Contents

Lesson 1

Lesson 2

Lesson 3

Lesson 4

Profiles of Scientists

Credits

# Lesson 4

### Using Transistors: Let’s Get Transistorized!

Overview

In this lesson, students build two circuits and explore how transistors function.

Objectives

• To observe how a transistor functions in a simple circuit

• To understand amplification—that a small current at the input of a transistor controls a larger current at its output

Background

When Bell Labs introduced the transistor in June of 1948, a spokesman proudly announced "This cylindrical object . . . can amplify electrical signals. . . . It is composed entirely of cold, solid substances."

The cold, solid substance that makes the transistor possible is the semiconductor, a class of material that includes silicon and germanium. Semiconductors are normally very poor conductors of electricity. But with the addition of tiny amounts of other elements, which provide carriers for electric current, they can become good conductors.

The first transistor, invented in 1947, was the point-contact transistor. William Shockley improved on this design with his junction transistor, a three-layer sandwich of different types of semiconductor.

The diagram illustrates the basic design of an NPN junction transistor. Two slices of N-type semiconductor, the emitter and the collector, form a sandwich with a layer of P-type semiconductor, called the base. P- and N-type semiconductors are made with different impurities, and the name indicates the dominant type of charge carrier.

The interface between the layers, called the P-N junction, allows the transistor to function as either an insulator or a conductor. If the collector and emitter are connected to a battery, the electrical charges at the P-N junctions form an electrical barrier and no current flows between the emitter and the collector. The transistor acts like an insulator or a switch that is turned off.

When a positive voltage is applied to the base, electrons are pulled out of the junctions and they no longer act as barriers. Now electrons can flow from the emitter through the base to the collector. The transistor acts as a conductor, or a switch that’s turned on. (If the voltage applied to the base is negative, the transistor turns off again.)

Transistors do not create electric current, they only control electric current supplied to them. The input current at the base controls the output current flowing between the emitter and the collector. The transistor can turn on or off if the base current turns on or off. If the base current varies, so does the output current, which is how a transistor functions as an amplifier. It’s similar to the way you control the flow of water with a faucet. With a small hand movement, you can turn the water on or off, or adjust the flow between a trickle and a rushing stream.

Most early commercial transistors were junction transistors and are the type used in the activity on the next two pages. However, the most common modern transistor, the one that is found by the millions in computer chips, is the metal oxide semiconductor (MOS) field effect transistor. The transistor has evolved since its invention, but the principle of a small current controlling a larger one is the same effect that Bardeen, Brattain, and Shockley first revealed in 1947.

Engage

As explained in Transistorized!, the invention of both the transistor and the vacuum tube grew out of the need to amplify weak electric current. Begin by demonstrating a weak current that students can recognize and experience. Connect a circuit using wire, a 9-V battery, an LED, a resistor, and a microammeter to measure current. Have students note what happens when they complete the circuit first by connecting the leads together (relatively large current and the LED lights), and then by holding the leads in their hands (very small current and the LED does not light). Safety: The current in this circuit is small enough to perform this activity safely, but caution students not to try this activity with other wires or power sources.

Have students offer their ideas on what an amplifier is and how to amplify a current. Point out that most electronic devices run on a small current that is amplified.

Explore

Have students perform the activity to see how transistors amplify current.

Evaluate

After the activity, discuss students’ results and the activity questions.

### LESSON 4 ACTIVITY

What You’re Going To Do

You’re going to build two simple transistor circuits, each using a single transistor. These circuits will allow you to observe the operation of a transistor as an amplifier, just as Walter Brattain did at Bell Labs in the winter of 1947. In the first circuit, you’ll use the transistor to control the brightness of a light; in the second, the transistor will turn the current flowing through your body into sound!

Part 1: Light Touch

Construct the first circuit using a single transistor, an LED, a power source, and a resistance. The brightness of the LED will indicate the relationship between the current going to the base of the transistor—its input—and the current flowing from the transistor’s collector to the emitter—its output.

