Generally , band-gap is the distance between the valley formed by the conduction band and valance band (a band is how close the orbitals are). If the bands are such that the peak of valance band and valley (or dip ) of conduction band coincide then they are "direct-band gap" e.g: GaAs while if the valley and peak are offset then it is "indirect-band gap" e.g: Silicon

Now, in optical domain we can say energy produced by an electron-hole pair if = to a photon then such a material is direct band gap.

Alternatively , radiative recombination (like the ones in photodetectors) the active material should be direct band gap and to ensure higher radiative recombination the carriers (electron and holes)needs to be of higher concentrations OR higher level of injection of the carriers into the active layer is a necessity (for which we bias the device) .

The active layer must have highly perfect in terms of its crystalline nature, for this there is something called "lattice matching" that is done . Lattice is matched to the substrate material on which active layer is epitaxially grown.

Dear Omar,

Semiconductor photodetectors may consist of two or more materials with different band gaps, therefore their response will not give you unambiguous information about the band gap (or gaps) of one or several materials involved. You have to know the type of your photodetector. The lower energy edge of sensitivity can tell you about the band gap of one material involved. For example, If sensitivity starts from about 1.1 eV, then you most probably have Si photodetector, and Si is an indirect semiconductor.

See the references below about types of photodetectors (from quick Google search)

http://www.iste.co.uk/data/doc_ckcxvperdelf.pdf

https://nanohub.org/resources/9143/download/PHOTODETECTORS.pdf

https://iopscience.iop.org/article/10.1088/0268-1242/18/4/201/pdf

https://www.psi.ch/lmu/DevPsdEN/lutz_sc_rad_detect.pdf

Article Progress in Infrared Photodetectors Since 2000

Best regards

Dear Gazi,

Thank you for your answer. I’m sorry that I have been not clear about the photodetcor type. The device is Metal-Semiconductor-Metal so the semicondctor channel consist of one material.

Kind regards

Omar, in the second reference which I have provided above (I have also added one more reference to an accessible online book), the absorption edges for different types of semiconductors commonly used in commercial photodetectors are given. Depending on lower energy edge of sensitivity of your detector, you can identify the material used in your detector. If you know the material used, then you know is it direct or indirect.

Kind regards

When a photodetectors is made of primarily one semiconducting materials, then spectral response may provide hints or some estimated value for the bandgap of the semiconducting materials. However, usually materials used for photodetectors, such as Si are extrinsic (p-type or n-type doped semiconductor), that impacts their optical response. Moreover, elements such as mid-bandgap absorption, or surface-state absorption, etc. may impact the optical response of a photodetector (Such processes are leveraged to build photodetectors that are sensitive in different energy / wavelength ranges) . As an example you may find the following review article about the Si-based photodetectors and adjusting the optical responses useful. http://article.sapub.org/10.5923.j.optics.20120201.01.html#Sec3.1

Generally , band-gap is the distance between the valley formed by the conduction band and valance band (a band is how close the orbitals are). If the bands are such that the peak of valance band and valley (or dip ) of conduction band coincide then they are "direct-band gap" e.g: GaAs while if the valley and peak are offset then it is "indirect-band gap" e.g: Silicon

Now, in optical domain we can say energy produced by an electron-hole pair if = to a photon then such a material is direct band gap.

Alternatively , radiative recombination (like the ones in photodetectors) the active material should be direct band gap and to ensure higher radiative recombination the carriers (electron and holes)needs to be of higher concentrations OR higher level of injection of the carriers into the active layer is a necessity (for which we bias the device) .

The active layer must have highly perfect in terms of its crystalline nature, for this there is something called "lattice matching" that is done . Lattice is matched to the substrate material on which active layer is epitaxially grown.

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