sx05_TE_(nc8-20)c04N.fm Page 810 Thursday, June 2, 2005 8:13 PM
Section
1
1
Electronic
Signals and
Semiconductors
Electronic Signals and
Semiconductors
Objectives
Reading Preview
After this lesson, students will be able to
N.4.1.1 Describe two types of electronic
signals.
N.4.1.2 Explain how semiconductors are
used to make electronic components.
Key Concepts
Target Reading Skill
• electronics • electronic signal
• analog signal • digital signal
• semiconductor • diode
• transistor • integrated circuit
• What are two types of electronic
signals?
• How are semiconductors used to
make electronic components?
Key Terms
Asking Questions Explain that changing
a head into a question helps students
anticipate the ideas, facts, and events they
are about to read.
Answers
Sample questions and answers:
What are analog and digital signals?
(Analog signals are currents that are varied
smoothly to represent information, while
digital signals are pulses of current used to
represent information.) What are
semiconductor devices? (Semiconductor
devices are electronic devices that use
semiconductors to vary the current in
a circuit.)
Target Reading Skill
Asking Questions Before you
read, preview the red headings. In
a graphic organizer like the one
below, ask a what question for
each heading. As you read, write
the answers to your questions.
Electronic Signals and Devices
Question
Answer
What are analog
and digital
signals?
Analog signals
are . . .
Teaching Resources
• Transparency N43
Preteach
L2
Ask: What are appliances or devices at
home that use electrical energy? (Sample
answer: Refrigerator, lamps, microwave oven)
What devices use electrical energy and also
give you information? (Sample answer:
Clock, telephone, radio, television, answering
machine) Explain that in this section,
students will learn what distinguishes
electrical devices from electronic devices.
1. Write a short sentence on a sheet of paper.
2. Morse code is a language that uses dots and dashes to convey
information. Convert your sentence to dots and dashes using
the International Morse Code chart at the right.
3. Turn a flashlight on and off quickly
to represent dots. Leave the
flashlight on a little longer to
represent dashes. Practice using
the flashlight for different letters.
4. Use the flashlight to transmit your
sentence to a partner. Ask your
partner to translate your message
and write down your sentence.
Think It Over
Inferring Were you able to transmit
information using light? How does
your light message differ from the
same message read aloud?
Every day, you use devices that run on electric current. But not
all these devices are the same. Light bulbs and toasters are
examples of electrical devices. An electrical device relies on a
continuous supply of electric current.
When you watch television or talk on a cell phone, you are
using electronic devices. The difference between electronic and
electrical devices is in the way that they use electric current.
Electronics is the use of electricity to control,
communicate, and process information. How do electronic devices work? Electronics is based on electronic
signals. Any information that can be measured or
numbered, whether it is electrical or not, can be converted to a signal. An electronic signal is a varying
electric current that represents information.
Build Background
Knowledge
Can You Send Information With a Flashlight?
810 ◆
L2
Skills Focus Inferring
Materials flashlight
Time 15 minutes
Tips To save time, have students transmit
a single word rather than an entire
sentence.
810
Cameras can use electronic signals
to take photographs.
Expected Outcome Most students
should be able to transmit a message using
Morse code.
Think It Over Sample answer: The
message was transmitted. A message read
aloud would be composed of sounds. The
light message uses patterns of flashing
light to transmit information.
sx05_TE_(nc8-20)c04N.fm Page 811 Thursday, June 2, 2005 8:13 PM
Instruct
Digital Signal
Temperature ( C )
Temperature ( C )
Analog Signal
20
15
10
5
0
6
a.m.
8
a.m.
10
a.m.
12
p.m.
2
4
p.m. p.m.
Analog and Digital
Signals
20
15
10
Teach Key Concepts
5
0
6
a.m.
Time
8
a.m.
10
a.m.
12
p.m.
2
p.m.
4
p.m.
Time
Analog and Digital Signals
There are two basic kinds of electronic signals: analog signals
and digital signals. The two types of signals represent information in different ways.
Analog Signals In analog signals, a current is varied
smoothly to represent information. An analog signal varies in
much the same way that temperature varies in a liquid-filled
thermometer. This kind of thermometer shows temperature as
the height of a liquid in a tube. The height of the liquid rises
and falls smoothly with the temperature. The “analog signal”
from the liquid-filled thermometer can be represented by a line
graph like the one in Figure 1.
