Investigation of semiconductors

Lynne Shenk, Crown College, St. Bonifacius, MN, based on an original activity from MATEC Teachers Guide, Semiconductor Science Activities for High School Physics
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Summary

In this physics investigation, the students will first do a web quest to get a basic understanding of semiconductors, diodes, and photovoltaic cells. They will then do two simulations to illustrate what is happening to the electrons in different situations. The first simulation simply illustrates a pn junction. The second simulation shows what will happen when an electromotive force is applied in a forward bias, and then in a reverse bias. The last part of the lesson uses a web simulation program that is similar to the second part of the activity. The students will try to explain what is happening as they change the variables in the simulation.

Learning Goals

1. This activity is designed to give students a basic understanding of the composition of semiconductors and diodes.
2. Students should be able to predict and explain the effect of an EMF on the pn junction.
3. Students will be exposed to some of the important applications of this technology.
Key concepts: 1. Students will learn how semiconductors are made and how the electrons migrate within them. 2. Students will learn about the relationship of the width of the depletion zone and the current that flows in the context of a forward biased pn junction and reversed biased pn junction.
Vocabulary: semiconductor, doping, pn junction, depletion zone, photovoltaic cell

Context for Use

This is probably a two or three day activity that would be appropriate for high school students in a college prep physics class within a unit on electricity. It requires some previous knowledge of chemistry. It could be used for any size class as long as computers are available. The research could be done both in school and at home as time allowed. Students should already be familiar with the concepts of valence electrons and bonding and should already have been exposed to simple electronic circuits.

Subject: Physics:Electricity & Magnetism:Semiconductors and Tubes
Resource Type: Activities:Classroom Activity
Grade Level: High School (9-12)

Description and Teaching Materials

Do a web investigation of semiconductors and photovoltaic cells using the following websites:
http://www1.eere.energy.gov/solar/index.html
http://electronics.howstuffworks.com/diode.htm
http://www.electronics-tutorials.ws/diode/diode_3.html

Use the information from the websites to answer the following questions:
What is the primary element used to make semiconductors?
Describe what it means to use doping in reference to silicon? Why is this necessary?
Describe N-type doping.
Describe P-type doping.
Describe a diode and how it is used.
What is happening in a photovoltaic cell?
Describe some of the common uses of photovoltaic cells.
Why are photovoltaic cells an attractive way to produce electricity?

Lab Activity (This activity is a slight modification of the original activity developed by MATEC Teachers Guide, Semiconductor Science Activities for High School Physics)

Materials/group of 2:
Take a normal sheet of paper and cut it into 4 equal pieces.

Draw ~ 2 cm circles on each paper so that you have 6 across the longer side and 4 down the shorter side.

Each group will need 2 cards.
Each group needs 24 pennies or washers.

Procedure:
Part 1 The p-type and n-type junction

1. Use the pennies or washers, and fill every hole on one of the cards. This simulates the n-type material with every washer or penny representing excess electrons. Place the second card next to it without any "electrons". It represents the p-type material.

2. Move the first column of electrons from the first card to the first empty column on the second card. You now have a pn junction and corresponding depletion zone. Assume no more electrons are able to migrate.

Questions: You may need to refer to the internet articles in order to answer the questions.
1. Draw the final position of the pennies/washers on the cards.
2. Explain why the electrons would move from the n-type material to the p-type material.
3. Why don't the electrons continue to move?
4. Circle and label the pn junction on your diagram.
5. What is necessary to keep the electrons migrating across the depletion zone?

Part 2 What is the effect of an outside EMF source being added to a pn device?

Materials:
Use the same cards and pennies/washers used in part 1.
An EMF (electromotive force such as a battery) card with a power supply symbol that shows the positive and negative end:
2 pieces of string that are about the same length as the card.
Tape
Procedure for the forward biased pn junction:
1. Attach a piece of string to each side of the EMF card. This represents the wire leads.
2. Arrange a pn junction by putting pennies/washers in the first 3 rows of each card and arranging the cards side by side, touching each other.
3. Attach the negative lead, from the EMF, to the p-type material and the positive lead to the n-type material. Remember that electrons flow from the negative to the positive pole of a battery or EMF force.
4. On the p-type card, move the outer "electrons" towards the positive lead wire and remove them from the card. This represents the electrons being attracted to the positive lead and traveling through the wire to the battery or EMF source.
5. On the n-type card, shift all calumns of "electrons" one step towards the pn junction. Fill the open holes with the "electrons" that passed through the wire leads.
6. Repeat steps 4-5. Draw the final position of the pennies/washers on the card and circle the depletion zone.

Procedure for the reverse biased pn junction:
1. Return the pennies/washers to the original setup for the forward biased pn junction.
2. Attach the negative lead, from the EMF, to the n-type material and the positive lead to the p-type material.
3. Move all columns of "electrons" on the n-type card one step towards the positive lead, resulting in one column of "electrons" moving off of the card. This represents the electrons being attracted to the positive lead and traveling through the wire to the battery or EMF source.
4. Fill the next open column of "holes", on the p-type card, with the "electrons" that have passed through the wire leads.
5. Repeat steps 3-4. Draw the final position of the pennies/washers on te card and circle the depletion zone.

Questions:
1. Compare the size of the depletion zone in the forward and reversed biased pn junctions.
2. If the depletion zone is small, the resistivity of the device will also be small. If the depletion zone is large, the resistivity of the device will also be large. Given this information, will the forward biased or reversed biased pn junction allow a current to flow? Explain your answer.

Closing activity:
Go to the following website to access a simulation program for semiconductors:
http://phet.colorado.edu/simulations/sims.php?sim=Semiconductors
a. Start with a voltage of 0 V. Drag the p-type material to the lead attached to the negative terminal of the battery. Drag the n-type material to the lead attached to the positive terminal of the battery. Record any observations. Increase the voltage 0.1 V at a time and record observations. Try to explain your observations.
b. Do the same thing, but drag the p-type material to the lead attached to the positive terminal of the battery. Drag the n-type material to the lead attached to the negative terminal of the battery. Record your observations and give an explanation for what you observed. Illustrations for simulation activity setup (Microsoft Word 31kB Jul19 09)

Teaching Notes and Tips

Be prepared to allow some discussion of the web based articles and to answer questions that students may have. It may be necessary to review some of the chemical bonding concepts. Encourage the students by letting them know that the lab type activity should help them to visualize some of the abstract concepts presented. I have not yet had the opportunity to teach physics, so I have never used this activity.

Assessment

The students will turn in a written copy of the answers to the questions in the web quest as well as the lab activity and computer simulation.

Standards

9.2.3.2.1 Energy – Identify the energy forms and explain the transfers of energy involved in the operation of common devices. A photovoltaic cell is used in many common devices.
9.2.3.2.4 Energy – Explain and calculate current, voltage and resistance, and describe energy transfers in simple electric circuits. The study of semiconductors relates resistance to voltage and current flow.

References and Resources