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Transcript
Teacher’s Guide Transistors Overview Students learn that field effect transistors (FETs), like mechanical switches in circuits, control electric current. However, unlike mechanical switches, FETs control electric current by using the field effect. Students explore how the gate of a junction field effect transistor (JFET) turns an electric current through a semiconductor on and off by changing the voltage. Students learn how to use two transistors to build a logic AND gate as well as a logic OR gate. In addition, students are introduced to MOSFETs and carbon nanotube transistors. Learning Objectives Students will be able to: Compare a transistor to a mechanical switch Explain how electric fields obstructs the flow of electric currents Describe how JFETs work Explain how logic gates can be built from transistors Prerequisite Knowledge Students should already have a basic understanding of: Semiconductors: charge carriers, P-type and N-type semiconductors, and depletion regions (See ET “Semiconductor” activity) Coulomb’s Law Logic operations Electronics Background Resources A review of semiconductors and doping: http://www.hscphysics.edu.au/resource/Semiconductors%20Lesson%20outline.pdf The PBS documentary Transistorized!, which discusses the history and operation of the transistor: http://www.pbs.org/transistor/ An explanation of Boolean logic gates, including how to construct a simple adder: http://www.howstuffworks.com/boolean.htm Approximate time for lesson completion: 60 minute Activity Answer Guide Page 1: 1. Describe what happens in the circuit when you click the switch to turn the light off. When the switch breaks the circuit, the electric current is interrupted and the light is off. 2. Describe what happens in the circuit when you click the switch to turn the light on. When the switch connects the circuit, the electric current flows and the light is on. Page 2: 1. What happens to the flow of electrons when you change the amount of negative charge on the island? 2. Take a snapshot that shows a pinched channel. When the island is more negatively charged, the size of the pink area in which the electrons are forced out increases. When the island is less negatively charged, the pink area shrinks. The size of the area determines how much the flow of the electrons is obstructed. Tell students that an analogy of this is a log in a river A log with a large cross section will resist more water flow than a log with a small cross section. 2. What happens to the flow of electrons after the island is moved to a different location? The electrons flow around the island wherever it is placed. 3. What causes the electrons to flow around the island (but not enter it)? According to Coulomb's Law, there is repulsion between the electrons and the negatively charged island. This force prevents the free electrons from entering the pink area. Page 3: 1. Take a snapshot that shows a narrowed channel. 3. Explain why a junction field effect transistor can be used as a switch. When the voltage is increased on the gate, the region that forces electrons out is increased. This reduces the area in which the current can flow. A junction field effect transistor can be used as a switch because it can increase the amount of voltage on the gate, pinch the channel shut, and break the current flow. Alternatively, a junction field transistor can reduce the amount of voltage on the gate and allow the current to flow. 4. The gate-source voltage slider has negative readings. Why? An N-type semiconductor contains an abundance of mobile electrons that carry negative charges. In order to pinch the flow of electrons, you need to apply a negative voltage to the gate. This creates a repulsive electric field for the electrons. Were the voltage positive in this case, the electrons would be attracted to the gates. When you use a light switch to turn the light on, the electrons flow in the circuit. When you use a light switch to turn the light off, the electrons stop flowing. Similar to a light switch, a gate of a field effect transistor can be used to allow the flow of the electrons or stop the flow of electrons. 2. Which of the following graphs best describes the relationship between the drain current and the gate voltage in a junction field effect transistor shown on Page 3? Page 4: (C) 1. Explain how an AND logic gate works based on your observation. 3. In the field effect transistors you have seen in this activity, the flow of electrons is controlled by the voltage applied to the gate. If the voltage of the gate is sensitive to the presence of certain types of molecules or ions, then one can make a sensor based on this effect. Reflect on this idea. If possible, come up with your own idea about how to design a field effect biosensor. I observed that the current can only flow when voltage is applied to both gates (both set at 1).If one of the gates has voltage (set to 0), the circuit is broken. Page 5: 1. Explain how an OR logic gate works based on your observation. I observed that the current could flow when one of the gates has a voltage applied to it (set at 1). When both gates have no voltage (set to 0), the circuit is broken. Page 6: 1. Why does there need to be a layer of oxide in a MOSFET? The oxide layer prevents the electrons in the conduction channel from flowing out of the transistor through the metal gate. Tell students that the oxide in a MOSFET performs the same function as a dielectric in a capacitor—the oxide separates charge. Page 7: No questions. Page 8: 1. Compare a gate of a field effect transistor to a light switch. A field effect biosensor would be monitoring the presence of specific molecules. If such molecules were detected, the sensor would either open or close a transistor. The change in the state would signal the presence of the molecule, and an alarm would go off. Further Extensions Boolean logic Use a logic gates simulator on the Internet, or use TTL chips and breadboards, to assemble a simple binary adder. Transistors as amplifiers Explore how transistors can amplify signals or act as variable resistors. Transistor fabrication Explore the chemistry and quantum theory behind photolithography.