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Teachers Guide Semiconductors Overview Students are introduced to semiconductors and the conditions under which a semiconductor will act like a conductor or insulator. The concept of holes is explained. Students are shown how dopant impurities in a semiconductor crystal create conditions for electron conduction. These ideas are then used to describe the operation of a p-n junction (diode) in forward and reverse bias. The quantum-mechanical nature of semiconductors is also explored. Learning Objectives Students will be able to: Explain how increasing temperature allows semiconductors to change from insulators to conductors Describe what a hole is and how it helps us explain electrical conduction in semiconductors Compare and contrast the effect of n-type and p-type dopants on semiconductor conduction Define the depletion region of a p-n junction, explain how it is formed between a p-type and n-type semiconductor, and how the depletion region allows current flow under forward bias but not under reverse bias. Explain how energy levels in atoms form into valence and conduction bands in a crystal and how n-type and p-type dopants donate electrons or holes to these bands Prerequisite Knowledge Students should already have a basic understanding of: Electricity and electric circuits Conductors and insulators (See ET Activity “How Electrons Move”) Crystalline structures and covalent bonds Quantum mechanics (See ET Activity “Introduction to Quantum Mechanics”) Background and resources http://hyperphysics.phy-astr.gsu.edu/Hbase/solids/sselcn.html - Semiconductor Physics for Solid State Electronics http://www.allaboutcircuits.com/vol_3/chpt_3/1.html - An Introduction to Diodes and Rectifiers Activity Answer Guide Page 3: Page 1: 1. The electric current in an N-typ semiconductor flows because 1. Explain why a semiconductor can conduct electricity at high temperature, but not at low temperature. As the temperature increases, more electrons gain enough energy to escape the covalent bonds. These electrons are now free to conduct. Page 2: 1. Select the "Hide electrons" check box and describe below what you observe. The hole moves towards the negatively-charged side of the electric field. A. there are extra electrons that can move from atom to atom under a voltage. 2. Explain your answer to the question to the left. If you dope a pure semiconductor with another type of atom that has more valence electrons, then these extra valence electrons are free to move or collide into the bound electrons facilitating their flow. 3. The electric current in a P-type semiconductor flows because D. all of the above. 2. Explain why holes are considered as positive charge carriers. Electrons will hop towards the positively-charged side of the electric field because they are negatively charged. Holes shift toward the negative charge, so they can be considered to be positively charged. 3. Select the "No hole" check box. Do you observe any electric current? Deselect the check box and observe again. Explain why there is a difference. There is no electric current without the presence of holes. Every space into which an electron could move is filled, so none of the electrons can move. When the hole is present, it acts as an empty space that an adjacent electron can move into, and so electrons can hop from one site to the next. 4. Based on your experiments and observations with the above model, explain why the concept of hole is useful. The movement of electrons between covalent bonds requires an empty space into which adjacent electrons can move. These empty spaces behave like positively-charged particles when an electric field is applied. The concept of a hole allows us to describe how these empty spaces move and discuss their location and population density. 4. Explain the results of the above simulation using the concept of holes. Holes provide places for electrons to move into. When an electric field is applied electrons move into a hole and leave behind a hole, so holes appear to “flow” opposite to electrons. So, you can think of an electric current as the movement of “positive” holes instead of negative electrons. Page 4: 1. Run the model for a while and take a snapshot picture of the P-N junction. Identify the depletion region using the rectangle tool. 2. Explain why electron movement is stopped by the depletion region. The electrons that flow into the p-type material are attracted to the holes and held in place. These electrons repel other electrons and thus block the flow of electrons from the n-type material. 3. Explain why electrons can flow continuously under a forward bias voltage. The battery in the circuit provides enough energy to the electrons to cause the electrons to continually jump from one hole to the next in the p-type region making room for the next group of electrons to flow from the n-type region to the ptype region. 4. Explain why electrons cannot flow continuously under a reverse bias voltage. 3. In a P-type semiconductor, the extra holes provided by the dopant atoms can be found in the: (a) Valence band 4. At very low temperatures, electrons in an intrinsic semiconductor will be found in the: (a) Valence band The energy from the battery pushes electrons away from the junction, and all of the electrons collect on the far side of the n-type region. This inhibits electron flow. Page 6: 1. Take a snapshot picture of the energy band structure of an N-type semiconductor and annotate it to illustrate how electrons would move when temperature increases. Page 7: 1. Which of the following about semiconductors are true? Select all that apply. A. They are sometimes insulators and sometimes conductors. D. Their electrons are bound to atoms but can become loose when temperature increases. 2. Which of the following about holes are true? Select all that apply. B. A hole moves independently of atoms like a free electron. C. A hole hops from one bond site to another. 3. Check all statements that are TRUE regarding the depletion region. 2. Take a snapshot picture of the energy band structure of a P-type semiconductor and annotate it to illustrate how holes would move when temperature increases. (a) A depletion region is formed wherever p-type and n-type semiconductors are in contact. (d) Applying forward bias to a p-n junction allows electrons to flow across the depletion region. 4. An N-type semiconductor initially exists at a very low temperature. As the temperature is slowly increased, which of the following events will happen first? (b) Electrons in the energy gap due to the dopant will enter the conduction band. 5. The power plugs in your home use an alternating voltage to run your electronic devices. This means that the polarity of the voltage switches back and forth many times a second, unlike a battery. The upper image to the left shows a graph of how the voltage changes. If a P-N junction were to be connected to the alternating voltage from your wall socket, describe how current would flow through the P-N junction and why it would flow that way. Current would flow through the p-n junction when the alternating voltage created a forward bias condition in the p-n junction, and would not flow when the p-n junction was in reverse bias. The depletion region would block the flow of current when the p-n junction was in reverse bias, but would not block the current flow in forward bias. Further Extensions Examine the current-voltage behavior of p-n junctions by measuring the ideality factor of diodes Explore the current flow through a diode visually with a light-emitting diode