▁terms ▁of ▁depth . ▁Know ing ▁the ▁background ▁car rier ▁concentration ▁ ▁and ▁the ▁surface ▁imp urity ▁concentration , ▁the ▁sheet ▁resistance - j unction ▁depth ▁product ▁ ▁can ▁be ▁found ▁using ▁Ir vin ' s ▁curves , ▁which ▁are ▁numerical ▁solutions ▁to ▁the ▁above ▁equation . ▁ ▁Me asure ment ▁A ▁four - point ▁pro be ▁is ▁used ▁to ▁avoid ▁contact ▁resistance , ▁which ▁can ▁often ▁have ▁the ▁same ▁magnitude ▁as ▁the ▁sheet ▁resistance . ▁ ▁Typ ically ▁a ▁constant ▁current ▁is ▁applied ▁to ▁two ▁prob es , ▁and ▁the ▁potential ▁on ▁the ▁other ▁two ▁prob es ▁is ▁measured ▁with ▁a ▁high - imp ed ance ▁volt meter . ▁ ▁A ▁geometry ▁factor ▁needs ▁to ▁be ▁applied ▁according ▁to ▁the ▁shape ▁of ▁the ▁four - point ▁array . ▁Two ▁common ▁arrays ▁are ▁square ▁and ▁in - line . ▁For ▁more ▁details ▁see ▁Van ▁der ▁P au w ▁method . ▁ ▁Me asure ment ▁may ▁also ▁be ▁made ▁by ▁applying ▁high - con duct ivity ▁bus ▁bars ▁to ▁opposite ▁edges ▁of ▁a ▁square ▁( or ▁rect angular ) ▁sample . ▁Res istance ▁across ▁a ▁square ▁area ▁will ▁be ▁measured ▁in ▁ Ω / sq . ▁For ▁a ▁rectangle ▁an ▁appropriate ▁geometric ▁factor ▁is ▁added . ▁Bus ▁bars ▁must ▁make ▁o hm ic ▁contact . ▁ ▁Ind uct ive ▁measurement ▁is ▁used ▁as ▁well . ▁This ▁method ▁measures ▁the ▁shield ing ▁effect ▁created ▁by ▁ed dy ▁curr ents . ▁In ▁one ▁version ▁of ▁this ▁technique ▁a ▁conduct ive ▁sheet ▁under ▁test ▁is ▁placed ▁between