Lecture 5-6
Disclaimer
Objectives:
Meaning of the term “Optoelectronics”
Optoelectronic Devices (Examples)
Optoelectronic Devices (Applications)
Basic Principles of Photo-detector
The study materials/presentations are solely meant for academic purposes and they can be
reused, reproduced, modified, and distributed by others for academic purposes only with
proper acknowledgements.
As informed earlier that this has been prepared Solely for Teaching/Learning purposes and not for
wide circulation in any social media etc. You can share/discuss with Enrolled Students for OE
PHO852) of Optoelectronic Instrumentation classmates only.
What is the meaning of “Optoelectronics” ?
Optoelectronics is a word combining optics and electronics.
In the literature, "optronics" and "electro-optics" are used in the same
meaning.
Optoelectronics is the scientific field that deals with the
operation and application of light controlled electronic
devices and systems.
Optoelectronics is usually considered a sub-field of
photonics.
Photonics is the physical science of :
Photon generation,
Detection,
Transmission and Amplification.
Optoelectronics Covers various APPLICATIONS involving
Light emitting devices and elements,
Information displays,
Image pickup devices,
Optical storages,
Optical communications,
Remote sensing systems etc.
What are different types of Optoelectronic Devices?
Based on Electrical Energy as Input and Light Energy
as Output:
LED
Laser Diode
Based on Light Energy as Input and Electrical
Energy as Output:
Solar Cell
Photodetector
1st Edition
Photonics and Fiber Optics:
Foundations and Applications
Ed. by Tarun Kumar Gangopadhyay,
Pathik Kumbhakar,
Mrinal Kanti Mandal
Copyright 2020
Type of Semiconductors based on band structure:
Direct (Examples, GaAs, ZnS, ZnO, GaP etc.) and
Indirect bandgap (Si, Ge)
Whether a semiconductor has a direct or indirect
bandgap is of fundamental importance to its
optoelectronic applications.
If the conduction band minimum (CBM) occurs at the
same point in k-space as the valence band maximum
(VBM), which is usually at the center (Γ-point) of the
Brillouin zone for conventional semiconductors,
then the energy gap is referred as direct bandgap,
otherwise as indirect bandgap (shown later).
If a semiconductor has a direct bandgap and
the electric dipole transition from VBM to
CBM is allowed, the electron-hole pairs will
recombine radiatively with a high probability.
As a result, high-quality direct bandgap
semiconductors, such as GaAs and InP, are
used to make highly efficient light emitters.
They are essential materials for LASERs, Light
Emitting Diodes (LEDs) and other photonic
devices.
Whereas, in indirect bandgap semiconductors,
such as Si and Ge, optical transitions across an
indirect bandgap are not allowed, and, thus,
these materials are not used as efficient light
emitters, in general.
Direct and Indirect Nature and
Light Emission in Semiconductors
E
conductionelectron
band -
Band h
gap Electron-hole
recombination
+
valence
band hole
Si: Eg = 1.1 eV
GaAs: Eg = 1.4 eV, = 880 nm
AlAs: Eg = 2.2 eV, = 565 nm
Si: indirect bandgap, ineffective
GaAs: direct bandgap, effective
Electron-Hole
Recombination
and photon Energy
If the electron-hole
recombination occurs
not at the minimum gap
point, the emitted photon h > Eg
can have a higher energy.
When the recombination
involves an impurity state
in the gap, lower-energy
photon is generated.
LIGHT ABSORPTION
Attenuation:
A media in which light is propagating may be lossy. For a
given wavelength, the irradiance will decrease at a rate
proportional to the irradiance magnitude.
The resulting loss may be represented by an attenuation
constant αL in units of inverse meter (m-1 ) or in cm-1.
The attenuation constant is positive for a lossy media, zero for
a lossless media, and negative for a media with gain.
For one-dimension, the defining differential equation for the
irradiance (irradiance I with units of W/m2) amplitude is
dI/dx = - αL I.
The solution is :
I = Io exp(- αL x)
Where, Io is the value of I at the position (x = 0).