What You’ll Need

• 9-V battery and clip with leads

• hook-up wire

• LED

• 220-ohm resistor

• 100K-ohm resistor

• transistor, 2N2222A (Si type, NPN, Radio Shack part number 276-2009)

• microammeter (0–50 000 ľA range)

How To Do It

1. Work in groups of three or four. Assemble the circuit shown in the diagram. Match the leads on the transistor to the diagram, and identify the base, emitter, and collector. Check with your teacher if you are unsure of the connections.

2. Complete the input circuit with the two leads, using each method listed below.

• gently squeezing the leads

• tightly squeezing the leads

• dipping the leads in water

• increasing the distance between the leads in water

• making a dark line with pencil and touching the leads to it.

• increasing the distance between the leads on the pencil streak

In your lab book, make a table similar to the one shown in which to record the intensity of the light for each method. You might use terms such as dim, average, and bright, or develop a number scale with 1 = 5 very dim and 5 = 5 very bright. (In your table, include a column for sound intensity for Part 2.)

3. Draw a copy of the circuit diagram in your lab book. Use arrows to show the direction in which the current flows through the circuit. Remember that current flow is from positive to negative. Label the input circuit and the output circuit of the transistor.

4. Repeat one of the methods that gives a reasonably bright light. Place the microammeter in series with the input leads and record the reading. Then move the microammeter so that it is in series with the LED and record that reading.

FYI

The P and N in transistor nomenclature indicate the type of charge carriers that exist in the materials that form the transistor. In an N-type material, the carriers are negatively charged electrons, and in P-type material the carriers are positively charged. These are places where electrons could exist and are called holes.

What Did You Find Out?

1. Which methods allowed the light to glow the brightest? the dimmest?

2. Which methods allowed the most current to pass through them? the least? How do you know?

3. How good an amplifier was your circuit? How much larger was the output current than the input current? Where did the "additional" current come from?

Part 2: The Human Sound Machine

Now, you’ll modify your circuit by adding new parts. The transistor is very responsive to changes in its input. The input current can fluctuate thousands—even millions—of times per second, and the output current will respond accordingly. The additions to the circuit will produce an oscillating current, varying several thousand times per second, at the transistor’s input. You’ll hear the resulting output through the speaker.

What You’ll Need

(in addition to Part 1 materials)

• wire

• 10K-ohm resistor

• 100K-ohm resistor

• switch

• capacitors (0.1 microfarad and 0.01 microfarad)

• 1K CT: 8-ohm transformer (Radio Shack Cat # 273-1380)

• 8 ohm speaker

How To Do It

1. Assemble the circuit shown in the diagram. You may choose to solder or use common IC Experimenter boards.

2. Complete the circuit with the leads using each method listed in Part 1. Record the intensity of the sound for each method. You might use terms such as hum, squawk, and squeal, or develop a number scale with 1 5 very low and 5 5 very loud.

3. Draw a copy of the circuit diagram in your lab book. Use arrows to show the direction in which the current flows through the circuit. Label the input circuit and the output circuit of the transistor.

FYI

The MOS transistor—the modern transistor used in computer chips—is similar in operation to the one that Shockley first proposed. It consists of a semiconductor through which current can flow, and an electrode that is insulated from this semiconductor. The voltage applied between the insulated electrode and the semiconductor controls the current through the semiconductor. The principle is similar to water flowing through a piece of flexible tubing. When the tubing is squeezed, the flow of water is decreased. Squeeze hard enough, and the flow stops. In the MOS transistor, the voltage applied to the control electrode is what does the squeezing.

What Did You Find Out?

1. Which methods produced the loudest sounds? the softest?

2. Which methods allowed the most current to pass through them? the least? How do you know?

3. Discuss with your group the advantages you think transistor switches might have over mechanical switches. What quality of transistors—high reliability, small current amplification, or instant response—do you feel is the most important for transistors used in computers? in medical equipment such as pacemakers? in guided missiles?

Try This!

• Use your circuit to test how well other methods and materials conduct electricity.
• If possible, attach an oscilloscope to your circuit and analyze the waves you are hearing.
• Using Ohm’s Law, I = V/R, calculate the currents in the first circuit.
• Reverse the polarity of the battery and repeat each activity. What happens?

These educational materials are made possible by a grant from The Lucent Technologies Foundation and may be duplicated for educational non-commercial use.

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