APPLICATIONS OF SEMICONDUCTING MATERIALS Karen Porter-Davis Chamblee Charter High School CLASS LEVEL High School Advanced/Honors/Gifted Physics /AP Physics LESSON TIME 3 to 4 days (with extended activities) PROBLEM What are semiconductors and why are they important for integrated circuits in microelectronics? ABSTRACT Semiconductors are solid crystalline substances that tend to have greater electrical conductivity than insulators, but less than good conductors. The valence band of a semiconductor is full similarly to that of an insulator, but the band gap is much smaller (about 1 eV compared to about 5 eV). In fact, the band gap in several semiconductors is so small that electrons are easily able to be thermally excited into the conduction band. This means that the electrical conductivity of many semiconductors is strongly reliant on temperature. Even though conductivity is not dependent only on the number of free electrons, materials with less than one free electron per million atoms will not easily be able to conduct electricity. To have practical uses for semiconductors the conductivity must be greatly increased and raising the temperature is not a very reliable way to achieve this goal. However, it is accomplished by doping (adding a very small amount of other atoms in with the semiconductor), which increases conductivity by adding either electrons or holes to a semiconductor. By putting together n-doped and p-doped semiconductors diodes and transistors can be created. In these devices, voltage and current can be varied in more complicated way than directed by Ohm’s Law. To build a practical circuit it is important to have switches (on/off switches are related to binary code) that can control current, voltage, and resistance. Semiconductors can easily be manipulated to become conducting or insulating materials and can change their conductive properties very quickly. This allows for the possibility of building millions of tiny semiconducting “switches” on a single chip. NATIONAL STANDARDS ALLIGNMENT CONTENT STANDARD A: UNIFYING CONCEPTS AND PROCESSES • Systems, order, and organization. • Evidence, models, and explanation. • Change, constancy, and measurement. • Evolution and equilibrium.
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APPLICATIONS OF SEMICONDUCTING MATERIALS
Karen Porter-Davis
Chamblee Charter High School
CLASS LEVEL High School Advanced/Honors/Gifted Physics /AP Physics
LESSON TIME
3 to 4 days (with extended activities)
PROBLEM What are semiconductors and why are they important for integrated circuits in
microelectronics?
ABSTRACT Semiconductors are solid crystalline substances that tend to have greater electrical
conductivity than insulators, but less than good conductors. The valence band of a
semiconductor is full similarly to that of an insulator, but the band gap is much smaller
(about 1 eV compared to about 5 eV). In fact, the band gap in several semiconductors is
so small that electrons are easily able to be thermally excited into the conduction band.
This means that the electrical conductivity of many semiconductors is strongly reliant on
temperature. Even though conductivity is not dependent only on the number of free
electrons, materials with less than one free electron per million atoms will not easily be
able to conduct electricity. To have practical uses for semiconductors the conductivity
must be greatly increased and raising the temperature is not a very reliable way to
achieve this goal. However, it is accomplished by doping (adding a very small amount of
other atoms in with the semiconductor), which increases conductivity by adding either
electrons or holes to a semiconductor.
By putting together n-doped and p-doped semiconductors diodes and transistors can be
created. In these devices, voltage and current can be varied in more complicated way
than directed by Ohm’s Law. To build a practical circuit it is important to have switches
(on/off switches are related to binary code) that can control current, voltage, and
resistance. Semiconductors can easily be manipulated to become conducting or insulating
materials and can change their conductive properties very quickly. This allows for the
possibility of building millions of tiny semiconducting “switches” on a single chip.
NATIONAL STANDARDS ALLIGNMENT
CONTENT STANDARD A: UNIFYING CONCEPTS AND PROCESSES
• Systems, order, and organization.
• Evidence, models, and explanation.
• Change, constancy, and measurement.
• Evolution and equilibrium.
• Form and function.
CONTENT STANDARD B: SCIENCE AS INQUIRY
• Understanding of scientific concepts.