Digital Signals In digital signals, pulses of current are used
to represent information. Rather than varying smoothly to
represent information, a digital signal carries information in
pulses, or steps. If you did the Discover activity, you used
pulses of light to represent letters.
A digital signal varies much the same way the numbers on a
digital thermometer vary. You have probably seen a digital
thermometer in front of a bank. The number on the thermometer is constant for a while and then changes suddenly by a
whole degree. Of course, the temperature doesn’t really change
so suddenly. But the thermometer can only show the temperature to the nearest degree, and so the temperature seems to
jump. The digital signal from a digital thermometer can be
represented by a bar graph, as shown in Figure 1.
How is the changing temperature on a liquid-filled
thermometer like an analog signal?
FIGURE 1
Analog and Digital
An analog signal varies smoothly.
A digital signal varies in steps.
Predicting How would the bar
graph be different if it showed
temperature measurements made
every minute?
L2
Smooth Variation vs. Pulses
Focus Tell students that two types of signals
can represent information in different ways.
Teach Ask: What are the two kinds of
electronic signals? (Analog and digital)
What do digital signals use to represent
information? (Pulses) Point out the
traditional thermometer in Figure 1. Ask:
How is the way this thermometer shows
temperature like an analog signal? (The
height of the liquid rises and falls smoothly,
just as in an analog signal a current is varied
smoothly to represent information.)
Apply Have students study Figure 1. Ask:
Why would a line graph representing the
readings of a digital thermometer be
misleading? (Digital thermometer readings
change suddenly by whole degrees, with
nothing in between. A line graph implies that
temperature readings change smoothly.)
learning modality: visual
Teaching Resources
• Transparency N44
For: Links on electronic
signals
Visit: www.SciLinks.org
Web Code: scn-1441
Download a worksheet that will guide students’ review
of Internet sources on electronic signals.
For: Links on electronic signals
Visit: www.SciLinks.org
Web Code: scn-1441
Chapter 20 ◆ 811
Independent Practice
L2
Teaching Resources
• Guided Reading and Study Worksheet:
Electronic Signals and Semiconductors
Student Edition on Audio CD
Differentiated Instruction
Monitor Progress
L3
Gifted and Talented
Ask
students
to
Testing Thermometers
investigate whether analog and digital
thermometers give the same readings. Each
student should design an experiment
comparing the two. A typical experiment
would involve reading thermometers in the
same place at regular intervals. learning
L1
Special Needs
Take
apart
an
Diodes and Transistors
old, nonworking electronic device such as a
transistor radio. Show students the diodes
and transistors that once made the radio
work. As students examine these
components, have a volunteer read the text
descriptions aloud. learning modality:
modality: logical/mathematical
visual
L2
Writing Ask students to explain the
difference between analog and digital signals.
Answers
Figure 1 Sample answer: The bar graph
would have 60 bars for each hour instead of
1 bar per hour.
The height of the liquid rises
and falls smoothly, just like
an analog signal varies smoothly.
811
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Semiconductor Devices
Teach Key Concepts
L2
Conducting Under Certain Conditions
Focus Tell students that an electronic device
must be able to vary the current through a
circuit to be able to transmit an electronic
signal.
Teach Ask: How is a semiconductor
different from a conductor? (Current
through a conductor is continuous, but a
semiconductor conducts current only under
certain conditions.) What is a diode? (An
electronic component that consists of an n-type
and a p-type semiconductor joined together)
Have students examine the images of a diode
and a transistor in Figure 3 to differentiate
between the two components. Ask: What
can a transistor do that a diode cannot?
(Amplify an electronic signal)
Apply Ask: What component contains
both diodes and transistors? (An integrated
circuit) How is a chip different than an
integrated circuit? (They aren’t different. An
integrated circuit and a chip are two names for
the same thing.) learning modality: verbal
Teaching Resources
• Transparency N45
L1
Modeling a Semiconductor
Materials flour, sieve or colander, paper
towel, mixing bowel, scissors
Time 10 minutes
Focus Tell students that a model can
demonstrate the function of a
semiconductor.
Teach Fill a sieve lined with a paper towel
with flour and hold it over the mixing bowl.
