What is Photodiode?
Working Principle of Photodiode
When a diode is in reverse biased condition, there would be a reverse saturation current flowing through it from positive to the negative terminal of the diode. The unavoidable minority charge carriers cause this reverse saturation current in the semiconductor crystal. The value of this reverse saturation current does not depend on the applied reverse voltage across the diode rather it depends on the concentration of minority charge carriers in the semiconductor crystal. Hence for a certain range of reverse voltage across the diode, this current remains almost constant. We can control the reverse saturation current in a diode by controlling the concentration of minority charge carriers in the semiconductor crystal. We can change the concentration of minority charge carriers in a semiconductor by supplying external energy to the crystal.
In the photodiode, we do the same to control the conductivity of the device. As the name suggests in the photodiode, the pn junction gets exposed in the light. Depending on the intensity of the light, the covalent bonds in the crystal get broken and generate free electron-hole pairs across and nearby the pn junction. As a result, the reverse current in the diode gets increased or in other words the conductivity of the device increases.
NB: – Here it is to be noted that in a photodiode, only the pn junction portion of the diode must be exposed in light this is because if the light falls away from the junction, the electron-hole pairs created away from the junction get sufficient time to recombine hence they can not contribute reverse current. But electron-hole pairs created in the junction or very nearby to the junction, can propagate easily towards opposite polarity due to the influence of electric field across the junction and hence the current through the photodiode gets increased.
Construction of Photodiode
The photodiodes are available in a metallic package. The diode is a p n junction, mounted in an insulated plastic substrate. Then we seal the plastic substrate in the metal case. On the top of the metal case, there is a transparent window, which allows light to entire up to the PN Junction. Two leads, anode and cathode of the diode come out from the bottom of the metal case. A tab extending from the side of the bottom portion of the metal case identifies the cathode lead. 
Symbol of Photodiode
The symbol of a photodiode is just like ordinary diode except for two downward inclined arrows to symbolise the light.
Characteristic of Photodiode
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Dark Resistance of Photodiode
It is true that there are always some minority charge carriers in the semiconductor crystal even in extreme dark condition — these minority charge carriers in the semiconductor crystal present due to unavoidable impurities and natural thermal excitation of the crystal. So even in dark condition, there would be a tiny and constant reverse saturation current in the diode. This current is fixed for a photodiode, and the current is known as dark current. The ratio of maximum withstandable reverse voltage to the dark current of a photodiode is called dark resistance of that diode.
- When we apply light to the diode, the reverse current increase. This relation is linear. The value of reverse current is directly proportional to the intensity of incident light energy.
- If we go on increasing the light intensity, after a certain value of reverse current. The current will not increase further with increasing light intensity. We call this maximum value of reverse current as saturation current of the photodiode.
Applications of Photodiode
There are many applications of the photodiode. We don’t have the scope of discussing all the applications of the photodiode in this short article. We shall discuss here only two popular application of photodiode.
Alarm Circuit Using Photodiode
Hence, we set a light source in such a way, the light always falls on a photodiode. As long as the light has strikes on the photodiode function, there would be a reverse current through the diode, as the diode is already connected in reverse biased condition in the alarm circuit. If an obstruction occurs in the light source, the reverse current in the photodiode comes down to dark current level. The circuit is so designed when the reverse current comes down, an alarm bell starts sounding. We may fit this arrangement in the doorway to defect the entry of people. In this arrangement, the light beam crosses the doorway from one side to another. We fit the photodiode in opposite side of the light source. When any person enters through the door, the light beam breaks and alarm sounds.
Counter Circuit Using Photodiode
When numbers of items go through a conveyer belt, these can be easily counted by using photodiode. Here, we attach a light source on one side of the conveyer belt and a photodiode in opposite side of the belt. The light source and photodiode are fitted in such a way that, the light comes directly to the photodiode. As the light falls on the photodiode there will be reverse recovery current in the circuit. We connect the photodiode with a counter circuit. The light beam breaks, the counter gets one increment. When one item passes the light beam, it breaks the light beam and counter counts the item.
