# Diodes and Transistors

## SPECIFIC OBJECTIVES

To determine some of the operating characteristics of semiconductor diodes and transistors.

## EQUIPMENT

Circuit board, D-cell batteries (2), wires, resistors, multimeter, diode, transistor, and probe leads.

## PROCEDURE

The underlined passages below require an answer or a sketch in your notebook.

### Part 1 - Diodes

1. When you are not taking data, please disconnect the battery; this will increase its lifespan.

2. Connect the circuit shown above using the 1000-ohm resistor. Orient the diode with its band closer to point B. Record this data under "Forward Bias".

3. With the switch closed and the current flowing, adjust the potentiometer until there is voltage of 0.05 volt across the resistor. Measure the voltage across the diode. Record your values in the table below.

4. Adjust the potentiometer to attain voltages across the resistor between 0.10 volts and the maximum battery voltage. For each of these settings, measure the voltage across the diode. Record your values in the table below. Plot the points as you go. Take more data where the plot is interesting, less where it is predictable.

5. Reverse the orientation of the diode. Record this data under "Reverse Bias".

6. Set the diode voltage (not the resistor voltage this time) to values between 0.50 volts and the maximum battery voltage. For each of these settings, measure the voltage across the resistor. Record your values in the table below. Plot the points as you go. Take more data where the plot is interesting, less where it is predictable. (Sometimes zero is the correct answer.)

7. Calculate the current flowing in this series circuit by dividing the resistor voltage by the resistance. Record your values in the table below. This must also be the current flowing through the diode.

8. Graph the current through the diode (vertical axis) vs. voltage across the diode (horizontal axis). Plot the forward bias voltages on the positive horizontal axis and the reverse bias voltages on the negative horizontal axis. This graph is called the current-voltage characteristic of the diode.

9. Repeat the exercise with the 330-ohm resistor in place of the 1000-ohm resistor.

10. Discuss the shape of the graph. Describe in words how a diode behaves.

11. Plot the 1000-ohm and the 330-ohm graphs on top of each other. Is there any difference in the two curves? What does this tell you about the diode?

12. Sketch the current-voltage characteristic of a resistor and compare it to that of the diode. Does the diode obey Ohm's Law?

13. Suggest an application for a diode.
These tables are for illustrative purposes only; the tables on the separate printable page have more rows for data entry.

R = 1000 ohms
Forward Bias
 Vdiode Vresistor Current

R = 1000 ohms
Reverse Bias
 Vdiode Vresistor Current

R = 330 ohms
Forward Bias
 Vdiode Vresistor Current

R = 330 ohms
Reverse Bias
 Vdiode Vresistor Current

### Part 2 - Transistors

1. When you are not taking data, please disconnect the battery; this will increase its lifespan.

2. Connect the circuit shown above using R1 = 1000 ohms and R2 = 100 ohms. Be sure that the transistor is seated properly in its socket.

3. Adjust the potentiometer until the voltage across the large resistor VAB is approximately 0.002 volts (=2 millivolts). Measure the voltage across the small resistor VCD. Record your values in the table below.

4. Adjust the potentiometer to attain voltages VAB between 0.000 volts and 0.250 volts. Record the corresponding VCD in the table below. Plot the points as you go. Take more data where the plot is interesting, less where it is predictable.

5. Note that VAB divided by R1 gives the current flowing into the base of the transistor IB, while VCD divided by R2 gives the current flowing out of the collecter IC. Record your current values in the table below.

6. Graph the collector current IC (vertical axis) vs. the base current IB (horizontal axis).

7. Repeat the exercise with R2=330 ohm instead of 100 ohm.

8. Discuss the shape of the graph. Describe in words how a transistor behaves.

9. Find the slope of the straight-line region near the origin. This ratio IC / IB is referred to as the "current amplification" of the transistor.

10. What does the leveling off of the graph indicate? This region is called "saturation" of the transistor.

11. Plot the 100-ohm and the 330-ohm graphs on top of each other. Is there any difference in the two curves? What does this tell you about the transistor?
These tables are for illustrative purposes only; the tables on the separate printable page have more rows for data entry.

R1 = 1000 ohms; R2 = 100 ohms.
 VAB IB VCD IC

R1 = 1000 ohms; R2 = 330 ohms.
 VAB IB VCD IC

Bonus: "b" stands for "base"; "c" stands for "collector". What does "e" stand for on the transistor? (See why it pays to read the lab ahead of time?)

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