Electronic Devices
BITS Pilani Dr. Navneet Gupta
Pilani Campus Department of Electrical and Electronics Engineering
BITS Pilani
Pilani Campus
Course No. EEE F214/INSTR F214/ECE F214
Module-10
Special Purpose Diodes
Disclaimer
The content used in this presentation is collected from
various standard textbooks. The copyright is held with
respective publishers/authors. The content is used for
academic purpose only and to disseminate the
knowledge in the field of electronic devices.
Navneet Gupta
BITS Pilani, Pilani Campus
Optoelectronic Devices Lecture 36
Basic particle of light i.e Photon-plays a major role. Photons Energy:
Optical sources, detectors & data transmission E ph (eV )
1.24
( m)
Optoelectronic Devices
1. Photodiodes/Photodetectors 3. Light Emitting Diodes
2. Solar Cells 4. Laser Diodes
optical electrical electrical optical
E ph Eg not absorbed (semiconductor is transparent).
E ph Eg are absorbed (semiconductor is opaque).
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Prof. Navneet Gupta BITS Pilani, Pilani Campus
Photon Absorption
Photons may travel a considerable distance in the semiconductor before being
absorbed. The light intensity decreases exponentially with the distance of travel x
I ( x) I 0e x
absorption coefficient
1
penetration depth
I ( x)
gop optical generation rate
h
If α is very large then most of the absorption will be close to the surface.
if α is very small, then most of the light will pass though without absorption.
Prof. Navneet Gupta BITS Pilani, Pilani Campus
Photodiodes E ph Eg
Iop
In n-region: pop gop p
Prof. Navneet Gupta BITS Pilani, Pilani Campus
Optically generated current: I op qAgop ( Lp Ln W )
Dp D
Thermally generated current: I th qA pn np
n
L L
p n
qV
Total reverse current with illumination: I I th
e kT
1 I op
When device is short circuited (V=0)
When device is open circuited (I=0) and V=Voc
For symmetrical junction:
Discuss the effect of
pn = np and τp = τn Doping
Temperature
pn/τn = gth Band Gap
If we neglect the generation within W and gop >> gth; then ??
7
Power is delivered from the Power is delivered to the
external circuit external circuit
A photodiode may be used to detect optical signals as we observe the change
in current with change in light intensity when a reverse bias voltage is applied.
Why are PDs used in reversed biased?
Performance Parameters: Quantum efficiency and Responsivity
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Prof. Navneet Gupta
Solar cell: (unbiased PDs)
Structure
A solar cell’s structure is identical to a PN junction diode but
with finger-shaped or transparent electrodes so that light can
strike the semiconductor.
Prof. Navneet Gupta BITS Pilani, Pilani Campus
…Solar cell
Solar power in India is a fast
developing industry. The
In 2021 country's solar installed
capacity was 61.625 GWAC as
of 31 October 2022. Solar
power generation in India
ranks fourth globally in 2021
High quality mono-crystalline solar panels Solar panel array as the roof on a house
Prof. Navneet Gupta BITS Pilani, Pilani Campus
Solar Cell:
Principle of Operation
The amount of absorption reduces
with depth and hence the depletion
region must be close to the surface
to maximize absorption. This is
achieved by having a thin n region.
Adapted from Principles of Electronic Materials - S.O. Kasap
Prof. Navneet Gupta BITS Pilani, Pilani Campus
Solar cell
Equivalent circuit
Iop
Ith q
The total diode (solar cell) current is the sum of the current generated by the
voltage and that generated by light.
qV
I I th e 1 I op
kT
Output Power = P = IV
BITS Pilani, Pilani Campus
Solar Cell:
IV Characteristics
Solar cell operates in the fourth quadrant of the IV plot. Since I and V have opposite signs,
solar cell generates power.
I mVm
Pm I mVm
P I V
m m m FF
Pin Pin I scVoc
BITS Pilani, Pilani Campus
Light Emitting Diodes Lecture 37
LED is essentially a pn junction diode typically made from a direct bandgap
semiconductor.
The electrons and holes recombine by emitting photons (light) with hv ≈ Eg.
By adjusting the composition of the semiconductor, Eg can be altered to
make blue, green, yellow, red, infrared, and UV LEDs possible. They are
made of compound semiconductors involving In, Ga, Al, As, P, and N.
Prof. Navneet Gupta BITS Pilani, Pilani Campus
Light Emitting Diodes
Light emitting diodes (LEDs)
LEDs are made of compound semiconductors such as InP and GaN.
Light is emitted when electron and hole undergo radiative recombination.
Ec
Radiative Non-radiative
recombination recombination
through traps
Ev
Prof. Navneet Gupta BITS Pilani, Pilani Campus
Light Emitting Diodes
Performance Parameters
• Frequency (colour) of the photons: Eg (eV )
1.24
( m)
• External quantum efficiency: no. of photons escaped
ext.
no. of injected charge carriers
• Internal efficiency: no. of photons radiated
int
no. of injected charge carriers
no. of photons escaped
• Extraction efficiency: extraction
no. of photons radiated
• ηext = ηint x ηextraction
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Prof. Navneet Gupta BITS Pilani, Pilani Campus
Loss Mechanisms
Absorption within the LED material
Fresnel loss: Reflection loss when light passes from
semiconductor to air due to difference in R.I.
Critical angle loss: TIR at angles > critical angle (θc)
Hence , typically LEDs are made with a dome-type encapsulation,
which acts as lens so that more of the photons can be extracted.
Prof. Navneet Gupta BITS Pilani, Pilani Campus
LED Materials
Compound semiconductors
Lattice
Eg(eV)
E (eV ) Wavelength
(μm)
Color constant
(Å)
binary semiconductors:
InAs 0.36 3.44 6.05 - Ex: GaAs, efficient emitter
InN 0.65 1.91 infrared 3.45
ternary semiconductor :
InP 1.36 0.92 5.87 - Ex: GaAs1-xPx , tunable Eg (to vary
the color)
GaAs 1.42 0.87 Red
red 5.66
yellow
Yellow
GaP 2.26 0.55 blue
Green 5.46 quaternary semiconductors:
violet
Blue - Ex: AlInGaP , tunable Eg and lattice
AlP 3.39 0.51 5.45 constant (for growing high quality
GaN 2.45 0.37 3.19 epitaxial films on inexpensive
substrates)
AlN 6.20 0.20 UV 3.11
Light-emitting diode materials
1.24 1.24
LED wavelengt h ( m)
photon energy Eg (eV )
Prof. Navneet Gupta
LED Material
Eg AB1 x Cx Eg ( AB) Eg ( AC ) Eg ( AB) x
Eg (GaAs1-xPx)
Direct Band gap:
GaAs = 1.43 eV
GaP = 2.75 eV
Indirect band gap:
GaAs = 1.86 eV
GaP = 2.26 eV
Prof. Navneet Gupta BITS Pilani, Pilani Campus
New Technology
OLED
OLED’ stands for Organic Light-Emitting Diode - a technology that
uses LEDs in which the light is produced by organic molecules.
Flexible OLED
BITS Pilani, Pilani Campus
Advance ED Course
EEE F419: Flexible and Stretchable Electronics
Course Description:
Introduction to flexible and stretchable electronics (FSE), material systems
and scaling issues; materials and substrates for flexible and printed
electronics, Material Considerations and various properties, techniques
for fabrication and characterization of FSE devices, mechanics of thin-films
and flexible devices, various flexible and stretchable devices: solar cells,
displays, thin-film transistors, sensors, artificial skin and actuators;
human-machine interfaces, wearable electronics for emerging
applications.
BITS Pilani, Pilani Campus