Ask: Does this container “conduct” flour
well? (No. The flour is contained inside the
sieve.) Using scissors, cut a hole in the center
of the paper towel, and then repeat the
procedure. Students will observe some of the
flour passes through the sieve.
Apply Ask: How is this sieve like a
semiconductor? (A semiconductor conducts
electricity only under certain conditions. The
sieve passes flour through it only under certain
conditions.) learning modality: visual
812
FIGURE 2
Semiconductors
The electrical resistance of pure
silicon is reduced by adding
atoms of other elements to it.
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e e
e e
e e
Electron
e
e
e
e
e e
Hole
e e
e
e
e
e
e
e e
e
e
Extra
electron
e e
N-type Semiconductor
Adding an element with an
extra electron to silicon
creates a n-type semiconductor.
e
e
e
e
e
e
e
e
e e
e e
e
e
e
e
h
e
e e
e e
P-type Semiconductor
Adding an element with fewer
electrons, or holes, creates a
p-type semiconductor.
Semiconductor Devices
How can an electronic device transmit electronic signals? To
transmit an electronic signal, an electronic device must be able
to vary the current through a circuit. To vary current, electronic devices use semiconductors. A semiconductor is a
material that conducts current better than insulators but not as
well as conductors. A semiconductor conducts current only
under certain conditions.
Communicating
How do you make someone
understand how tiny a chip is
or how fast an electronic
signal travels? An analogy
can help communicate what
a measurement means. An
analogy uses a similarity
between two things that are
otherwise unlike each other.
For example, “a chip is as
small as a baby’s fingernail”
is an analogy. So is “an
electronic signal moves as
fast as a bolt of lightning.”
Write your own analogies to
describe how many diodes
there are in one integrated
circuit chip.
How Semiconductors Work How can a material conduct
current only under certain conditions? Silicon and other semiconductors are elements that have extremely high resistance
in their pure forms. However, if atoms of other elements are
added to semiconductors, the resulting material can conduct
current much more easily.
By controlling the number and type of atoms added, scientists produce two types of semiconductors. In Figure 2, you
can see that adding atoms with extra electrons to silicon produces an n-type semiconductor. “N,” for “negative,” indicates
that the material can release, or give off, electrons. Look again
at Figure 2. Notice that adding atoms with fewer electrons, or
holes, to silicon produces a p-type semiconductor. “P,” for
“positive,” indicates that the material has room for and can
receive an electron.
Scientists combine n-type and p-type semiconductors in
layers. This layered structure allows for the delicate control of
current needed for many electronic devices. The two types of
semiconductors can be combined in different ways to make
diodes, transistors, and integrated circuits. These components control current in electronic devices.
812 ◆
L2
Skills Focus Communicating
Materials dictionary
Time 15 minutes
Tips You may wish to pair students still
mastering English with native speakers for
this activity. Advise students to brainstorm
a list of possible analogies and write a
sentence for each.
Expected Outcome Students’ analogies
should rely on the idea that a chip may
contain hundreds of thousands of
components. Sample analogy: There are
more diodes in one integrated circuit chip
than there are pennies in a million dollars.
Extend Have students choose the best
analogies and make a poster about chips
with written analogies and cartoon
illustrations. learning modality: verbal
sx05_TE_(nc8-20)c04N.fm Page 813 Thursday, June 2, 2005 8:13 PM
Diodes An electronic component that consists of an n-
type and a p-type semiconductor joined together is a
diode. A diode, shown in Figure 3, allows current in one
direction only. If you connect a diode in a circuit in one
direction, there will be a current. But if you turn the
diode around, there will not be a current. Diodes can be
used to change an alternating current to a direct current.
Diodes can also be used as a switch.
Transistors When a layer of one type of semiconductor
is sandwiched between two layers of the other type of
semiconductor, a transistor is formed. Figure 3 shows the
structure of a transistor. A transistor has two uses: it
either amplifies an electronic signal or switches current
on and off.
When electronic signals travel great distances, they
gradually grow weak. When they are received, signals
must be amplified, or made stronger, so that they can be
used. Transistors revolutionized the electronics industry
by making amplifiers much cheaper and more reliable.
When a transistor acts as a switch, it either allows a
current or cuts it off. Millions of transistors that act as
switches are what make computers work.
n
FIGURE 3
Diodes and Transistors
Diodes (top) allow current in only one
direction. Transistors (bottom) can
amplify electronic signals or act as
switches. Comparing and Contrasting
How are diodes and transistors similar?
