Calculation of Two-Phonon Conductivity in Semiconductors

Photoconductivity produced in semiconductors by the two-photon excitation process is analyzed theoretically by considering surface and volume recombinations. The photoconductivity is shown to depend, in general, on the intensity of the excitation light, the thickness $L$ of the crystal, the diffusion length $\frac{1}{\lambda }$ of the free carriers, and the surface-recombination velocity. When the excitation intensity is such that $\lambda$ is much greater than the absorption constant of the two-photon process and $\lambda L\gg 1$, the expression for the photoconductivity reduces to a very simple form and is found to be relatively independent of the surface-recombination velocity. Calculations using various values of surface-recombination velocity and diffusion length are presented.