Optical Detectors

Photonics and Optical Information Systems
3: Optical Detectors
Prof. Yaochun Shen
Email: [email protected]
• Most modern photodetectors operate on the basis of the
internal photoelectric effect – the photoexcited electrons and
holes remain within the material, increasing the electrical
conductivity of the material.
• Electron-hole photo-generation in a semiconductor.
• Absorbed photons generate
free electron-hole pairs
• Transport of the free electrons
and holes upon an electric
field results in a current
Absorption Coefficient
Absorption coefficient in different
semiconductor materials.
• Slow rise in semiconductors with
indirect bandgap (Si and Ge).
• Different absorption coefficients
for different materials and photon
energy. That is Responsivity (R) is
dependant on the photon energy.
Choice of Photodiode Materials
• A photodiode material should be chosen with a bandgap
energy slightly less than the photon energy corresponding to
the longest operating wavelength of the system.
• This gives a sufficiently high absorption coefficient to ensure
a good response.
• Direct-bandgap III-V compound semiconductors can be
better material choices (GaAs).
• Their bandgaps can be tailored to the desired wavelength by
changing the relative concentrations of their constituents.
• They may also be fabricated in heterojunction structures
(which enhances their high-speed operations).
• Indirect bandgap (Si): Yes, but slow rise in absorption
coefficient. High energy photons are needed.
A photodiode consists of an active p-n junction. When a photon of
sufficient energy strikes the diode, it excites an electron-hole pair
thereby creating a mobile electron and a positively charged hole. If the
absorption occurs in or near the depletion region, these carriers are
swept from the junction by the built-in field of the depletion region. Thus
holes move toward the anode, and electrons toward the cathode, and a
photocurrent is produced which is proportional to the illumination.
• Photoconductive mode
• Photovoltaic mode (solar cell)
Photovoltaic mode– a solar cell
When used in zero bias or photovoltaic mode, the flow of photocurrent out
of the device is restricted and a voltage builds up. The diode becomes
forward biased and "dark current" begins to flow across the junction in the
direction opposite to the photocurrent. This mode is responsible for the
photovoltaic effect, which is the basis for solar cells—in fact, a solar cell is
just an array of large area photodiodes.
Q: why there is an energy conversion efficiency limit of solar cells?
Photoconductive mode of operation circuit examples.
Photoconductive mode:
In this mode the diode is usually operated in reverse bias. This increases the
width of the depletion layer, which decreases the junction's capacitance
resulting in faster response times. When light falls on the junction, a reverse
current flows which is proportional to the illuminance. The linear response to
light makes it an element in useful photodetectors for some applications. It is
also used as the active element in light-activated switches.
• Diodes; PN, PIN….: Photons with sufficient energy are
absorbed to produce e-h pairs.
• Un-biased
• Built-in field sweeps carriers apart
• Current flows through external circuit
• Solar cell
• Reverse biased
• Large field sweeps carriers apart
• Short transit time & low depletion capacitance –high speed
• Detector
PN Junction
What will happen when p-type & n-type
semiconductor are placed in contact (single crystal with
different dopants)?
q High concentration of holes on the p-side diffuse
towards n-side; high concentration of electrons on
the n-side diffuse towards p-side
q Holes from the p-side and electrons from the nside combine at the junction, forming a depletion
q n-side becomes positively charged because it has
lost holes; the p-side negatively charged because
it has lost electrons; A potential (built-in voltage)
is formed at the junction which inhibits further
diffusion of electrons and holes (equilibrium)
PN Junction Photodiode unbiased
• The semiconductor photodiode
detector is a p-n junction structure
that is based on the internal
photoelectric effect.
• The photoresponse of a photodiode
results from the photogeneration of
electron-hole pairs through band-toband optical absorption.
• The threshold photon energy of a semiconductor photodiode is the
bandgap energy Eg of its active region.
• The photogenerated electrons and holes in the depletion layer are
subject to the local electric field within that layer. The electron/hole
carriers drift in opposite directions. This transport process induces an
electric current in the external circuit.
Energy-band Diagram
PN Photodiode
• In the depletion layer, the internal electric field sweeps the
photogenerated electron to the n side and the photogenerated hole
to the p side.
• a drift current that flows in the reverse direction from the n side to
the p side.
• Within one of the diffusion regions at the edges of the depletion
layer, the photogenerated minority carrier (hole in the n side and
electron in the p side) can reach the depletion layer by diffusion and
then be swept to the other side by the internal field.
• a diffusion current that also flows in the reverse direction.
Light to electron conversion basic understanding
The carriers generated near the junction is separated by the electric field
and moved (swept – drift) away from the junction, with electrons and
holes moving to their respective areas, n and p type respectively.
This induces a current in the external circuit which is in addition to the
leakage current.
The implication of this is that there needs to be enough photons to
produce a current greater than the leakage current before there is a
measurable signal.
