4 Point Probe (4PP)

4 Point Probe

A Four Point Probe (“4PP”) is typically used to measure the sheet resistance of a conducting or semi-conducting material.

  • Easy to use
  • Inexpensive measurement
  • Well understood
  • Probes damage soft materials
  • Can give anomalous results if probes are straddling grains boundaries in polycrystalline material

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Example Outputs

Example plots showing Applied Voltage vs Measured Current (top left), Applied Current vs Measured Voltage (top right) and the resulting computed Sheet Resistance across 25 different measurements (bottom).

Instruments Used for 4PP
Ossila Four Point Probe System

Ossila Four Point Probe System

  • Probe spacing: 1.27 mm
  • Rectangular Sample Size Range:
    • Long-edge Minimum: 5 mm
    • Short-edge Maximum: 60 mm
  • Circular Sample Size Range (Diameter): 5.0 – 76.2 mm
  • Maximum Sample Thickness: 10 mm
  • Acccuracy as low as ±1% for sheet resistance in 10 Ω – 10 kΩ range
  • High reproducibility, with deviation below 1% for most measurements
Sample Requirements
  • Solid phase
  • Must be conducting or semi-conducting
How 4PP Works

In 4PP systems, a set of 4 probes mounted in a line of known dimensions is brought into contact with the sample surface. Typically, the probes are spring-mounted to allow for small variations in the surface topography. A known current is injected and returned through the outer two probes and the voltage drop across the material is measured as a function of this current.

Using Ohm’s law, one then calculates the resistance of the material and with some modeling and analysis can determine the sheet resistance.

This technique is often used in the solar industry to measure transparent conductor sheet resistance. The 4 point probe is one of the main methods for determining metal coating thickness and resistance.

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