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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 spaces move and discuss their location and population density. Page 1: Page 3: 1. Why does a semiconductor's ability to conduct electricity increase when temperature rises? (c) 2. Explain your answer to the question to the left based on your observation of the two simulations on this page. 1. The electric current in an N-typ semiconductor flows because (a) there are extra electrons that can move from atom to atom under a voltage. 2. Explain the results of the above simulation using the concept of holes. As I heat up the atoms they shake around more and more electrons are freed up from their nuclei. I can see that when I raise the temperature more current flows, because more electrons are free to move. 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 2: Page 4: 1. Which of the following is NOT true about the simulation of hole movement? 1. Run the simulation for a while until no more electrons move into the P-type area. Take a snapshot of the P-N junction and then identify the depletion region using the rectangle tool. Drag your image into the box below. (d) 2. Why is an electron hole considered as a carrier of positive charge? (d) 3. Select the "What if there is 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 simulation, explain why the concept of a 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 2. Why does the electron movement stop after the depletion region is formed? (f) 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? 3. What causes the electrons to flow continuously under a forward bias voltage? (b) Electrons in the energy gap due to the dopant will enter the conduction band. (b) 4. What causes the electrons to stop flowing under a reverse bias voltage? (d) Page 5: No questions. Page 6: 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. No questions. 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. (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. 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