From an engineering perspective long drift times in the depletion region
results in lower speed of operation (i.e lower frequency operation). The
width of the depletion region can be made small but this will mean that
the photo-generation is less, that is fewer electrons/holes for a similar
photon stream (i.e lower sensitivity).
So a trade off between sensitivity and speed of response.
Light to electron conversion PN junction with reverse bias
This is a pn photodiode. It is reversed
biased. The electric field across the pn
junction moves electrons and holes to
their respective sides p and n respectively.
This results in a increase in strength of
depletion region electric field either side
of the junction. This stops majority
charges crossing the junction in the
opposite direction.
üThis field, however, accelerates minority charges from both sides of the
junction. This current is known as a reverse leakage current. A photon
incident, is absorbed in or near the depletion region. If the energy of the
photon is equal to or greater than the band gap energy (hc/λ ≥ Eg) an electron
is excited and moved into the conduction band leaving a hole in the valence
band (a).
Solar cell: https://www.youtube.com/watch?v=ZYO83TkM0To
PN Photodiode
Photons can be absorbed in
the diffusion regions.
Electrons/holes generated in
the depletion region
separate and drift. But
electron/holes generated in
the diffusion region diffuses
through the diffusion region.
This diffusion process is very
slow compared to the drift.
• The engineering implication of this is that its limits response
speeds. Therefore, it is important that photons are absorbed in the
depletion region and this is made as long as possible by reducing
the doping levels. Normally, this depletion region is ~ 1 – 3 µm.
PN Photodiode
This width is optimised for the efficient detection of light at a given
wavelength. For silicon devices this 400 – 700nm and for
germanium 700 – 900nm. Typical photodiode output characteristic
As the reverse bias is
increased there is a small
increase in the photo
generated current for the
same lighting conditions. As
the light level increases the
photo current increased.
Factors affecting speed of response
Drift time of charges through the depletion region: When the electric field
in the depletion region exceeds a saturation value then the carriers may be
assumed to travel at constant (maximum) drift velocity (vd). The longest
transit time (tdrift) is for charges that must traverse the full width (d) of the
depletion layer and is given by:
tdrift = d/vd
A field strength above 2 x 104 V.cm-1 in silicon gives maximum (saturated)
charge velocities of approximately 107 cm.s-1. The transit time through the
depletion layer width of 10µm is approximately 0.1 ns.
Diffusion time (tdiff) of charges generated outside of the depletion region
is a relatively slow process. The time taken for charges to diffuse a distance
(di) is:
tdiff = di2/(2Dc)
Dc is the minority charge diffusion coefficient.
Typically, the hole diffusion time through 10µm of silicon is 40ns and for
electrons this is 8ns.
Factors affecting speed of response
Junction Capacitance of the photo diode (Cj).
• The junction capacitance is dependant on the level of reverse bias applied to
the photo diode and is related to the variation in stored charge at the
Cj = εsA/d
εs is the dielectric constant of the semiconductor material.
A is the area of the diode junction.
d is the depletion layer width.
The engineering implication is that a smaller depletion width increases the
junction capacitance but reduces drift time. What is the compromise for
response time if that was an important factor in a design?
The overall capacitance of the photo diode (Cd) is the sum of the junction
capacitance (Cj) and that associated with the connections(Cc) and the
packaging (Cp).
Cd = Cj + Cc + Cp
This capacitance must be minimised to reduce the RC time constant to
improve the response of the photo detector.
Recap: PN Photodiode
Photon energy >bandgap of semiconductor
Photon absorptionà e- & h+
e- & h+ flow à photo-current
Depletion region (<1 µm)
Diffusion region
Lower detection sensitivity at
longer wavelength because
absorption decreases with the
increase of wavelength.
Drift in depletion region: fast
(0.1 ns)
Diffuse in diffusion region: slow
Low response speed
PIN photodiode
To extend the operation of
photo diodes to a longer
wavelength a wider depletion
region is required.
This is achieved by introducing
a lightly doped material
between the p and n in the
previous arrangement. This
region is so lightly doped that
it can be considered intrinsic.
All the absorption takes place
in the depletion region.
This means that there are relatively few charges diffusing across the
depletion region and therefore implies a faster response.
Note also that the junction capacitance will be reduced. So response time
is improved.
Schematic of energy level in a PIN photo diode
Wider depletion regionà
Increased absorption of photonsà
Higher sensitivity
Drift current dominateà
Faster speed
Typical device structure for PIN photo diode
a) Front illuminated device.
A depletion layer of 20 – 50μm is
required to give a quantum efficiency
of 85% for wavelengths between
800 – 900 nm. The typical response
time is <1ns and the dark current
< 1nA
b) Side illuminated device.
Light is injected parallel to the junction
plane. The absorption region is
relatively long (~500μm). The device
is more sensitive (why?) to wavelengths
close to the band gap limit
Hetrojunction Photodiode
Many III-V PIN photodiodes have heterojunction structures.
