A key objective of this experiment is to relate the solid-state structure to physical properties of these materials. This experiment relates observed properties such as the color, wavelength and energy of light, and excitation voltage for a series of compound semiconductors to composition and periodic trends. You will observe how electrical resistance of a metal changes with temperature and how electrical resistance of a semiconductor changes under illumination.
- Wear eye protection
- Thermal gloves recommended
- Caution: liquid nitrogen is extremely cold. Do NOT allow it to come into contact with skin or clothing, as severe frostbite may result
Step 1. When you open the box with “Periodic Properties and Light Emitting Diodes” on top, this is what you will see. Please note where pieces are so that you are able to return them in the correct place when you are finished. It is especially important to return the clear plastic diffraction grating sheet at the end of the experiment.
Step 2. Using the circuit containing the 1-kΩ resistor (it has a red stripe), connect the battery snap to a 9-volt battery. The circuit is “turned on” by inserting the LED into the socket. Does it matter which way the LED is inserted?
Step 3. Obtain an LED reference strip box. View the LEDs through the diffraction grating sheet oriented so that the light from the LEDs is diffracted away from the reference strip.
Step 4. Plug the white LED into the circuit containing the 1-kΩ resistor and view it through the diffraction grating.
Step 5. With the LED plugged into the circuit containing the 1-MΩ resistor (it has a green stripe) with the battery attached, measure the voltage across the LED using the multimeter. To do so you must attach the red alligator clip to the side of the LED that is plugged into the red wire and the black alligator clip to the black side. The meter must be set to measure in the 20 Volts DC setting. After taking a room temperature measurement, plunge the LED into a foam coffee cup containing liquid nitrogen. Caution: liquid nitrogen is extremely cold. Do NOT allow it to come into contact with skin or clothing, as severe frostbite may result. With the LED still submerged, take a low temperature voltage reading.
Step 6. Place each LED into the 1-kΩ circuit and attach the 9V battery. Dip the LED into liquid nitrogen in a foam coffee cup and then view it through the diffraction grating. Caution: liquid nitrogen is extremely cold. Do NOT allow it to come into contact with skin or clothing, as severe frostbite may result.
Step 7. Resistance measures the difficulty with which an electron moves through a material. Use a digital voltmeter on the resistance setting (200 Ω) to record the resistance of a copper RF coil. The small resistance to electrical current (flow of electrons) in metals is due to vibrations of the atoms that interfere with the flow of electrons. Dip the copper RF coil into liquid nitrogen. Caution: liquid nitrogen is extremely cold. Do NOT allow it to come into contact with skin or clothing, as severe frostbite may result. Did the resistance change?
Step 8. Use a digital voltmeter on the resistance setting (2000 Ω) to record the resistance of a CdS semiconductor photocell. Cover the photocell with your hand to block out most of the light. Does the resistance change with light exposure? Do not dip the photocell in liquid nitrogen; the plastic casing will crack!