LED End of Unit Quiz Answer KEY

Part I: Concepts & Content

1. Transverse
2. Semiconductors
3. Decrease,Increase
4. Phosphorus
5. Located in atomic nuclei
6. Green LED
7. Decrease the band gap of the LED
8. All travel at the same speed
9. True
10. False
Part II: Problem Solving

1. (a) 5.46e14 Hz (b) 3.62e-19 J (c) 3.62e-19 J = 2.26 eV = GaP or Gallium Phosphide
2. (a) holes are positive and in abundance on the P-Type semiconductor material; electrons are negative in charge and in abundance on the N-Type region of the semiconductor material. (b) Electrons will flow from negative to positive. (c) No, electrical current will not flow. Notice that in the diagram the electrons pushed from the negative end of the power source will only result in attracting the holes from the p-type material, thus making the depletion zone larger, which means the LED will not illuminate. (d) Since we know that the material is Gallium Nitride, or GaN, which has a bandgap of 3.4 eV, we can use that information to calculate the type of electromagnetic radiation that could be emitted. This LED would emit ultraviolet waves with a frequency of 8.22e14 Hz.
Part III: Critical Thinking

1. (a) In order for a hydrogen atom to emit light, an electron must gain energy and be excited to a higher energy level (greater thann n=1). Light is emitted by the electron when it falls back down to the ground state from the excited state, as energy must be conserved. (b) Based upon where you choose to place the red "energy at collision" arrow, answers may vary. For instance, if the red arrow is at/above n=3, a total of three different photons could be emitted (3 to 1, 3 to 2 or 2 to 1).
2. In answering how a LED works, students should indicate the following:
• Electrons flow from negative to positive
• Energy is absorbed by electrons and then some of it is later released as light
• Electrons move through a circuit and combine with holes within the semiconductor
• For the illustration, students should show electron movement through the crystalline structure (exceptional answers may include valence and conduction bands) and indicate where light is produced (at the N/P type boundary within the semiconductor material
• The color emitted is related to the energy bandgap
• The energy bandgap is determined by the semiconductor material

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