How are they different?
Diode A diode is a combination of
an n-type and a p-type semiconductor.
Transistor A transistor is a combination
of three layers of semiconductors.
Address Misconceptions
L1
Transistors Are Not Radios
Focus Some students may have heard the
term transistor radio and believe that a
transistor is a device that allows a radio to be
small or that the radio itself is a transistor.
Teach Explain that a transistor is a
component of a radio that amplifies the
signal. The use of transistors did allow radios
to become much smaller than they once
were, but other components—including
small batteries—also allowed radios to
become smaller.
Apply Ask: What electronic devices are
transistors found in other than in radios?
(Millions of transistors that act as switches are
what make computers work.) learning
modality: verbal
p
FIGURE 4
Combining Electronic Components
Diodes and transistors can be combined
to carry out specific tasks within
electronic devices. The singing fish uses
n
p to move
electronics
and make sounds
when a person walks by.
p n
Circuit
board
Transistor
Transistor
Transistors amplify the signals,
causing the fish to move and
to give off sound.
Diode
A photodiode acts as a
switch to turn on the fish.
Chapter 20 ◆ 813
Monitor Progress
L2
Oral Presentation Call on students to
explain what a semiconductor is, what diodes
are, and what transistors are.
Answer
Figure 3 Both control current, and both
can be used as a switch. Diodes can change
an alternating current to a direct current.
Transistors can amplify electronic signals.
813
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Monitor Progress
L2
Answer
A chip is another name for
an integrated circuit, which
is a thin slice of semiconductor that contains
many diodes, transistors, and other
electronic components.
Assess
Reviewing Key Concepts
1. a. Analog signals and digital signals
b. Both represent information. Analog
signals use smoothly varying current, while
digital signals rely on pulses of current.
c. The swinging pendulum represents an
analog signal. The pendulum moves
continuously and smoothly, much like an
analog signal.
2. a. Semiconductors control the current
in electronic devices. b. A transistor consists
of a layer of either ntype or p-type semiconductor sandwiched
between two layers of the other type of
semiconductor. c. Transistors could amplify
the signals that represent sounds and switch
the current that produces sounds on and off.
Reteach
L1
Help students make a concept map that
shows how the section’s key terms are
related.
Performance Assessment
L2
Writing Ask students to explain in a
paragraph why semiconductors are used in
electronic devices.
Teaching Resources
• Section Summary: Electronic Signals and
Semiconductors
• Review and Reinforce: Electronic Signals
and Semiconductors
• Enrich: Electronic Signals and
Semiconductors
FIGURE 5
Integrated Circuits
An integrated circuit chip is
smaller than an ant. Yet the
integrated circuit contains
hundreds of thousands of
diodes and transistors.
Integrated Circuits Individual electronic components can
be combined into larger groups, called integrated circuits, to
increase their usefulness. An integrated circuit is a thin slice of
semiconductor that contains many diodes, transistors, and
other electronic components. Integrated circuits are also called
chips. Figure 5 shows a magnified view of a chip from a computer. A chip smaller than one millimeter on each side can contain hundreds of thousands of components. Electronic signals
flow through integrated circuits at tremendous speeds because
the various components are so close together. On some chips,
the space between two components can be one hundredth as
thick as a human hair. The high-speed signals of integrated circuits make possible devices from video games to spacecraft.
The small size of integrated circuits has allowed the size of electronic devices such as computers to be greatly reduced.
What is a chip?
1
Section 1 Assessment
Target Reading Skill Asking Questions Use
the answer to the questions you wrote about the
section headings to help you answer the questions
below.
2. a. Reviewing How are semiconductors used in
Reviewing Key Concepts
1. a. Listing What are the two basic kinds of
electronic signals?
b. Comparing and Contrasting How are the
two types of electronic signals similar? How
are they different?
c. Classifying A grandfather clock uses a
pendulum that continuously swings to
control the clock’s hands. What type of
signal does the swinging pendulum
represent? Explain.
changes electronic signals into sounds.
Why would transistors be useful parts of
a loudspeaker?
Directions Review the Morse code at the
beginning of the section. Write directions a
friend could use to send you messages using
light or sound.
