Shendi University

Faculty of Engineering & Architecture

Department of Electrical and Electronics
Engineering
Semiconductor Physics and Devices
Lecture 7

Solar Cells and Photodetectors

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 Lecture Outline
❑ Introduction
❑ Solar Cells
❑ Photodetectors
❑ Photodetector Noise
❑ Important Properties of Photodetectors
❑ Applications of Photodetectors
❑ Conclusions

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 Introduction
❑ A photoconductor is a device whose resistance (or conductivity)
changes in the presence of light.
❑ A photovoltaic device produces a current or a voltage at its
output in the presence of light.
❑ Photodiodes which are the most common type of photovoltaic
devices, can be used to realize a photodetector of the
photovoltaic type.
❑ Photodetectors are devices that convert an optical signal into a
signal of another form.
❑ They are respond to the power or intensity rather than field
amplitude, of an optical signal.
❑ There are two classes of photodetectors
✓ Photon Detectors and
✓ Thermal Detectors

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 Solar Cells
❑ cell (photovoltaic devices) is a P-N junction device with no
voltage directly applied across the junction (used with zero bias).
❑ The solar cell converts photon power into electrical power and
delivers this power to a load.
❑ We will initially consider a long diode in which excess carriers
are generated uniformly throughout the semiconductor device.
❑A solar cell has as substantial area
that means high capacitance is
there. It is kind of sensitive and can
produce a greater amount of power
from light.

Solar Cooker

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 Solar Cells
❑ Usually wide gap cells in series with narrow gap cells are
connected for high power.

EC

0.95 eV
1.6 eV

EV

High energy gap Low energy gap

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 Solar Cells

ONE YEAR OF
x 10,000 =
SOLAR ENERGY
NE YEAR OF EARTH’S
PROVIDED BY SUN
FOSSIL ENERGY
CONSUMPTION
SOLAR ENERGY = RENEWABLE RESOURCE
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 Photodetectors
❑ When excess electrons and holes are generated in a
semiconductor, there is an increase in the conductivity of the
material. This change in conductivity is the basis of the
photoconductor, perhaps the simplest type of photodetector. If
electrons and holes are generated within the space charge region
of a P-N junction, then they will be separated by the electric
field and a current will be produced.
❑ The P-N junction is the basis of several photodetector devices
including the photodiode and the phototransistor.
✓ Photodiode is a P-N junction diode operated with an applied reverse-
biased voltage.
✓ Phototransistors are similar to photodiodes, but exploit internal
amplification of the photocurrent. They are less frequently used than
photodiodes.

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 Photodetectors
❑ Photodetectors are devices used for the detection of light – in
most cases of its optical power. More specifically,
photodetectors are usually understood as photon detectors,
which in some way utilize the photo-excitation of electric
carriers; thermal detectors are then not included by the term, and
are also not treated in this article.
Detectors examples
❑ Strip detectors
Scientific applications

❑ Charge Coupled Devices (CCD)

Imaging, scientific and consumer applications

❑ Monolithic Active Pixel Sensors (MAPS)

Imaging, consumer applications
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 Photodetectors
The Detection Chain
Sensing/ Charge transport Signal
E Charge creation and collection
Conversion
processing
Data TX

Si physical Si device Si device topologies properties

properties properties
all the boxes of the detection chain process based upon Silicon
❑ Detection principles: Ionization: by imparting energy to
break a bond, electrons are lifted from VB to CB then made
available to conduction. Well established concept ( ionization
chambers, microstrip, hybrid pixels, CCD, MAPS…)
❑ Photodetectors usually deliver an electronic output signal – for
example, a voltage or electric current which is proportional to the
incident optical power. They are thus belonging to the area
of optoelectronics.
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 Photodetectors
❑ Photoconductive detectors are also based on certain
semiconductors, e.g. cadmium sulfide (CdS). They are cheaper
than photodiodes, but they are fairly slow, are not very
sensitive, and exhibit a nonlinear response. On the other hand,
they can respond to long-wavelength infrared light.
❑ Phototubes are vacuum tubes or gas-filled tubes where
the photoelectric effect is exploited (→ photo emissive-
detectors).
❑ Photomultipliers are a special kind of phototubes, exploiting
electron multiplication processes for obtaining a much
increased responsivity. They can also have a high speed and
large active area. Some of them are based on multichannel
plates; they can be substantially more compact than traditional
photomultipliers.

