A Photosensitive device Principle: Photo conductive effect A pn junction and junction is coated with one of the photosensitive material like Cds, se, zns, Pbs. In a photodiode, photosensitive coating Provided only at the junction. is If light falls slightly from the junction, the photodiode will not respond to the light. Photodiode has a very larger depletion layer width and this is obtained by reducing the cloping conc. of p- & N-region Photodiode has higher sensitivity due to larger depletion layer width →Photodiode can be operated with open circuit or short-circuit condition but the conductivity very small and there are no practical. application. Photodiode is always operated at reverse bias Ge photodiode will respond to visible light si photodiode will respond to IR light. When compare to a normal diode, photodiode: (1) 10 times fostere (11) 100 tighe fasternsitivity Advantage (!!!) Low power handling I disadvantage. →II + Iphoten Light Convex lens working as a window for the flow of light rays Reverse brass is kept under darckness Photochrode will be working as a normal I diode under reverse bias & the current → Reverse saturation current Thermally generated current List for One photodrocle for si photodiode Dark cwirent (Idark). ↓ Thermally generated current ↓ Duft current. When light falls on the convex lens, the max. intensity of this light will be focus at the junction and because of the photon energy a large large no no of covalent bonds are broken and of minority queries are generated and this will inc. the conductivity. This i's called didde a photo conductive effect. The photo i's ON -State $ the current ius I = Io + Iphoton. ↓ davck cwerent Photodiode has two current components (1) To → Thermally generated current OR dark current & it is due to temperature (in Iphoton It is the current parsing in photocliode because of photon energy Photodiode current is the sum of thermally generated current & photon current In a photodiode, photon current is added to thermally generated current.. Iphoton > 7o or Idark. (MA) (ША) Photodiode current flows from N to p Photodiode current is Photodiode cuerent ins current. a reverse current a minority carveror Photodiode current is a diffusion coerent I = Io + Iphoton T Drift L Proton Orffusion I Iphoton (Siffusion cwerent) Photodiode current & Light flux Photodiode cwerent depends on no. of phot falling at the junction. -Photodiode los basically a light operated swit → Photodiode iss a minority carrier injector Photodiode current equation is I = Is + Io [1--eny] whore Is short coreult covient of photo diode. Intensity of light Short circuit current of photodiode linear Inc. with intensity of light falling at junction exponentially increasing function Photodiode characteristics will be plotted in third quadrant. Idark Lumen → Unit for intensity of light →The magnitude of reverse current in the Photodiode increases with intensity of light.. falling at the junction. →For a good photocliode the essential require- -ment is larger. Iphoton ration. Idark Iphoton GIC PO SI PO Better pedformance Iclark 103:1 106:1 Application!一 As a remote control sensor As I light operated switch NOTE!- → If photo cliode is forward brased and light is focussed at the junction → Whe A forward biased photochrode will be working as current ios tavege a normal chiode under FB and i''s due to majority carriers and it. ie: MA When light is focussed at the junction minors caveriers are generated and these minority carriers will be blocked in the FB Photochrode and therefore there ius no. on the forward current effect of light → The forward biased photodiode cannot work a ONI, light operated switch (as it is always never goes to offustate (RB)