Photoconductivity
When light of sufficient energy is absorbed by a semiconductor, the number of free electrons and holes changes and raises the electrical conductivity of the semiconductor. This increase is called photoconductivity and is dependent of the excess electron and hole concentrations and their mobility via the following equation.
[1] \(\Delta\sigma = e \cdot(\mu_{n}\Delta n + \mu_{p} \Delta p)\)
[2] \(\Delta\sigma = e \cdot G\)opt \(\cdot \tau \cdot(\mu_{n} + \mu_{p})\)
Gopt is the optical generation rate, which depends on the incident light intensity, the light spot on the sample and the wavelength.
[3] \(G\)opt \(= \alpha \cdot \phi \cdot(1 - R)e^{\alpha x}\)
Equation 2 implies that the photoconductivity is proportional to the product of lifetime t and the mobility µ. Therefore, it is also proportional to the square of the diffusion length L, which is defined as:
[4] \(L = \sqrt{D \cdot \tau} = \sqrt{\frac{e}{kT} \cdot \mu{\tau}}\)
