Surface photovoltage
Surface photovoltage (SPV) measurements are a widely used method to determine the minority carrier diffusion length of semiconductors.
Since the transport of minority carriers determines the behavior of the p-n junctions that are ubiquitous in semiconductor devices, surface photovoltage data can be very helpful in understanding their performance.
As a contactless method, SPV is a popular technique for characterizing poorly understood compound semiconductors where the fabrication of ohmic contacts or special device structures may be difficult.
As the name suggests, SPV measurements involve monitoring the potential of a semiconductor surface while generating electron-hole pairs with a light source.
When a light source creates electron-hole pairs deep within the semiconductor, they must diffuse through the bulk before reaching the surface depletion region.
Even for well characterized materials, uncertainty about the value of the surface recombination velocity reduces the accuracy with which the diffusion length can be determined for thinner films.
Surface photovoltage measurements are performed by placing a wafer or sheet film of a semiconducting material on a ground electrode and positioning a kelvin probe a small distance above the sample.
On a semiconductor whose spectral absorption coefficient is known, the minority carrier diffusion length can in principle be extracted from a measurement of photovoltage versus wavelength.
The temperature of the semiconductor must be carefully controlled during an SPV measurement test thermal drift complicate the comparison of different samples.
Typically SPV measurements are done in an AC-coupled fashion using a chopped light source rather than a vibrating Kelvin probe.
The minority carrier diffusion length is critical in determining the performance of devices such as photoconducting detectors and bipolar transistors.
Even so the SPV is a convenient method of measuring the density of impurity-derived recombination centers that limit device performance.
