Cut-off wavelength of photo detector

A cut of the wavelength of any photodetector can be defined as if any particular semiconductor can be absorbed photon over a limited wavelength range. The highest bandwidth is known as cut-off wavelength.

Cut off wavelength is determined by band gap energy Eg of a material.

      Cut of wavelength = hc / Eg  = 1.24 / Eg  

Where,

Eg  = Electron volts (eV) 
A typical value of cut of wavelength for silicon is 1.06 micrometer and germanium is 1.6 micrometer.

For example :

In PIN photo diode  Eg  = 1.43 eV, hc = 1.24 , find cut of wavelength = ?

 Cut of wavelength = hc / Eg  = 1.24 / Eg
                                                
                                               = 1.24 / 1.43
                    
                                                = 0.867 micro meter
                       
                                                = 867 nm

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