# Question About Indirect vs. Direct Bandgap Semiconductors

eNtRopY
Let's say I have an unknown material and I wanted to determine whether it is has an indirect or a direct bandgap. Is there a standard experiment for doing this without assuming we know the bandgap energy a priori?

eNtRopY

eNtRopY
This thought just occured to me....

Maybe you could bombard the surface of the semiconductor with a specific wavelength of light and look for emissions of another wavelength. If the emitted wavelengths have a different frequency than the impinging wavelengths, then there the bandgap must be indirect because some of the added energy would have to be converted to phonon energy in order for an emission to take place.

Any thoughts?

eNtRopY

So what is

the definition of direct and indirect bandgap? I can't say that I've heard the terms.

eNtRopY

Originally posted by Tyger
the definition of direct and indirect bandgap? I can't say that I've heard the terms.

Well, a direct bandgap has a conductance band minima and a valence band maxima at the same wavevector. An indirect bandgap does not. This becomes important for optical reasons. In a direct bandgap semiconductor, like GaAs, you can have absorption and emission of photons without requiring phonons for momentum conservation.

eNtRopY

indirect bandgap semiconductor:
semiconductor in which bottom of the conduction band does not occur at k=0 at which top of the valence band occur; energy released during electron recombination with a hole is converted primarily into phonon; e.g. Si, Ge, GaP.
(Phonon: A quasiparticle which is a quantized sound wave.)

direct bandgap semiconductor:
semiconductor in which the bottom of the conduction band and the top of the valence band occur at the momentum k=0; energy released during band-to-band electron recombination with a hole is converted primarily into radiation (radiant recombination) wavelength of which is determined by the energy gap; e.g. GaAs, InP.

So, give the semiconductor some energy and if it makes light-it's direct. If you measure the wavelength/frequency of the light, you can get the energy gap of a direct bandgap semiconductor.
I don't think an indirect bandgap semiconductor will produce light, or at least much less than a direct bandgap semiconductor.

P.S. Direct bandgap semiconductors are sometimes reffered to as "optically active" and indirect as "optically inactive".