Question

Q/ Silicon photodiodes, CCDs, and PMTs are used as detectors (signal transducers) in UV Visible spectrometers. Describe the principles of operation of each and their advantages and limitations.

Answer 1

Q. Silicon Photodiodes are silicon doped chips those cause current to fluw thourgh them when photons strike them.A photodiode is a p–n junction or PIN structure. When a photon of sufficient energy strikes the diode, it creates an electron-hole pair. This mechanism is also known as the inner photoelectric effect. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced.

CCD are capacitors. A charge-coupled device (CCD) is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time. CCDs move charge between capacitive bins in the device, with the shift allowing for the transfer of charge between bins.

PMT detector ( photomultiplier) members of the class of vacuum tubes, and more specifically vacuum phototubes, are extremely sensitive detectors of light in the ultraviolet, visible, and near-infrared ranges of the electromagnetic spectrum. These detectors multiply the current produced by incident light by as much as 100 million times (i.e., 160 dB), in multiple dynode stages, enabling (for example) individual photons to be detected when the incident flux of light is low.

hence photodiodes are good for detecting presence of light, CCD are good for use of access to pixel data ( the light frequency in each pixel of reciever) and PMT can be used for detecting presence of light of very small intensity