der pauw patterns available for the wafer measurements to be made the measurement system includes an automatic test prober apparatus modified to incorporate a powerful permanent magnet providing a magnetic field to produce a hall effect in the gaas wafer a parametric measurement system coupled through test probes to the van der pauw patterns is programmed to measure sheet resistivity hall voltage and magnetic field strength from which are derived values of sheet hall concentration and mobility that are stored and mapped application_number US 76373691 A description fig1 illustrates the on wafer hall effect measurement system according to the invention a gaas wafer 20 is mounted in a prober test apparatus 22 such as an electroglas model 2001x semi automatic prober manufactured by electroglas inc 2901 coronado drive santa clara calif 95054 the automatic prober test apparatus 22 includes a support table 26 on which a chuck 28 is movable in x y and z directions the automatic prober test apparatus 22 is modified to include a flat permanent magnet 30 on chuck 28 beneath the gaas wafer 20 being tested the magnet 30 is preferably made of samarium cobalt having a field strength in the range of 1 2 kilogauss the magnet 30 has a diameter at least the size of the gaas wafer 20 and a flat upper surface adapted for mounting.