The operation of the P/N Tester is based on the SPV (Surface Photovoltage) technique. The excitation is represented by chopped light, which periodically generates free carriers (electron-hole pairs) in large concentration. A high sensitivity probe is capacitively coupled to the sample close to the sample surface. The surface potential barrier of the sample is periodically changed by the chopped light and can be measured by the p/n probe. When the light is on, the sample is near to the flat-band condition. When the light is off, the sample has a larger surface potential barrier. The signal stands for the difference between the potential values measured during the bright and the dark periods of the excitation. Since the sign of the signal depends on the conduction type of the sample, this method can be used for conduction type determination.

The technique has certain limitations:

• Thermal donors (oxygen) can yield false measurements
• Oxide charge on the surface can also adversely affect the measurement
• The measurement result on p-n or n-p structured wafers depends on the layer thickness (The P/N Tester measures either the substrate or the layer, depending on the layer thickness.)
• Residue of passivating materials, acids, and other active chemicals can adversely affect the measurement as well.
• It is difficult to measure as-cut samples in some cases, when the damaged layer is much thicker than the penetration depth of the light used by the P/N Tester