V. Semiconductor Photodetectors (PD)
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Introduction
Photoconductors
P-i-N photodiodes
Schottky Barrier (M-S) Photodetectors
Avalanche photodiodes (APD)
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Introduction to the Photodetectors
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Photodetectors are semiconductor devices that can convert optical signals into electrical signals.
Three steps for the operation of a photodetector:
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Basic requirement for the photodetectors:
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(i) carrier generation by incident light  absorption and generation
(ii) carrier transport (and/or multiplication) by current gain mechanism  photocurrent
(iii) interaction of current with the external circuits to provide the output signal  current collection
high sensitivity at the operating wavelengths
high response speed
low noise
compact (small size)
low biasing voltages or current
high reliability
Development for advanced photodetectors:
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tunability
speed
integration
Intrisic
Extrisic by a deep
level
Extrisic by intersubband in a QW
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Photoconductivity
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Photoconductivity
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Important parameters in a photoconductive process:
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absorption coefficient ()
cut-off wavelength (λC)
responsivity (RS)
dark current (ID)
quantum efficiency (ηe)
Strong absorption condition:
 L (absorption thickness of the sample) > 1/
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When incident light impinges on the surface of the photoconductor, EHPs are generated either by band-to-band transition (intrinsic detector) or by transitions involving forbidden-gap energy levels (extrinsic detector),
resulting in an increase in conductivity.
Typical spectrum of a photodetector
the absorption coefficient for the extrinsic absorption is quite small (~10 cm-1) , so a thick sample is needed for
extrinsic detectors.
Extrinsic detectors are important for detection of long wavelength radiation
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Intrinsic detectors need a very narrow bandgap for long wavelength detection and it is difficult to fabricate
high-quality devices from such materials.
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Important semiconductor materials for light detection
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Photoconductors
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The photoconductor is the simplest type of
the detectors and consists of a simple slab of
semiconductor across which a bias is applied.
An important benefit of the photoconductor is
the gain in the device, i.e., one can collect
more than one electron (or hole) for each
photon impinging.
The response time is determined by the
transit time.
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To achieve short transit time, small electrode
spacing and a high electric field must be used.
The photoconductor suffers from the presence
of a large dark current noise in the detector.
 A reverse biased p-n diode has a very low dark
current. (p-n or p-i-n photodiode)
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The photoconductors are extensively used for
infra-red detection especially for wavelength
greater than a few microns.
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Photodiodes — the basic concepts
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A p-n junction diode operated under reverse bias.
The electric field in the depletion region of the diode serves to separate photo-generated
EHPs, and an electric current will flow in the external circuit.
The thickness of the depletion layer WD, the quantum efficiency η , but transit time
(response speed )
 Hence, there is a trade-off between the response speed and quantum efficiency.
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Photodiodes — the quantum efficiency
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The quantum efficiency η is defined as the number of
e-h pairs generated for each incident photon.
One of the key factors that determines the quantum
efficiency η is the absorption coefficient α.
Since α is a strong function of the wavelength, the
wavelength range in which appreciable photocurrent
can be generated is limited.
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For light wavelength λ > λc, the values of  are too
small to give appreciable band-to-band absorption.
For short wavelengths, the values of  are very large
(~105 cm-1), and hence the radiation is mostly absorbed
by surface recombination.
The UV and visible region ⇒ The M-S photodiodes
The near-infrared region ⇒ The Si photodiodes
The 1.0 ~ 1.6-μm region ⇒ The Ge photodiodes
and GaInAs photodiodes
For longer wavelengths ⇒ cooled photodiodes
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Photodiodes — the response speed
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The response speed is limited by three factors:
(1) diffusion time for carriers generated outside
the depletion region
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minimized by making the junction very close to
the surface
(2) the drift time of carriers through the
depletion region (charge collection time)
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the depletion width (WD) , carrier mobility (μ) ,
and applied bias (V)  ⇒ drift time 
(3) RC time constant of the device
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the depletion width (WD)  ⇒ capacitance of the
depletion region (C)  ⇒ RC 
The optimal compromise is the width at which
the depletion layer transit time is approximately
one half the modulation period.
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For a modulation frequency of 2 GHz, the optimal
WD in a Si photodiode (with a saturation velocity
of 107 cm/s) is 0.5  (2 GHz)-1  107 cm/s =
0.0025 cm = 25 m
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The p-i-n Photodiodes
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For a p-i-n photodiode, the depletion region
thickness can be tailored by controlling the ilayer thickness to optimize the quantum
efficiency and frequency response.
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When the device is reverse biased, the
applied voltage appears almost entirely across
the i region (thus depleted entirely), width of
the depletion layer is approximately equal to
that of the i-layer.
