LED – Band gap Module
Background
 Attended a TI uController development event where they
demonstrated a uC using an LED as a light detector to
control its own brightness.
 Use this frequently in electronics class
 One day a student wanted to really get some signal so they
aimed a red laser pointer into a blue led – no signal
 I used this as an opportunity to explain how the photons
had to have and energy > band gap to excite a voltage.
 Seemed like a way to demonstrate some quantum
phenomena tied to everyday objects.
Interesting Property of LEDs
LED
 Usually LEDs are thought of as
light sources
 LEDs are also light detectors
 We use this property to
investigate the band gap
structure of LEDs
 Use inexpensive LEDs and
digital meter
 More advanced version have
been developed
 http://laser.physics.sunysb.ed
u/~tanya/report2/
Meter
Objectives
Primary
 Explain how quantum behavior of light and matter
interact
 Band gaps
 Photons
 Identify LEDs as an example of nano-scale structures
 Three challenges in designing LEDs
 Efficiently producing light Thin film (nano-scale)
 Efficiently extracting light deposition techniques
Selection of semiconductor
 Wide range of colors
materials/dopants
Objectives
Secondary
 Instrumentation
Technology
 Moving from manual to
automated
instrumentation
 “Beyond Moore” –
semiconductor devices
integrating more and
more functionality
 Design and fabrication
 Basic soldering skills
 Board layout and design
project
Overview of Lab
 Use LEDs in pairs, one an illuminating LED, one a detecting LED.
Fill in a table, also use an intense red source (laser pointer) as an
illuminator:
Overview of Lab
 Use LEDs in pairs, one an illuminating LED, one a detecting LED.
Fill in a table, also use an intense red source (laser pointer) as an
Detector
illuminator:
Energy
Color
Illuminator
Energy
IR
RED
YELLOW
GREEN
Blue
Color
IR
RED
YELLOW
GREEN
Blue
Laser Ptr
Observations
(voltage difference
with/without illuminator)
Overview of Lab
 Use LEDs in pairs, one an illuminating LED, one a detecting LED.
Fill in a table, also use an intense red source (laser pointer) as an
Detector
illuminator:
Energy
Color
Illuminator
Energy
IR
RED
YELLOW
GREEN
Blue
Color
IR
RED
YELLOW
GREEN
Blue
Laser Ptr
Observations
(voltage difference
with/without illuminator)
Review of Prior Knowledge
 Photons –
wavelength,
color and energy
Color
Wavelength
Photon Energy
E=hc/λ
3.1 eV
2.5eV
2.1eV
1.8eV
Review of Prior Knowledge
 Single energy well
(single atom) –
discrete energy
levels
 Several energy
wells (molecule) –
Each atomic levels
splits to closely
spaced levels
separated by gaps
 Infinite array
(crystal) – levels
merge into bands
separated by gaps
.
.
.
http://www.falstad.com/qm1d/
Prior Knowledge
 Number of good sources
for review of
semiconductor diodes
and related topics:
 nanoHUB.org
http://nanohub.org/
 MRSEC http://mrsec.org/
 Excellent YouTube
overview
 Video at
http://powerofthesun.ucs
b.edu/
Working With LEDs
 LEDs as diodes – (Long lead = +)
 Show need for current limiting resistor (sacrifice an
LED)
 Options for using LEDs
 Simply twist leads of LED and resistor
 Solder LED and resistor
Simple soldering skills project
 Design & build a small board
Working With LEDs
 Use LEDs with clear
plastic cases
 Color cases add a
confounding filtering
effect
√
X
Lab – Color, Wavelength, Energy
~470Ω
V
Illumination
Detection
Use of a section
Of drinking straw or other tubing helps
direct the light and exclude ambient light
Results
Detector
Energy
Color
Illuminator
Energy
IR
RED
YELLOW
GREEN
BLUE
IR
.3V
0
0
0
0
RED
.2V
.4V
o
0
0
YELLOW
.2V
.2V
.2V
.1V???*
0
GREEN
.4V
.3V
.2V
.4V
0
BLUE
.2V
.5V
.3V
.3V
.6V
Laser Ptr
.8V
.6V
0
0
0
Color
*Green and Yellow LED emission spectra overlap
Explanation
Egap
Photon
 For an LED with a given band gap energy, illuminating
photon has to be greater than this energy to excite a
signal.
 Although green light is higher energy than yellow, we
see a yellow LED does excite some signal in the green
LED – LED colors are not sharp lines but bands, they
yellow and green spectrum overlap.
Extensions
 Automation
 Use uController with ADC to automate measurements
 For students in courses where they are already familiar
with uC
 Design and fabrication
 LED boards are available for http://ice.chem.wisc.edu/
but students can build their own
 Simple soldering project
 Design a simple board with software such as EagleSoft
(free education version)
Extensions
 Male vs. Female color perception - who can match colors
to a color chart best
 White and pink LEDs – view spectrum in a diffraction
grating (or use a CD/DVD) and explain the use of
phosphors.
 Economic/Environmental impact of LED lighting
 Candle vs. incandescent vs. CFL vs. LED
 Mercury emissions
 Coal fired electricity- Incandescent
 CFL – trace mercury