• p+-AlGaAs/GaAs/n+-AlGaAs, p+-InP/InGaAs/n+-InP, or p+AlGaAs/GaAs/n+-GaAs, p+-InGaAs/InGaAs/n+-InP.
• AlGaAs/GaAs (0.7 – 0.87 µm)
• InGaAs/InP (1300 – 1600 nm).
• A typical InGaAs PIN photodetector operating at 1550 nm has
a quantum efficiency η ≈ 0.75
Hetrojunction Photodiode
Heterojunction structures offer additional flexibility in optimizing the
performance of a photodiode.
In a heterojunction photodiode, the active region normally has a
bandgap that is smaller than one or both of the homogeneous regions.
A wide-bandgap homogeneous region, which can be either the top p+
region or the substrate n region, serves as a window for the optical
signal to enter.
The small bandgap of the active region determines the long wavelength
cutoff of the photoresponse, λth.
The large bandgap of the homogeneous window region sets the shortwavelength cutoff of the photoresponse, λc.
For an optical signal that has a wavelength λs in the range λth > λs > λc,
the quantum efficiency and the responsivity can be optimized.
Quantum Efficiency
Responsivity of diodes
Different absorption
coefficients for different
materials and the
wavelength of light. That
is Responsivity (R) is
wavelength dependant
[ R(l)].
photocurrent/input optical
Noise considerations
1. Electrons in a semiconductor
move randomly with a
certain distribution function.
2. The probability of finding N
electrons crossing an area A
in time interval Dt.
3. The average particle current
is “a” so that the mean value
of the particle number is N =
a Dt. A schematic of the
current flow is shown. The
statistical variations result in
noise in the current.
Noise in Photodiode
Diode Circuits
Equivalent Circuit
A photodiode has an internal resistance Ri and an internal capacitance Ci
across its junction.
The series resistance Rs takes into account both resistance in the
homogeneous regions of the diode and parasitic resistance from the
The external parallel capacitance Cp is the parasitic capacitance from the
contacts and the package.
The series inductance Ls is the parasitic inductance from the wire or
transmission-line connections.
The values of Rs, Cp, and Ls can be minimized with careful design,
processing, and packaging of the device.
Photodiode Arrays
For current / voltage delivery:
Photodiode Arrays
For 2D image capture, pixels from separately addressed diodes:
Charged Coupled Devices (CCDs)
High Sensitivity and Fast Response Optical
Vacuum Photodiodes (Rapid Response):
A photon strikes a photo sensitive material. The energy is
absorbed and an electron is emitted. This electron is moved to a
collection electrode. When absorbed at this electrode a current
flows in the external circuit.
High Sensitivity and Fast Response Optical
Photomultiplier operation
A photon is absorbed by the photosensitive material. An
electron generated is moved towards the first dynode. The
collision produces more electrons.
These electrons are then accelerated towards the next
dynode. Where they produce more electrons. When they
reach the collection electrode they are absorbed and a
current flows in the external circuit.
Avalanche photodiode
1 photon à 1 electron
1 photon à M(>>1) electrons
A powder snow avalanche in the Himalayas near Mount Everest.
Avalanche photodiode
The avalanche photodiode has
a more sophisticated structure
than the PIN device.
The electric fields are much
larger: (~ 3 x 105 V.cm-1).
Most photons are absorbed in
the depletion region. Where
primary charges (hole,
electrons) are produced.
The high electric field causes these charges to move rapidly colliding with
neutral atoms causing the production of a secondary hole/electron pair
(called impact ionisation). These and the original charge carrier are then
accelerated and further ionisation may occur.
This gives the “avalanche” photo diode an internal gain known as the
multiplication factor (M). M = I/Ip (I = total current generated, Ip = initial
primary photo current)
Avalanche photodiode
to impact
from impact
Avalanche photo diodes require
relatively high reverse bias
voltages (100 – 400 V).
Multiplication factors of 104 are
Response times are ~ 100ps.
For high speed operation there is full depletion of the absorption region. This
achieved with an electric field of ~ 104 V.m-1. All the charges drift at the
saturation velocities. The response of the device is then limited by three
i. The transit time of the charges across the depletion width.
ii. The time taken for the charges to perform the avalanche multiplication
iii. The RC time constant of the junction capacitance of the diode and its
external load.
• PN junction
– Material: Only the absorbed light is detected
(reflected or transmitted light will be lost)
– Energy Band Structure of a PN Junction
– Photon absorption
– Un-biased, Forward and reverse biased
– Photovoltaic and photoconductive modes
– Responsivity (wavelength dependent); efficiency;
speed; noises
– Specification data sheet and photodiode circuits
• PN photodiode
Photovoltaic mode:
• Can be Solar cells
• Build in field sweeps electrons & holes apart
Photoconductive mode:
• Reversed biased to increase depletion region & hence field.
• PIN photodiode
Better performance at longer wavelength because of increased
• Avalanche photodiode and photomultiplier tube (vacuum
• Diode arrays and CCD camera
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