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The i-region need not be truly intrinsic, as long
as the resistivity is high.
The light absorption region is extended as
compared to a p-n photodiode, so higher
quantum efficiency can be obtained.
The device response is fast since the
photocurrent is primarily due to the prompt
photocurrent (in contrast to the diffusion
photocurrent)
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Anti-reflection coating is used to increase the
quantum efficiency by minimizing the surface
reflection.
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Heterojunction p-i-n Photodiodes
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Advantages:
(1) Using the large-bandgap material as a
window for the transmission of optical
power, the quantum efficiency does not
depend critically on the distance of the
junction from the surface.
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Broad absorption range since the efficiency is flat
in the wavelength as a result of lack of near
surface absorption.
Ga0.47In0.53As (Eg = 0.73eV), InP (Eg = 1.35 eV)
(2) The heterojunction can provide unique
material combinations so that the quantum
efficiency and response speed can be
optimized for a given optical-signal
wavelength.
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Because absorption is in the depletion region, no
light generated diffusion current occurs. It is a
very fast device.
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Schottky Barrier Photodetectors
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A thin metal layer (~ 100 Å) replaces the p-layer
in the p-n diode to avoid the surface absorption.
 Metal-Semiconductor photodiode
The absorption region is the depletion layer
formed by the Schottky barrier between the M/S
junction
 Schottky-Barrier photodiode
A key advantage is it is a majority carrier device,
it does not suffer from speed delay arising from
minority carrier lifetime issues.
 very fast response speed (up to 150 GHz)
The diode can response in two important
regimes:
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(i) hν > qφbn : electrons are excited in the metal
barrier to overcome the Schottky barrier. (a source
of dark current)
(ii) hν > Eg : e-h pairs are created in the depletion
and swept out to produce photocurrent.
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Metal-Semiconductor-Metal (MSM) Photodetectors
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A planar version of the Schottky barrier
photodiode. (The two metal contacts are on the
top of the device)
The spacing between the interdigitated finger
contacts is small (~ 1 to 5 μm)so that, the
region between the fingers can be completely
depleted when biased.
When biased, the device represents two diodes
in series, one forward biased and the other
reverse biased.
Advantages of the MSM photodiode over the
standard Schottky one:
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planar structure is easy to fabricate.
The parasitic capacitance between contacts is
much small than for a vertical diode
 smaller RC time constant
smaller contact spacing due to the narrow the
finger spacing
 shorter carrier transit time
easier integration with amplifiers or waveguide
 very attractive for OEIC applications
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Avalanche Photodiodes (APD)
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An APD is operated under a reverse-bias
voltage which is sufficient to enable
avalanche multiplication (impact ionization)
to take place.
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The Avalanche Multiplication Process
vary large internal current gain
fast response to the light modulated at
microwave frequencies
high sensitivity to low-level optical signals
with about 1 ns response time  particular
useful in fiber optic communication.
Important parameters
– Breakdown voltage : VB
– Ionization rate (coefficient) : e and h
 The number of EHPs generated by an electron
(or hole) per unit distance.
– Multiplication factor : M
 The ratio of the number of electrons leaving
the junction to those entering it.
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M
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 dx
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Comparisons between PIN and APD detectors
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APDs have greater sensitivity for a given material
– detection of weaker signals over longer distances of fiber at a higher bit rate.
APDs have higher dark current.
– PIN diode may have greater sensitivity for long wavelength detection than an APD.
PIN diodes are easier to fabricate.
– Hence produce higher yield devices and array at lower cost.
PIN diodes are easier to integrate (with amplifiers)
APDs are very temperature sensitive for dark current and multiplication factor.
– Temperature stabilization circuitry is usually needed to increase their reliability.
APDs require a high (tens or hundreds of volts) operating voltage.
 The use of APDs is thus presently limited to applications where high gain is of paramount important.
– In an APD one has to maintain very stable voltage and temperature values which males the
system costly and somewhat unreliable especially if it is placed in a region that is difficult to
access (like in undersea regenerators for long distant optical communications)
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Phototransistors
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The phototransistor provides high gain due to
the transistor action and is a low noise device
device as compared to an APD.
The device usually operated with the base
open-circuited.
Holes generated in the reverse biased basecollector junction region provide a base
current which causes the electrons to be
injected from the emitter.
Due to the transistor action, a small
photocurrent induced base current produces a
large collector current.
The device does not have a good high
frequency response due to very large
capacitance associated with the base-collector
junction.
The phototransistor finds important uses due
to its low noise and high gain.
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Comparison of various photodetectors
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