The Four Probe Method is one of the standard and most widely used method for the measurement of resistivity of semiconductors. The experimental arrangement is illustrated above. In its useful form, the four probes are collinear. The error due to contact resistance, which is specially serious in the electrical measurement on semiconductors, is avoided by the use of two extra contacts (probes) between the current contacts. In this ment the contact resistance may all be high compare to the crystal resistance, but as long as the resistance of the crystal and contact resistances are small compared with the effective resistance of the voltage measuring device (Digital Multirange Volt Meter), the measured value will remain unaffected. Because of pressure contacts, the arrangement is also specially useful for quick measurement on different crystals or sampling different parts of the crystal.
Apparatus Required
Probes Arrangement : It has four individually spring loaded probes. The probes are collinear and equally spaced. The probes are mounted in a teflon bush, which ensure a good electrical insulation between the probes. A teflon spacer near the tips is also provided to keep the probes at equal distance. The whole arrangement is mounted on a suitable stand and leads are provided for the voltage measurement. Fine adjustment (Screw type) is provided to adjust the pressure more precisely of spring loaded probes.
Sample : Germanium crystal in the form of a chip.
Oven : It is a small oven for the variation of temperature of the crystal from the room temperature to about 200° C (max.)