• An appreciation of "how we know" what we know in science.
• Understanding of the nature of science.
• Skills necessary to become independent inquirers about the natural world.
• The dispositions to use the skills, abilities, and attitudes associated with science.
CONTENT STANDARD C: PHYSICAL SCIENCE
• Structures of atoms.
• Structure and properties of matter.
• Interactions of energy and matter.
CONTENT STANDARD E: SCIENCE AND TECHNOLOGY
• Abilities of technological design
• Understandings about science and technology
CONTENT STANDARD F: SCIENCE IN PERSONAL AND SOCIAL
PERSPECTIVES
• Science and technology in local, national, and global challenges
CONTENT STANDARD G: HISTORY AND NATURE OF SCIENCE
• Science as a human endeavor.
• Nature of scientific knowledge.
• Historical perspectives.
GEORGIA PERFORMANC STANDARDS ALLIGNMENT
SCSh1. Students will evaluate the importance of curiosity, honesty, openness, and
skepticism in science.
SCSh2. Students will use standard safety practices for all classroom laboratory and
field investigations.
SCSh3. Students will identify and investigate problems scientifically.
SCSh4. Students will use tools and instruments for observing, measuring, and
manipulating scientific equipment and materials.
SCSh5. Students will demonstrate the computation and estimation skills necessary
for analyzing data and developing reasonable scientific explanations.
SCSh6. Students will communicate scientific investigations and information clearly.
SCSh7. Students will analyze how scientific knowledge is developed.
SCSh8. Students will understand important features of the process of scientific
inquiry.
SCSh9. Students will enhance reading in all curriculum areas.
SP5. Students will evaluate relationships between electrical and magnetic forces.
a. Describe the transformation of mechanical energy into electrical energy
and the transmission of electrical energy.
b. Determine the relationship among potential difference, current, and
resistance in a direct current circuit.
OBJECTIVES
• To understand the unique properties of semiconductors and why they work
well in electronics. • To understand how diodes and transistors work in electronics. • To understand the future direction of semiconductor technology.
ANTICIPATED LEARNER OUTCOMES
a. Students should be able to describe and demonstrate how electrons and holes
move throughout a semiconductor.
b. Students should understand and demonstrate the idea of “doping” a
semiconducting material and the difference between p and n doping.
c. Students should understand how diodes are formed and their applications.
d. Students should demonstrate how LEDs work in a series and parallel circuit.
e. Students should be able to describe the uses and importance of transistors in
our modern world.
BACKGROUND
Atoms consist of a dense, positively charged nucleus surrounded by a cloud of negatively
charged electrons. The electron in an atom can possess only certain amounts of energy
(quantized). Due to this, electrons can occupy only certain allowed energy levels.
Usually the electrons in an atom occupy the lowest possible energy levels available to
them. This condition is referred to as the ground state. An atom can sometimes absorb
outside energy, which if the energy is sufficient enough, one of the atom’s electrons can
move to a higher energy level. The atom is then in its excited state. The electron may
absorb so much energy that it is no longer bound to the atom and is now free.
When identical atoms are far apart they have the same energy levels and wave functions,
but as the atoms are brought closer together, their wave functions overlap. Because no
two electrons in the same system can occupy the same state, the energy level in an atom
is altered by the influence of the electric field of another atom. This causes energy levels
to split. Adding a few more nearby atoms causes further splitting and when many atoms
interact, the energy levels are so closely spaced that they can be represented as energy
bands. The bands are separated by values of energy that no electron can possess. These
energies are called forbidden gaps. For atoms in the ground state, the lower energy
levels are completely full. The outermost band that holds electrons is called the valence
band. The lowest band that is not filled to capacity with electrons is called the
conduction band. Electrical conduction in solids explained in terms of these energy
bands and forbidden gaps is called the band theory of solids. This band theory explains
why solids fall into three categories: conductors, insulators, and semiconductors.
