SPV stands for Surface PhotoVoltage, a phenomenon referring to the creation of a voltage at a semiconductor surface as a result of light hitting the semiconductor. In 1961, A. M. Goodman published a paper explaining how to calculate diffusion length by comparing SPV signals at various wavelengths. The technique became popular, and when people talk about SPV they are usually referring to the technique to measure diffusion length rather than the SPV phenomenon.
Light hitting a semiconductor is absorbed by the semiconductor material, and its intensity decays exponentially as a function of depth. Longer wavelengths penetrate deeper into the semiconductor than short ones. For wavelengths below 1.13µm (IR), each photon of light creates one hole-electron pair.
There is no electric field within the bulk semiconductor material, so holes and electrons created there drift randomly. At the surface of the semiconductor material there is often a depletion region, and the electric field in the depletion region is such that it accelerates minority carriers to the surface. Thus, electrons drifting in the bulk of p-type material are accelerated to the surface if they enter the depletion region, whereas in n-type material it is the holes that are accelerated to the surface if they enter the depletion region.
As excess holes and electrons, created by light, drift in the bulk of the semiconductor material they may recombine. Recombination tends to occur at recombination centers, caused by contamination or defects in the crystal lattice, or at surfaces. Of course carriers that recombine in the bulk can never reach the surface to form a surface photovoltage.
The SPV technique for measuring diffusion length uses multiple wavelengths of light. The longer the wavelength the deeper the penetration into the semiconductor material and the more likely excess carriers will recombine and not reach the surface to produce a surface photovoltage.
