Hands-on in MT102

I teach an introductory course on semiconductor devices and am never sure of the backgrounds of my students. I thought it might be helpful if we completed some hands-on activities that would demonstrate some of the class principles.

Experiment 1: What conducts?
This was a simple experiment to help students see a closed-loop circuit and to explore what materials will conduct.
Supplies:

• Battery (D-cell, AA)


• Battery holder


• Test leads with alligator clips


• Low voltage buzzer (my collection came from Radio Shack)


• Random samples (pencil, aluminum foil, conductive dough, mylar film)

Students built a circuit to make the buzzer sound and then tried adding one of the samples to the loop.

The experiment went well. Students were surprised that the mylar film did not conduct but looks like it is metallic.

Experiment 2: Measure the resistivity.

Supplies:

• Conductive dough


• Multimeter


• Ruler

I gave the students blocks of conductive dough and had them measure the dough with the ruler. Then then measured the resistance of the dough using the multimeter. A simple conversion:

R = resistivity * l/A

enables the students to determine the resistivity. The students then varied the geometry of the dough to see if the resistivity value was the same.

This experiment was not as successful. The dough does not have a static resistivity. I think it might be because the dough uses salt for conduction - so is really somewhat of an electrolyte.

I also had the non-conductive dough (which conducted for some reason - I had added food coloring, this might be the problem) and this seemed to be more stable.

Experiment 3: Measure the approximate band-gap energy of an LED

Supplies:

• 2 AA-batteries


• Potentiometer


• Leads with alligator clips


• Multimeter


• LEDs of various colors (clear housings)

The students created a circuit with the batteries, potentiometer and LED. They measured the voltage across the LED as they varied the potentiometer - watching for the point at which the LED lit. They repeated this going up to the point of the LED turning on and going down to the point of the LED turning off. The voltage measured is approximately an measure of the band-gap energy (eV).

This experiment went well. The students were able to make the measurement and see some variation in the the band-gap energy with the wavelength of light emission. This was good practice for orienting diodes in circuits.

Experiment 4: Measure the I-V characteristics of a pn-diode

We did this experiment before I introduced the I-V curve.

Supplies:

• AA or D-cell battery


• Test leads with alligator clips


• Potentiometer


• Diode


• 2 Multimeters

The students built a circuit with the battery and diode. They placed one multimeter in series with the diode, and set the multimeter to measure current. The other multimeter was placed across the diode and set to measure voltage. As the potentiometer was turned, they recorded voltage and current a few times.

We combined everyone's data and generated an I-V curve for the diode. I am not sure how this experiment went. I will know more at the next class.