Conductors: When a potential difference is placed across a substance, the resulting
electric field exerts a net force on the electron. The electron then accelerates and gains
energy (the field does work on the electrons). If there are bands with in the material that
are only partially filled, then there are energy levels available that are only slightly higher
than the electron’s present level. Therefore, the electron can move from one atom to the
next in what is referred to as the conduction band. Such movement of electrons from
one atom to the next is called electric current, and the entire process is known as
electrical conduction. Materials with partially filled bands conduct electric current easily
and are considered conductors.
The electrons move rapidly and randomly (106 m/s) in a conductor due to collisions with
the cores of the atoms. However, if an electric field created by a potential difference is
applied there will be a net force pushing the electrons in one direction. Although their
motion is not greatly affected, they have a slow overall movement directed by the field
called drift velocity (10-5
m/s or slower). If temperature is increased, the speeds of the
electrons also increase which causes them to collide more frequently with the atomic
cores. Therefore, as the temperature increases, the conductivity of metals decreases
because the drift velocity decreases. As conductivity is reduced, a material’s resistance
rises.
Insulators: In an insulating material the valence band is filled to capacity and the
conduction band is empty. In these materials the valence band and the conduction band
are separated by a forbidden gap. For an electron to move from the valence band to the
conduction band it must gain a large amount of energy (5-10 eV). Though electrons
possess some kinetic energy due to their thermal energy, the average kinetic energy of
electrons at room temperature is not enough for them to jump the forbidden gap. Even
with a small electric field, almost no electrons gain enough energy to reach the
conduction band, so there is no current.
Semiconductors: Semiconductors have a smaller forbidden gap than insulators and
therefore need less energy for their electrons to jump into the conduction band. Some
electrons reach the conduction band on their own as a result of their thermal kinetic
energy and even more make it when an electric field is applied to the material. Unlike
metals, as the temperature increases the electron movement and conductivity increases.
An atom from which an electron has broken free from its valence band is missing an
electron is said to contain a hole. A hole is an empty energy level in the valence band.
The atom now has a net positive charge. If an electron breaks free from another atom, it
can land on the hole and become bound to an atom once again. When the hole and a free
electron recombine, their opposite charges cancel each other. The electron, however, has
left behind another hole on its previous atom. The negatively charged, free electrons
move in one direction and the positively charged holes move in the opposite direction.
Pure semiconductors that conduct as a result of thermally freed electrons and holes are
called intrinsic semiconductors. Because so few electrons or holes are available to carry
charge, conduction in intrinsic semiconductors is very small; thus, their resistance is very
PROBLEM: How can you design a circuit so that changing the direction of the current
changes the LED that light up?
SAFETY:
MATERIALS:
0- to 12-V variable power supply
Red LED
Green LED
Bi-colored LED
Wires
470- resistor
Voltmeter
PROCEDURE:
1. Connect a series circuit with the power supply, the resistor, and the red and
green LEDs to them both. Do not bypass or omit the resistor with an LED.
Always have the resistor between an LED and one side of the power
supply.
2. Reverse the direction of the current in the circuit and note the result.
Measure the voltage across an LED.
3. Connect a parallel circuit with the power supply, the resistor, and the red
and green LEDs to light them both. Do not bypass or omit the resistor
with an LED. Always have the resistor between an LED and one side
of the power supply.
4. Reverse the direction of the current in the circuit and note the result.
Measure the voltage across an LED.
1 (Glencoe Physics: Principals and Problems 2002)
5. Repeat steps 1-4 with the bi-colored LED instead of the separate red and
green LEDs. Remember to leave the resistor connected to the power
supply.
DATA AND OBSERVATIONS:
1. What voltage was needed to light the LEDs in each circuit?
2. Describe what happened when the current was reversed in each of the
circuits?
3. Make a drawing of the stoplight circuit that will allow: the red on, green
off; green on, red off.
