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Photodiode - Definition, Principle, Applications



Definition of Photodiode

The photodiode is an optoelectronic device whose region of operation is limited to the reverse bias region. It carries current in reverse bias condition that depends upon the intensity of light falling on a P-N junction

Principle of Photodiode

The photodiode is a simple P-N junction diode that is reverse biased with a battery of voltage V (Figure 4,9) Under normal conditions, a reverse saturation current (lo) flows due to thermally generated minority carriers. It is called dark current. The reverse saturation current is independent of applied reverse voltage (reverse voltage opposes the flow of majority carriers and diffusion of minority carriers is unaffected). Minority carrier depends upon temperature, therefore, reverse solution current is a function of temperature When light falls at the P-N junction, light photons impart their energy to valence electron to break the covalent bond. So electron and hole pairs are generated and their concentration depends upon the energy of photons (i.e. intensity of light). These additionally generated electron-hole pairs are added to already present minority carriers and the total quantity of minority carriers increases. Thus increasing the reverse
current and total reverse saturation current l-lo +
Where lo is due to thermally generated minority carriers and I, is due to photogenerated minority ca Since I, is a function of light intensity, so current () of photodiode depends upon
the intensity of Iight

V-I Characteristic Of Photodiode

Characteristics of photodiode have following salient features

V-I  Characteristic Of Photodiode

  1. Current with no light fallng on the junction is called dark current Uo) It is a function of temperature only.
  2. Reverse current mcruases with incrcase un the ntensity of light (mcasured in foot candles).
  3. Reverse current is dependent on the amount of reverse voltage.
  4. if the forward voltage is applied. forward current due to majority carriers flows opposite to reverse current, therefore, the magnitude ofthe resultant current flowing through the photodiode decrease. 
  5. The magnitude of reverse current also depends upon the dıstance of the point of light falling from the junction Larger is the distance lesser will be the current (Since chances of recombination are higher for a point away from the junction).
  6. Forward voltage at which resultant current is zero is called the photovoltaic potential.

Applications Of Photodiode

The photodiode has follow ing application
(1) Light detection s stem
(2) Counting of objects
(3) Rcading of film soundtrack
(4) Light opcrated switches
(5) High speed reading of computer punched cards and tapes
(6) Alarm sy stem
(7)In octocoupler

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