materials
Multi-Layer Coating Materials
nH Materials
nL Materials
Zirconium Dioxide:
2.1
Magnesium
Fluoride *
1.38
Titanium Dioxide:
2.4
Cerium Fluoride
1.63
Cryolite
1.35
Zinc Sulfide:
2.32
* Very durable, hence frequently used.
© 2012 Henry Zmuda
AR Coatings
17
Anti-Reflection Coatings
Generally d is chosen so that the electrical length is a quarterwave in the yellow-green portion of the spectrum where the eye
is most sensitive.
At wavelengths on either side of the central
yellow-green region, the reflectivity increases
and the coated optical component will appear
blue-red in reflected light.
Of the materials just cited, none satisfy the condition for zero
reflection for an air-glass interface, but they do substantially
reduce reflections (see next slide). The durability of MgF2 makes
it a popular material to use.
© 2012 Henry Zmuda
AR Coatings
18
Anti-Reflection Coatings
Glass-air interface:
ns − no
ns + no
uncoated
nM g F2 = 1.38
no ns = 1.225
© 2012 Henry Zmuda
AR Coatings
19
Anti-Reflection Coatings
Glass-air interface:
© 2012 Henry Zmuda
AR Coatings
20
Anti-Reflection Coatings
Glass-air interface:
ns − no
ns + no
uncoated
nM g F2 = 1.38
no ns = 1.225
© 2012 Henry Zmuda
AR Coatings
21