# Photoconductance decay

The photoconductance decay method, in which the excess conductance is measured after a very short light pulse (less than 100 ms), is used to measure the effective minority charge carrier lifetime in silicon wafers. This method has been widely used to sense the photoconductivity without making electrical contact with the wafer. The most conventional method to measure the effective lifetime is using the quasi-steady-state photoconductance (QSSPC) measurement method using Sinton Lifetime measurement tool [1].

The total recombination in a sample of thickness W can be expressed in terms of the average excess minority carrier density, Δnav, and an effective minority carrier lifetime, τeff,

$J_{ph}=\frac{\mathrm{\Delta}n_{av}qW}{\tau_{eff}}$, (1)

where under steady state illumination, the photogenerated excess electron and hole densities are same (Δn = Δp). Therefore, the excess conductance σL can be expressed as:

$\sigma_L=q\left(\mathrm{\Delta n}\mu_n+\mathrm{\Delta p}\mu_p\right)W=\ q\mathrm{\Delta n}\left(\mu_n+\mu_p\right)W$, (2)

where μp and μn in are the hole and electron mobilities in silicon respectively. Therefore, the effective minority carrier lifetime can be expressed as:

$\tau_{eff}=\frac{\sigma}{\left[J_{ph}\left(\mu_n+\mu_p\right)\right]}$.   (3)

Below a short video is shown of a minority carrier lifetime measurements using a Sinton WCT-120 tool at UNSW Sydney.

[1] – R. A. Sinton and A. Cuevas, “Contactless determination of current-voltage characteristics and minority-carrier lifetimes in semiconductors from quasi-steady-state photoconductance data,” Applied Physics Letters, vol. 69, p. 2510, 1996. Available: https://doi.org/10.1063/1.117723