at optical wavelengths photon absorption takes place through the creation of electron hole pairs and the absorption coefficient determines the probability that this occurs the absorption coefficient is a strongly decreasing function of wavelength there is a wavelength range in which appreciable photocurrent can be generated a long wavelength cutoff exists at the wavelength associated with the semiconductor bandgap 1 1 micrometers for silicon photons of larger wavelengths do not create electron hole pairs and thus do not contribute to the photocurrent a short wave length cutoff also occurs for sufficiently short wavelengths the absorption coefficient is so large that essentially all electron hole pairs are created so close to the surface where the recombination rate is large that they therefor cannot escape and reach the depletion region to be swept across it the short wavelength cutoff increases to longer wavelengths for deeper junctions when the depth of the depletion region is greater than about the bulk minority carrier diffusion length the photon current generation efficiency decreases with increasing depth at all wave lengths but in such a way as to favor long wave lengths thus the flexibility of adjusting the junction depth gained by using an additional photodiode implant mask gives rise to the ability to.