MEMO
To: David Thacker
From: Jake N. Querubin
Date: February 19, 2014
Subject: Writing to Define and Writing to Describe Light Emitting Diode (LED)
Organization:
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Department of Electrical Engineering Undergrads
Group Ethos (credibility) – sophomores in the electrical engineering department know very few
about circuits, schematics, energy, and digital logics.
This definition and description touches the surface of courses they will be taking in the future;
also it gives a small insight of what they will be dealing with and at the same time gaining
knowledge of about the technology on the rise.
Term or Phrase:
Light Emitting Diodes
Audience:
LEDs and semiconductors are not introduced until undergrad students majoring in computer
science and electrical engineering take the course Digital Logics (ECE 240). The level of education for this
term is specialized towards freshmen’s that are entering their sophomore year.
Problem or Need:
Sophomores taking Digital Logics needs to be familiar how a diode and semiconductors operate.
Furthermore general knowledge of LEDs gives the audience a better scope of what is coming in the
future classes.
Placement:
This type of document describing LEDs would be found in a website. For the ambitious
sophomores they can look ahead into their future courses about micro controllers and lighting designs.
At an electrical engineering course website is where they will likely encounter a similar document
containing a simple definition of an LED. If sophomores desire more information from the definition, the
description would follow along the page or another link would be provided.
Visuals:
I chose the visuals shown below so my non-specialized audience can get a better grasp of the
description presented. Especially for sophomore’s, some of the terminology used might be there first
time hearing about a semiconductor or a diode, let alone how it operates. I used these visuals so they
can see what I am trying to describe, and enhance the definition.
Rhetorical Moves:
Querubin 1
The rhetorical situation for this document is freshmen’s entering their sophomore year. At this
point in their major they would only know limited engineering terminology. I took the approach of
breaking it down to using headings and bullet points to simply describe the many characteristics of LEDs.
Using bullet points and headings emphasizes on what I think is important for them to know about LEDs.
Querubin 2
Technical Definition
Light-Emitting Diodes (LEDs) are semiconductors (meaning they are able to conduct electricity) that
converts electrical energy to visible light. LEDs come in dozens of different shapes and sizes. The most
common are the through hole LEDs with two wires; one wire is the anode (positive) and the other is
cathode (negative). The positive and negative wires attached have a current flowing only in one
direction known as a “diode”. LEDs are unique because it uses solid-state lighting. Unlike incandescent
or fluorescent lamps, which create light with filaments and gases encased in a glass bulb, solid-state
lighting consists of semiconductors that convert electricity into light.
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Technical Description
Semiconductor:
How does LEDs Work?
LED’s are solid-state semiconductors
that convert electrical energy to visible light.
The semiconductors are made up of two layers:
an n-type (anode) and a p-type (cathode).
When the layers combine, they form the die or
chip of the LED. The anode is made up of
electrons that flow to the holes of the cathode
layer. When the n-type and p-type layers are
joined, light, or photons are created.
Basic Characteristics of an LED
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Materials: Colors of Light
LEDs are assembled with nitrides and phosphides of gallium, indium, and aluminum, each
producing different colors of light. The color of light emitted from the LED depends on the
material of the chip. AlGalnp (aluminum gallium-indium phosphide) is used to create red,
orange, yellow, yellow green light. InGaN (indium-gallium nitride) is used to create green, blue
green, and blue light.
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Energy output
LEDs use low voltage devices, making them much safer than sources requiring high voltages. The
visible light emitted from LEDs also does not generate significant amounts of ultra-violet or infra-red
technology. However, LEDs tend to get hot inside and the heat must be conducted away from the die so
the LEDs do not fail.
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Life Span
An incandescent lamp life averages around 1,000 hours and compact fluorescent lamp life averages
around 10,000 hours. LEDs have 50,000 hours lamp life on average; newer models of LEDs have 100,000
hours. However, light energy emitted from the LED will decrease over time, depending on the color and
materials used in the production of the LED.
Sizes of different LEDs
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Size
LEDs vary drastically in size, with
some as small as tenths of millimeters.
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Temperature
Heat generated by an LED source is a real enemy to the quality of that LED source. However, LEDs
are cool to touch whereas halogen lamps run around 600-700 degrees Fahrenheit and incandescent
lamp run around 375-400 degrees Fahrenheit.
Advantages
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Energy Efficient
Solid-State
Environmental Friendly
Instant Start-up
Long Life
Range of Colors
No UV emissions
Low Voltage
Disadvantages
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Initial Cost
Temperature Dependence
Voltage Sensitivity
Current Sensitivity
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References
Information was summarized from the following websites:
http://blog.batteriesplus.com/2013/learning-leds
http://science.howstuffworks.com/environmental/green-tech/sustainable/led-light-bulb1.htm
http://learn.adafruit.com/all-about-leds/what-is-an-led
http://www.jimonlight.com/2013/04/26/friday-facts-20-really-awesome-facts-about-leds-light-emittingdiodes-everyone-should-know/
http://led.linear1.org/why-are-leds-different-colors/
Images were borrowed from the following websites:
https://www.google.com/search?q=LED&safe=off&source=lnms&tbm=isch&sa=X&ei=LjoGU9
jaJYSdyQGq3oEo&ved=0CAcQ_AUoAQ&biw=640&bih=679
http://en.wikipedia.org/wiki/Light-emitting_diode
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