. = nm The wavelength nm corresponds to red colour light. ii) Photodiodes A p-n junction diode which converts an optical signal into electric signal is known as photodiode. Thus, the operation forward biased.
Since electrical energy is converted into light energy, this process is also called electroluminescence. The circuit symbol of LED is shown in Figure . (a). The direction of arrows indicates that light is emitted from the diode.
(a) p – P-type N-type Hole Light Electron Conduction band E g Valence band Recombi- nation (b) Figure . (a) Circuit symbol of LED (b) Schematic diagram to explain recombination process When the p-n junction is forward biased, the conduction band electrons on n -side and valence band holes on p -side diffuse across the junction. When they cross the junction, they become excess minority carriers (electrons in p -side and holes in n -side). These excess minority carriers recombine with oppositely charged majority carriers in the respective regions, i.e.
the electrons in the conduction band recombine with holes in the valence band as shown in the Figure . (b). During recombination process, energy is released in the form of light (radiative) or heat (non-radiative). For radiative recombination, a photon of energy hv is emitted.
For non-radiative recombination, energy is liberated in the form of heat. - - - - - - - - Unit electronics and Communication This reverse current in the absence of any incident light is called dark current and is due to the thermally generated minority carriers. Applications The photodiodes are used in · alarm system · count items on a conveyor belt · photoconductors · compact disc players, smoke detectors · medical applications such as detectors for computed tomography etc. iii) Solar cell A solar cell, also known as photovoltaic cell, works on the principle of photovoltaic effect .
Accordingly, the p-n junction of the solar cell generates emf when solar radiation falls on it. The construction details and cross-sectional view are shown in Figure . . In a solar cell, electron–hole pairs