4. Is this a series or parallel circuit? Why does it work this way?
5. What change would you observe if you replaced the resistor with a 330-
resistor?
6. If the voltage across the LED was increased, what would happen to the
current?
2 (Glencoe Physics: Principals and Problems 2002)
7. What must be true for the graph of current vs. voltage to be a straight line?
8. How does an LED differ from a 60-W light bulb?
9. Sketch a graph of the following data describing the relationship between
the current and the voltage.
VOLTAGE (V) CURRENT (A)
0.5 0.001
1.0 0.002
1.5 0.030
What does the graph indicate about the resistance of the LED? Is this an
Ohmic or non-Ohmic material?
3 (Glencoe Physics: Principals and Problems 2002)
TEACHER INFORMATION AND RUBRIC FOR STOPLIGHT LAB
*Students likely have little familiarity with the basic structures of solid state devices.
Students will sometimes confuse filament lamps with LEDs.
*DO NOT omit the current limiting resistor – excessive current can destroy LEDs.
Depending on the number of students in your class and the amount of equipment
you have you can put the students into groups of two to four. If you do not have
enough equipment for groups you may perform this as a class activity.
Purchasing Equipment: Here are a few websites that you should be able to order
with, if you do not already have the supplies.
Exploratorium Museum Store - http://www.exploratoriumstore.com/ Science toys and games, puzzles, gifts, books, classroom resources, charts and posters, videos, and software.
PASCO Scientific - http://www.pasco.com/ Offers a variety of interfaces and sensors (probes) bundled with computer-based activities for chemistry, biology, physics and general science.
Flinn Scientific - http://www.flinnsci.com/ Sells educational science supplies. Site has especially useful information on lab safety and lab design.
Sargent-Welch - http://www.sargentwelch.com/ Distributor of thousands of grades K-14 scientific, educational items ranging from basic glassware to hands-on curriculum products.
Vernier Software - http://www.vernier.com/ Maker of science hardware and software for the classroom, especially CBL products, probes, and TI programmable calculator programs.
Educational Innovations - http://www.teachersource.com/ Source for inexpensive and hard-to-find science workshop supplies and materials for the lab, classroom, school workshop, university and home experimenter.
Fisher Science Education - http://www.fisheredu.com/ Thousands of science products geared toward the K-12 education market.
Cambridge Physics Outlet - http://www.cpo.com/ Equipment and software for inquiry-based hands-on teaching of integrated math, science, and technology. Also, there are interactive science puzzlers and an online products catalog
The Science Source - http://www.thesciencesource.com/ Offers science kits, toys, supplies, materials, classroom kits, and other science products. Much of the product line evolved out of curricula developed by the Physical Science Study Committee (PSSC) at MIT and Project Physics at Harvard University, and products are available through distributors or online
Frey Scientific - http://www.freyscientific.com/ Scientific supplies and other materials for middle and high schools.
Rubrics and Evaluations: The following pages include the answer sheet to the lab
and two different rubrics. The first rubric is for a non-formal lab report (class
activity) and the second is for a typed, formal report. It is the teacher’s discretion to
choose which one to use.
TEACHER ANSWER SHEET FOR STOPLIGHT
LAB QUESTIONS
1) What voltage was needed to light the LEDs in each circuit?
The LEDs will begin to glow around 1.8 V and be bright at 2.2 V.
2) Describe what happened when the current was reversed in each of
the circuits?
Reversing the current causes both LEDs to go out.
3) Make a drawing of the stoplight circuit that will allow: the red on,
green off; green on, red off.
The stoplight circuit will have the LEDs in parallel and reversed in
polarity.
4) Is this a series or parallel circuit? Why does it work this way?
It is a series circuit. However, to light both the red and green
LEDs at the same time, the LEDs must be connected in parallel.
By reversing the polarity of the LEDs, only one color can be on at
a time. Reversing the leads at the power supply will change the
color of the stoplight.
