Four-Point Probe Manual 1. Introduction The purpose of the 4-point probe is to measure the resistivity of any semiconductor material. It can measure either bulk or thin film specimen, each of which consists of a different expression. The derivation will be shown in this tutorial. The 4-point probe setup used in the EECS 143 instructional lab consists of four equally spaced tungsten metal tips with finite radius. Each tip is supported by springs on the other end to minimize sample damage during probing. The four metal tips are part of an auto-mechanical stage which travels up and down during measurements. A high impedance current source is used to supply current through the outer two probes; a voltmeter measures the voltage across the inner two probes (See Figure 1) to determine the sample resistivity. Typical probe spacing s ~ 1 mm. Figure 1. Schematic of 4-point probe configuration 2. Theory 1. Bulk Sample In our derivations for this section, we assume that the metal tip is infinitesimal and samples are semi-infinite in lateral dimension. For bulk samples where the sample thickness t >>s, the probe spacing, we assume a spherical protrusion of current emanating from the outer probe tips. The differential resistance is: We carry out the integration between the inner probe tips (where the voltage is measured):