Problem X13

For this problem, go to: https://phet.colorado.edu/en/simulation/semiconductor . You should be able to run this simulation in a Chrome browser.

This simulation allows you to create semiconductors, see how doping changes their energy level structure, and apply voltages. The energy level diagram for the system is shown on the left. In the simulation, electrons are blue circles. You might find it useful to compare what you’re seeing in the simulation to Figures 7.5 and 7.6 in the supplementary text.

The Energy Levels of Semiconductors & the Role of Dopants:

  1. To start, the simulation displays an undoped semiconductor. Look at the electrons shown in the energy level diagram. Why are there two in each level? Try adjusting the voltage, using a range of both positive and negative values. Can you get a current to flow? Why not? (Hint: to get a current to flow, some electrons need to move to a higher energy level. Why can that not happen in this simulation of the updoped semiconductor?
  2. Return the voltage to zero and dope both semiconductors to make them n-type semiconductors. To do this, click on the green 'N Type' dopant cluster and drag to both of the purple squares. How are n-type electrons shown on the energy level diagram? Increase the voltage to 0.1 V. Why is it that current can flow now whereas it couldn't for the undoped case? (Hint: how much energy is needed to get the electrons to go up a level in the n-type semiconductor? Watch the top row of electrons when the voltage increases from 0 to 0.1V.) Can you get a current to flow in both directions?
  3. Repeat part, (b) but using p-type semiconductors.
  4. Based on what you've seen in the simulation, summarize why doping a semiconductor makes it easier for current to flow. Your explanation should include the terms valence band, conduction band, and band gap.

PN Junctions, Diodes, and LEDs:

  1. Set up a PN junction with the p-type semiconductor on the right, the n-type semiconductor on the left (like Figure 7.6), and the voltage set to zero. You should see that a couple of extra electrons have moved to the p-type semiconductor. What do these electrons represent? Where did they come from?
  2. Change the voltage to -4. Why is current able to flow initially, but then stops?
  3. Change the voltage to 4. Why is current able to flow?
  4. When current is flowing across the PN junction, electrons are dropping from a higher energy to a lower energy. What happens to that lost energy? Hint: this is how LEDs work.
  5. Explain why a PN junction only allows current to flow in one direction. Your explanation should include a description of energy levels.