5) What change would you observe if you replaced the resistor with a
330- resistor?
The 330- resistor allows more current to flow through the circuit,
causing the LEDs to glow more brightly.
6) If the voltage across the LED was increased, what would happen to
the current?
Because V = IR (for Ohmic materials), student will likely indicate
that current increases as voltage increases.
7) What must be true for the graph of current vs. voltage to be a
straight line?
The graph will be a straight line only if the resistance remains
constant (if it is Ohmic). It is not. These LEDs are non-Ohmic
materials, meaning they do not follow Ohm’s Law.
8) How does an LED differ from a 60-W light bulb?
A lightbulb emits a broad range of the electromagnetic spectrum,
whereas a LED emits a single wavelength only. In addition,
current can pass either way across a lightbulb filament but only in
one direction through an LED.
9) Sketch a graph of the following data describing the relationship
between the current and the voltage.
VOLTAGE (V) CURRENT (A)
0.5 0.001
1.0 0.002
1.5 0.030
What does the graph indicate about the resistance of the LED? Is this an
Ohmic or non-Ohmic material?
The relationship is not linear. Therefore, resistance of the LED is not
constant and the material is non-Ohmic.
At 1.0 V the resistance is 500 and at 1.5 V the resistance is 50 .
All required elements are present and additional elements that add to the report (e.g., thoughtful
comments, graphics) have been added.
All required elements are present.
One required element is missing, but additional elements that add to the report (e.g., thoughtful
comments, graphics) have been added.
Several required elements are missing.
Question / Purpose
The purpose of the lab or the question to be answered during the lab is clearly identified and stated.
The purpose of the lab or the question to be answered during the lab is identified, but is stated in a somewhat unclear manner.
The purpose of the lab or the question to be answered during the lab is partially identified, and is stated in a somewhat unclear manner.
The purpose of the lab or the question to be answered during the lab is erroneous or irrelevant.
Spelling, Punctuation, Grammar
One or fewer errors in spelling, punctuation and grammar in the report.
Two or three errors in spelling, punctuation and grammar in the report.
Four errors in spelling, punctuation and grammar in the report.
More than four errors in spelling, punctuation and grammar in the report.
Drawings / Diagrams
Clear, accurate diagrams are included and make the experiment easier to understand. Diagrams are labeled neatly and
accurately.
Diagrams are included and are labeled neatly and accurately.
Diagrams are included and are labeled.
Needed diagrams are missing OR are missing important labels.
Participation Used time well in lab and focused attention on the experiment.
Used time pretty well. Stayed focused on the experiment most of the time.
Did the lab but did not appear very interested. Focus was lost on several occasions.
Participation was minimal OR student was hostile about participating.
Error Analysis Experimental errors, their possible effects, and ways to reduce errors are discussed.
Experimental errors and their possible effects are discussed.
Experimental errors are mentioned.
There is no discussion of errors.
Procedures Procedures are listed in clear steps. Each step is numbered and is a complete sentence.
Procedures are listed in a logical order, but steps are not numbered and/or are not in complete sentences.
Procedures are listed but are not in a logical order or are difficult to follow.
Procedures do not accurately list the steps of the experiment.
Summary Summary describes the skills learned, the information learned and
some future applications to real life situations.
Summary describes the information learned and a possible application to
a real life situation.
Summary describes the information learned.
No summary is written.
Calculations All calculations are shown and the results are correct and labeled appropriately.
Some calculations are shown and the results are correct and labeled appropriately.
Some calculations are shown and the results labeled appropriately.
No calculations are shown OR results are inaccurate or mislabeled.
Materials / Setup All materials and setup used in the experiment are
clearly and accurately described.
Almost all materials and the setup used in the
experiment are clearly and accurately described.
Most of the materials and the setup used in the
experiment are accurately described.
Many materials are described inaccurately OR
are not described at all.
