Hall Effect Measurements are Essential for Characterizing High Carrier Mobility in Materials
Monday, 01 August 2011
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Enhancing Hall Effect Measurement Confidence
Building good measurement practices into a Hall effect system’s design will enhance its measurement integrity.
- Always incorporate electrostatic shielding to minimize electrical interference. Electrostatic coupling or interference occurs when an external electric field injects error signals into the circuit under test. Minimize this by shielding the sample and all sensitive measurement circuitry, connecting the shield to the test system’s low reference, and using shielded cabling.
- Ensure the test system is properly grounded. Prevent ground loops by grounding all equipment in the test system at a single point: the circuit reference common point.
- Use guarding to reduce the effects of leakage current. This includes the use of a guarded current source, guarded voltmeter, and triaxial rather than coaxial cables.
- Allow sufficient settling time before making measurements. When measuring a high-resistance sample, the capacitance in the cabling, combined with the high-resistance sample, creates a large RC time constant, requiring significant time for the voltage to stabilize. To determine the amount of delay time needed, monitor the voltage as a function of time.
- Regularly verify the system’s performance. The simplest way to do this is to build a test structure using four resistors of equal value of similar magnitude to the sample’s resistance (Figure 7). Compare measurements on the test structure with expected results.
- Minimize sources of thermoelectric voltages. Whenever possible, use the same materials in all connections, such as all-copper connections and copper wire. Reverse the current source’s polarity and average the readings to eliminate EMF errors. Always allow the test equipment to reach thermal equilibrium before starting testing, and minimize temperature gradients across the sample.
Characterizing the mobility of new materials is essential to semiconductor technology innovations, so making accurate Hall effect measurements will continue to be important.
This article was written by Robert Green, Senior Market Development Manager at Keithley Instruments, Cleveland, OH. For more information, visit http://info.hotims.com/34457-121. For additional system configurations, view the Keithley online webinar, “Hall Effect Measurement Fundamentals,” available at www.keithley.com/events/semconfs/webseminars.