What is the minimum wavelength of light that can promote an electron from the valence band to the conduction band in silicon?

Answer 1

Here is a site that lists the energy gap between the valence band and the conduction band.

http://www-inst.eecs.berkeley.edu/~ee130/fa07/lectures/Semiconductor_fundamentals_lec1.pdf

For Si that is 1.12 eV. convert that to wavelength.
1 eV = 1.6 x 10^-19 joules and
E = hc/lambda
Plug in 1 eV (in joules) for E, h is Planck's constant, c is the speed of light at 3.0 x 10^8 m/s, and lambda is what you calculate. Post your work if you get stuck.