Conclusion Conclusion includes whether the findings supported the hypothesis,
Conclusion includes whether the findings supported the hypothesis
Conclusion includes what was learned from the experiment.
No conclusion was included in the report OR shows little effort and reflection.
possible sources of error, and what was learned from the experiment.
and what was learned from the experiment.
Variables The relationship between
the variables is discussed and trends/patterns logically analyzed. Predictions are made about what might happen if part of the lab were changed or how the experimental design could be changed.
The relationship between
the variables is discussed and trends/patterns logically analyzed.
The relationship between
the variables is discussed but no patterns, trends or predictions are made based on the data.
The relationship between the
variables is not discussed.
Safety Lab is carried out with full attention to relevant safety procedures. The set-up, experiment, and tear-down posed no safety threat to any individual.
Lab is generally carried out with attention to relevant safety procedures. The set-up, experiment, and tear-down posed no safety threat to any individual, but one safety procedure
needs to be reviewed.
Lab is carried out with some attention to relevant safety procedures. The set-up, experiment, and tear-down posed no safety threat to any individual, but several safety procedures need to be
reviewed.
Safety procedures were ignored and/or some aspect of the experiment posed a threat to the safety of the student or others.
Replicability Procedures appear to be replicable. Steps are outlined sequentially and are adequately detailed.
Procedures appear to be replicable. Steps are outlined and are adequately detailed.
All steps are outlined, but there is not enough detail to replicate procedures.
Several steps are not outlined AND there is not enough detail to replicate procedures.
Scientific Concepts
Report illustrates an accurate and thorough understanding of scientific
concepts underlying the lab.
Report illustrates an accurate understanding of most scientific
concepts underlying the lab.
Report illustrates a limited understanding of scientific concepts underlying the
lab.
Report illustrates inaccurate understanding of scientific concepts underlying the lab.
Questions Report has thoroughly and accurately answered all questions posed in lab
Report has accurately answered all questions posed in lab.
Report has accurately answered most questions posed in lab.
Report has accurately answered a few questions posed in lab.
Experimental Design
Experimental design is a well-constructed test of the stated hypothesis.
Experimental design is adequate to test the hypothesis, but leaves some unanswered
questions.
Experimental design is relevant to the hypothesis, but is not a complete test.
Experimental design is not relevant to the hypothesis.
Data Professional looking and accurate representation of the data in tables and/or graphs. Graphs and tables are labeled and titled.
Accurate representation of the data in tables and/or graphs. Graphs and tables are labeled and titled.
Accurate representation of the data in written form, but no graphs or tables are presented.
Data are not shown OR are inaccurate.
Appearance / Organization
Lab report is typed and uses headings and
subheadings to visually organize the material.
Lab report is neatly handwritten and uses
headings and subheadings to visually organize the material.
Lab report is neatly written or typed, but formatting
does not help visually organize the material.
Lab report is handwritten and looks sloppy with cross-
outs, multiple erasures and/or tears and creases.
Experimental Hypothesis
Hypothesized relationship between the variables and the predicted results is clear and reasonable based on what has been
studied.
Hypothesized relationship between the variables and the predicted results is reasonable based on
general knowledge and observations.
Hypothesized relationship between the variables and the predicted results has been stated, but appears to be based on flawed
logic.
No hypothesis has been stated.
TRANSISTOR WRITING ASSIGNMENT
This assignment is due: ________________.
Directions: Write a 2-3 page, typed essay (12 pt type, must start at
top of page, doubled spaced) on the topic of transistors. Be sure to
answer the following questions within your essay and include any
references (Reference page does not count towards paper!)
• When was the transistor developed and by whom?
What was the problem that the developers of the
transistor were trying to solve?
• Describe some early uses of the transistor (At least
three).
• What were the advantages of the transistor over the
vacuum tube?
• How are transistors used today?
• In what way has the transistor changed modern life?