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 Photodetector Noise
❑ Based on the internal photoelectric effect These are
semiconductor devices in which electron- hole pairs are
generated through absorption of incident photons.
❑ Devices include photoconductors, junction photodiodes,
photovoltaic devices, phototransistors and charge coupled
devices.
❑ Photodetector Noise is a fundamental phenomena in nature.
❑ In a photodetector it sets the fundamental limit on the detectivity
of the detector.
❑ This determines the usefulness of a detector for a particular
application.
❑ There are few different types of noise for a photodetector.
❑ Two types of noise, quantum noise and thermal noise originates
from the basic physical laws of nature.
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 Photodetector Noise
❑ Quantum Noise, described as shot noise in electronics and
photonics, result from the statistical nature of a quantum event
dictated by the uncertainty principle.
❑ Thermal noise is known as Johnson noise or Nyquist noise in
electronics. It is a consequence of thermal fluctuations and
directly associated with thermal radiation (heat).
❑ Noise of such fundamental nature can only be minimized but can
never be completely eliminated.
❑ The noise of a photodetector can come from:
✓ The detector itself
✓ The amplifier used with the detector,
✓ The circuit used to extract the electrical signal from the
detector.

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 Photo Detectors
Important Properties of Photodetectors
❑ Depending on the application, a photodetector has to fulfill various
requirements:
✓ It must be sensitive in a certain spectral region (range of
optical wavelengths). In some cases, the responsivity should be constant
or at least well defined within some wavelength range. It can also be
important to have zero response in some other wavelength range; an
example are solar-blind photodetectors, being sensitive only to short-
wavelength ultraviolet light but not to visible sunlight.
✓ The responsivity tells how much electrical signal is obtained per unit
optical power. It depends on the optical wavelength.
✓ In some cases, not only a high responsivity, but also a high quantum
efficiency is important, as otherwise additional quantum noise is
introduced. This applies e.g. to the detection of squeezed states of light,
and also affects the photon detection probability of photon
counting detectors.

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 Photodetectors
Important Properties of Photodetectors
✓ The detector must be suitable for some range of optical powers. The
maximum detected power can be limited e.g. by damage issues or by a
nonlinear response, whereas the minimum power is normally determined
by noise. The magnitude of the dynamic range (typically specified as the
ratio of maximum and minimum detectable power, e.g. in decibels) is often
most important. Some detectors (e.g. photodiodes) can exhibit high
linearity over a dynamic range of more than 70 dB.
✓ The active area of a detector can be important e.g. when working with
strongly divergent beams from laser diodes. A high enough uniformity of
the responsivity may be important. For light sources with very high and/or
non-constant beam divergence, it is hardly possible to get all the light onto
the active area; an integrating sphere may then be used (with appropriate
calibration) for measuring the total power.

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 Photodetectors
Applications of Photodetectors
❑ Photodetectors have a very wide range of applications. Some examples:
✓ In radiometry and photometry, they can be used for measuring properties
like optical power, luminous flux, optical intensity and irradiance, in
conjunction with additional means also for properties like the radiance.
✓They are used to measure optical powers e.g. in spectrometers, light barriers,
optical data storage devices, autocorrelators, beam profilers, fluorescence
microscopes, interferometers and various types of optical sensors.
✓Particularly sensitive photodetectors are required for laser
rangefinders, LIDAR, quantum optics experiments and night vision devices.
✓Particularly fast photodetectors are used for optical fiber communications,
optical frequency metrology and for the characterization of pulsed
lasers or laser noise.
✓Mostly two-dimensional arrays containing many identical photodetectors are
used as focal plane arrays, mostly for imaging applications. For example,
most cameras contain such devices as image sensors.

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 Conclusions
❑ The field of semiconductor detectors encompasses different
scientific and technology fields.
❑ Some of the issues relevant to radiation detectors.
❑ Development of new detection techniques based on novel
and well established semiconductor material: ( phonon-
based detectors, quantum detectors, compounds, low
dimensional)
❑ Integration with electronics (monolithic solution to achieve
more compactness and reduce cost)
❑ 3D structures
❑ Topologies optimization (power reduction, noise reduction)
❑ Radiation hardness

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Thank You

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