Auto synchronous bandpass filter based on indium-doped CdS

Samples were grown on glass substrates of CdS doped with different levels of indium via the chemical bath technique. The samples obtained were optically characterized to determine their band-gap. A capacitor of parallel plates is constructed with the CdS samples placed between the plates; afterward, light with energy above the band-gap is used to generate carriers in the CdS to modify its dielectric properties and therefore the capacitance. The sample that causes greater current through the capacitor is used to form part of the tank circuit of a bandpass filter because this tank circuit determines the frequency of resonance or passage of the filter, thus forming a bandpass filter whose resonance frequency depends on the intensity of light that impinges on the CdS sample. Finally, the frequency responses of the filter when the sample is not illuminated compared to that when it is illuminated are studied to demonstrate a filter whose resonance frequency can be effectively controlled by light via the use of indium-doped CdS.