Objectives Observe the differences in electrical behavior between metals and semiconductors Learn the influence of temperature on the resistivity of metals and semiconductors Learn the influence of defects on the conductivity of metals and semiconductors Required Readings Text book: Callister & Rethwisch: "Materials Science & Engineering – An Introduction" Wiley & Sons, 9th Ed. Sections 18.1-18.9; 18.10-18.13 Info sheet Summary of Activities Measure the resistivity of metal wires vs temperature and plastic deformation Use provided data to calculate the resistivity of silicon wafers vs temperature Data Analysis Plot resistivity vs temperature Fit data to calculate Temperature Coefficient of Resistivity Calculate effect of defects on resistivity Study Questions What are definitions of electrical conductivity, resistance, and resistivity? Which of these quantities are material properties and which are not material properties? What are the units of electrical conductivity, resistance, and resistivity? What are the relationships between conductivity, resistivity, and resistance? How does conductivity depend on carrier density and carrier mobility? How do these quantities depend on temperature? What are intrinsic and extrinsic semiconductors? What atoms contribute the majority carriers to electrical conduction in each case? How do resistivity and conductivity depend on temperature for a semiconductor? How do the temperature dependencies of the conductivity and resistivity correlate with the temperature dependencies of carrier density and carrier mobility for metals and for semiconductors? How does conductivity of a metal depend on purity and temperature? How do defects such as dislocations and impurities impact the resistivity of metals? Segment 1 Instructions 1. Segment 1. Measuring the Resistivity of Metals vs T, and vs Deformation 1.1. The metal resistivity vs T samples are thin wires of the metals Al, Cu, and Ni. All of these wires are less than 0.5 mm in diameter so that measuring a resistance over a relatively short length will be easy. 1.1.1. The wires are threaded through 12" long ceramic tubes with 4 holes in them such that the wires make four passes through tubes. The "+" and "-" end of a wire sticks out of the same end of its tube for easy probing. The wire length in the tube is thus 48". 1.1.2. There are four samples for each metal to provide measurements at room temperature (RT), 150C, 200C, and 250C without waiting for a sample to heat from one temperature to the next. 1.2. Lab staff will show you how to set up a sample and the measurements. You will then complete the measurements yourselves, with help if needed. 1.3. The ohm meter needs to be connected to the leads on the sample. The measurement is done using a four point method, also known as a "Kelvin probe". In this method the probes at the very ends of the wire are used to force a current through the sample. Two more probes are connected between these forcing probes to measure the voltage created across the sample by forcing current through it (think about Ohm's Law). The meter "knows" the current, measures the voltage due to the current, and then calculates the resistance. This method eliminates errors due to contact resistance that occur with two point measurements, such with a hand held meter. 1.4. Collecting data for R vs T 1.4.1. Record the temperature and resistance values of the sample from the appropriate meters for sample as room temperature, and at the elevated temperatures. 1.5. Data analysis for R vs T Review the requirements for the presentation, particularly "You must explain the following physical phenomena". Use this to figure out what you need to do with your T and R data. Also review the chapter sections in your textbook about metals and their electrical properties. 1.6. The metal resistivity vs deformation samples are the same types of wires used in the resistivity vs temperature measurements. These wire samples will be rolled thin, introducing a great deal of deformation and defects. 1.6.1. The wire samples are about 150 mm to 200 mm long (6" to 8"). For each sample, measure the resistance of the wire over a known length using the 4 point method as previously done. 1.6.2. Measure and record the initial wire diameter. 1.6.3. Measure the resistance of each wire sample over a known length. 1.6.4. Record the length and resistance values. 1.6.5. Use the rolling machine to roll each wire sample to a final thickness that is from ¼ to 1/3 the initial wire diameter. Do not try to roll the wire as thin as you can. For example, if the initial wire diameter is 100 µm, roll the wire until it is about 25 µm to 33 µm thick. The wire will now be longer than it was initially. 1.6.6. For each sample, measure the final wire width and thickness and record them. 1.6.7. Measure the resistance of each rolled wire sample over a known length. 1.6.8. Record the length and resistance values. 1.7. Data analysis for R vs deformation 1.7.1. Calculate the resistivity of each sample before and after deformation. 1.7.2. You will need to discuss how plastic deformation affects the resistivity of metals in your presentations.