814 ◆
Keep Students on Track By this point,
groups should have brainstormed a list of
computer applications and chosen one to
research. Encourage students to find out how
the application was developed, how it works,
and who uses it. Make sure students consider
the inputs into the application and the
outputs that result. Provide library books on
computer applications to help them begin.
814
electronic devices?
b. Explaining What is a transistor?
c. Relating Cause and Effect A loudspeaker
Writing Mode Exposition
Scoring Rubric
4 Exceeds criteria; includes clear and
complete directions
3 Meets criteria
2 Meets some criteria; includes confusing
directions and/or lacks critical steps
1 Includes inaccurate and/or very
incomplete directions
sx05_TE_(nc8-20)c04N.fm Page 815 Thursday, June 2, 2005 8:13 PM
Design a Battery Sensor
Design a Battery
Sensor
Problem
Prepare for Inquiry
Key Concept
A light-emitting diode (LED) emits light only
when current is traveling in one direction.
How can an LED be used to tell if a battery is
installed correctly?
Skills Focus
evaluating the design, redesigning, observing,
drawing conclusions
Materials
• 2 D cells
• LED
• bicolor LED (optional)
• flashlight using 2 D-cells
• flashlight bulb and socket
• two insulated wires with alligator clips
Procedure
PART 1
LED Properties
1. Attach one wire to each terminal of the LED.
2. Tape the two cells together, positive terminal
to negative terminal, to make a 3-volt
battery.
3. Attach the other ends of the wires to the terminals of the battery and observe the LED.
4. Switch the wires connected to the battery
terminals and observe the LED again.
5. Repeat Steps 1–4, but substitute a flashlight
bulb in its socket for the LED.
PART 2
Sensor Design
6. Many electrical devices that run on batteries
will not run if the batteries are installed
backwards (positive where negative should
be). Design a device that uses an LED to indicate if batteries are installed backwards.
7. Draw your design. Show how the LED, the
device, and the battery are connected. (Hint:
The LED can be connected either in series or
in parallel with the battery and the device.)
8. Make a model of your sensor to see if it
works with a flashlight.
Analyze and Conclude
1. Observing What did you observe in Part 1
when you connected the LED to the battery
the first time? The second time?
2. Drawing Conclusions Based on your observations, is the LED a diode? How do you
know?
3. Evaluating the Design How did your observations of the LED’s properties affect your
design in Part 2?
4. Troubleshooting Describe any problems you
had while designing and building your
sensor.
5. Redesigning In what ways could you
improve your sensor?
Communicate
Write a product brochure for your battery sensor.
Be sure to describe in detail how your sensor can
be used to tell if batteries are installed correctly
in electrical devices. Include other possible uses
for your sensor. What practical application can
you see for such an LED?
Chapter 20 ◆ 815
Analyze and Conclude
1. Sample answer: The LED lights when
there is current in one direction but does not
light when the current is reversed.
2. The LED is a diode because it lighted only
when there was current in one direction.
3. Sample answer: Based on my
observations, I placed the LED in the circuit
so it lit when the batteries were installed
correctly.
L2
4. The device should be designed so that the
LED is connected to allow current only when
the battery is in correctly.
5. Sample answer: The sensor design could
be improved by adding another LED of a
different color in a parallel circuit. The LEDs
would be in opposite directions in the
circuit.
Skills Objectives
After this lab, students will be able to:
• observe what happens when an LED is
connected to a battery
• draw a conclusion about whether evidence
supports the hypothesis
• use their oservations to evaluate the design
of their sensor device
• list ways to redesign or improve their
battery sensor
Prep Time 15 minutes
Class Time 40 minutes
Advance Planning
Obtain 2 D cells, a 3-volt flashlight bulb and
socket, 2 insulated wires with alligator clips,
electrical tape, and an LED for each group.
Safety
Caution students to disconnect the
equipment when not in use. Review the
safety guidelines in Appendix A.
Teaching Resources
• Lab Worksheet: Which Light Source
Is a Diode?
Guide Inquiry
Introduce the Procedure
Have students read the procedure, and answer
their questions. Review diode.
Expected Outcome
The bulb will light with the current in either
direction; the LED will light when there is
current in only one direction. The LED is a
diode.
Extend Inquiry
Communicate Students’ brochures should
include a detailed description of how their
sensor works. Other possible uses should also
be described.
815