Photoconductive Properties of Semiconductors
Objective: The objective of this lab is to quantitatively determine how the electrical resistance of a cadmium sulfide photocell varies as a function of light intensity (distance).
Review of Scientific Principles:
Semiconductors often have the ability to respond to various forms of electromagnetic radiation. Silicon, germanium, gallium arsenide and cadmium sulfide are materials that can have opto-electronic effects, which means their electrical properties are responsive to light. This is due to the energy inherent in light radiation. Absorbing this energy can make some of the valence band electrons move to the conduction band. As a result, the conduction characteristics of the material change. In this lab, you will explore a cadmium sulfide "photoresistor" as it is exposed to various intensities and colors of light. You will be looking for possible connections between light intensity or energy and conductivity.
Opto-electronic devices such as photo-resistors, photo-diodes, and photo-transistors are being used more each year. These devices are commonly found in switches for outdoor lighting, on-off switches for pole lights and security systems. The "electric eye" can be used for a dimmer switch on the headlights of a car. The photo-resistor is also an important device in fiber optic communications. Photo-laser diodes are often found in the classroom in the form of pen sized laser pointers.
Time: One hour
Materials and Supplies:
cardboard tube (approximately one meter long and 3 to 5 cm diameter)
black electrical tape and duct tape
cadmium sulfide photocell
light bulbs- white, screw mini-base, 3 to 12 volt (Radio Shack)
miniature lamp socket- screw base
voltage supply ( 0 to 12 volts DC )
digital multimeter or ohmmeter
General Safety Guidelines:
* The leads of the photocell and the light bulb glass are fragile,
* Avoid electrical shock.
* The leads of the photocell and the light bulb glass are fragile, handle carefully.
* Avoid electrical shock.
1. Obtain a cardboard tube from wrapping paper (or make a roll from cardboard
into about a 60 to 70 centimeter tube with about a 5 cm diameter and tape so it
will hold its cylindrical shape). The diameter must be large enough to allow the
lamp socket to move through the tube while not allowing light to get in around
2. Press the electrodes of the CdS cell through a piece of duct tape large enough to
cover the open end of the tube so that the cell is inside the tube and the electrodes
are on the outside of the tube. (Several small pieces of black electrical tape work
3. Place this tape with the cell over one end of the tube and fold the tape onto the
tube so that no light can get into that end.
4. Connect one end of each of the wires to the lamp socket by bringing the wire up
through the holes in the base of the socket and securing under the screw.
5. Tape the socket to the end of the meter stick so that the bulb extends away from
the end of the stick.
6. Tape the wires along the meter stick allowing the wire to extend past stick.
7. Insert the socket end of the meter stick into the open end of the cardboard tube
and slide the socket toward the CdS cell at the opposite end. The socket should
slide freely, but not allow light to go past into the tube. If the space around the
socket is too large, wrap some layers of tape on the outside of the socket.
8. Withdraw the socket, place a bulb in the socket and connect the wires to the
power supply or batteries. Use the proper voltage for the light bulb being used.
9. Make sure the bulb works, then turn it off.
10. Connect the multimeter to the CdS photocell, and set to measure resistance in
kiloohms. There should be a reading on the meter.
11. With the light off, slide the bulb into the tube until it just touches the CdS cell.
12. Take a resistance reading and record in the data table for a dark measurement.
13. Turn the light on and immediately take a reading for 0 cm from the cell.
14. Pull the meter stick and light out of the tube a distance of 10 cm and take
15. Continue pulling the light out and taking readings for each 10 cm until reaching
about 60 or 70 cm.
16. Turn the light off and carefully peel the duct tape and photo cell from the end of
the tube in a way that it can be used again.
17. Using a small piece of colored filter supplied by your teacher, cover the CdS
cell and press the filter against the sticky tape to hold it in position over the cell.
18. Push the light into the tube and again take readings as in steps 11 through 16.
19. Repeat with the other colored filters.
Record the observed resistance values for each color at the given distances.
For each light, graph the resistance on the vertical axis and the distance on the horizontal axis. (Use one graph if the sizes of the resistances are comparable.)
1. What is the general shape of the graph?
2. What happens to the resistance as the distance increases (as
the light intensity goes down)?
3. What happens to the resistance as the color (energy) changes, assuming a fixed
distance, such as 20 cm?
4. Can you provide an explanation for either of the above phenomena?
*Teacher preparation time is approximately 30 minutes.
* Photocells and 6 volt miniature lights are available at Radio Shack.
* Filters of different colors may not allow the same amount of light intensity to pass so be careful in comparing results from different colors.
* Time may dictate whether all these colors can be done by all the groups. Perhaps different groups could be assigned different colors and class data collected. The experiment may be done as a demonstration if supplies are limited.
* The light intensity is inversely proportional to the square of the distance, and the resistance is inversely proportional to the intensity of the light.
* An extension exercise could be to have the students work on an electric eye project.
all data in kiloohms
1. What is the general shape of the graph? Exponential; the intensity of the light is inversely proportional to the square of the distance, so the graph should resemble a parabola (I = k * D2). Other factors can also influence the shape of the graph. It is not expected that the relationships will follow a perfect pattern.
2. What happens to the resistance, in general, as the distance increases, that is as the
intensity goes down? The resistance becomes larger.
3. What happens to the resistance as the color (energy) changes, assuming a fixed distance,
such as 20 cm?
4. Can you provide an explanation for either of the above phenomena? When the intensity of the white light goes down, fewer valence electrons are promoted into the conduction band. As a result, the resistance of the photocell increases. The variation of measured resistance because of different colors of light hitting the photoresistor may be dependent on several factors. One factor is that the intensity of the light passing through the filter may vary because of the optical
density and thickness of the material. Also, each color of light has a specific energy (E = hf). Therefore, the DMM readings for the colored light may differ from white light.
Experiment Design and Application Project
The Electric Eye
In the photoconductivity lab, where a cadmium sulfide photocell was used, you used only visible (optical) photon energy sources. That is, only light bulbs were used. Can you extend this laboratory concept by using non-visible energy sources? (For example, "black lights" or ultraviolet (UV) bulbs are now quite common. They could extend your energies even higher than your blue light did. Infrared (IR) diodes (available at Radio Shack) could be used to extend you energies to lower values.
Using these suggestions as a starting point, can you design another experiment where data can be taken for other kinds of electromagnetic radiation? How will you know if an infrared diode is working, if you cannot see it? Will you test other kinds of photocells besides CdS? What kinds of things do you hope to discover about these materials? Do all photocells respond to electromagnetic radiation in exactly the same way? Can you plot the curves? Can you explain why? Which devices would make the best "electric eyes"? Can you design a simple electric eye circuit and test it under actual conditions?
Today electric eyes are all around us on farms for turning on outside lights, on automatic street lights and on road constructions sites (for night time flashing lights).
Thermal Variation of the Resistivity of a CdS Photocell
Under constant lighting conditions, such as ordinary daylight, vary the temperature of the CdS photocell as much as possible. Using the DMM, measure its resistance for each thermal environment from low to high temperatures. What is the relationship between temperature and resistivity in the photocell?