A photo diode is a reverse-biased silicon or germanium pn junction in which reverse current increases when the junction is exposed to light. The reverse current in a photo-diode is directly proportional to the intensity of light falling on its pn junction. This means that greater the intensity of light falling on the pn junction of photo-diode, the greater will be the reverse current.
When a rectifier diode is reverse biased, it has a very small reverse leakage current. The same is true for a photodiode. The reverse current is produced by thermally generated electron hole pairs which are swept across the junction by the electric field created by the reverse voltage. In a rectifier diode, the reverse current increases with temperature due to an increase in the number of electron-hole pairs. A photodiode differs from a rectifier diode in that when its pn junction is exposed to light, the reverse current increases with the increase in light intensity and vice-versa. This is explained as follows.
When light (photons) falls on the **pn junction, the energy is imparted by the photons to the atoms in the junction. This will create more free electrons (and more holes). These additional free electrons will increase the reverse current. As the intensity of light incident on the on junction increases, the reverse current also increases. In other words, as the incident light intensity increases, the resistance of the device (photodiode) ***decreases. Photo-diode package. Fig. 7.10 (i) shows a typical photo-diode package. It consists of a on junction mounted on an insulated substrate and sealed inside a metal case. A glass window is mounted on top of the case to allow light to enter and strike the pn junction. The two leads extending from the case are labelled anode and cathode. The cathode is typically identified by a tab extending from the side of the case.
Fig. 7.10 (ii) shows the schematic symbol of a photodiode. The inward arrows represent the
Fig. 7.11 shows the basic circuit. The circuit has reverse biased photodiode, resistor R and d.c. supply. The operation of the photodiode is as under :
(i) When no light is incident on the pn junction of photo-diode, the reverse current Ir is extremely small. This is called dark current. The resistance of photo-diode with no incident light is called dark resistance (RR). Dark resistance of photo-diode, RR = Dark current VR
(ii) When light is incident on the pn junction of the photodiode, there is a transfer of energy from the incident light (photons) to the atoms in the junction. This will create more free electrons (and more holes). These additional free electrons will increase the reverse current.
(iii) As the intensity of light increases, the reverse current IR goes on increasing till it becomes maximum. This is called saturation current.
There are two important characteristics of photodiode.
(i) Reverse current-Illumination curve.
Fig. 7.12 shows the graph between reverse current (IR) and illumination (E) of a photodiode. The reverse current is shown on the vertical axis and is measured in µA. The illumination is indicated on the horizontal axis and is measured in mW/cm2. Note that graph is a straight line passing through the origin.
∴ IR = m E
where m = slope of the straight line The quantity m is called the sensitivity of the photodiode.
(ii) Reverse voltage-Reverse current curve. Fig. 7.13 shows the graph between reverse current (IR) and reverse voltage (VR) for various illumination levels. It is clear that for a given reverse-biased voltage VR, the reverse current IR increases as the illumination (E) on the pn junction of photodiode is increased.
There are a large number of applications of photodiodes. However, we shall give two applications of photodiodes by way of illustration.
(i) Alarm circuit using photodiode. Fig. 7.14
shows the use of photodiode in an alarm system. Light
from a light source is allowed to fall on a photodiode fitted in the doorway. The reverse current IR will continue to flow so long as the light beam is not broken. If a person passes through the door, light beam is broken and the reverse current drops to the dark current level. As a result, an alarm is sounded.
(ii) A counter circuit using photo-diode. A photodiode may be used to count items on a conveyor belt. Fig. 7.15 shows a photo-diode circuit used in a system that counts objects as they pass by on a conveyor. In this circuit, a source of light sends a concentrated beam of light across a conveyor to a photo-diode. As the object passes, the light beam is broken, IR drops to the dark current level and the count is increased by one.
Example 7.3. From the reverse current-Illumination curve for a photo-diode shown in Fig. 7.16, determine the dark resistance. Assume a reverse-biased voltage of 10 V.
Example 7.4. A photo-diode is exposed to light with an illumination of 2.5 mW/cm2. If the sensitivity of the photo-diode for the given conditions is 37.4 µA/mW/cm2, find the reverse current through the device.
Reverse current = Sensitivity × Illumination or IR = m × E = 37.4 × 2.5 = 93.5 µA
An optoisolator (also called optocoupler) is a device that uses light to couple a signal from its input (a photoemitter e.g., a LED) to its output (a photodetector e.g., a photo-diode). Fig. 7.17 shows a LED-photo diode optoisolator. The LED is on the left and the photo-diode is on the right. The arrangement shown in Fig. 7.17 is referred to as optocoupling because the output from the LED circuit is coupled via light to the photo-diode circuit. When the LED is energised, current flows through the LED. The light from the LED hits the photo diode and sets up a reverse current through resistor R2. The voltage across the photo-diode is given by :
Vout = VSS – I R2
The output voltage depends on how large the reverse current is. If we vary the LED supply, the amount of light changes and this causes the photo diode current to change. As a result, Vout changes. The key advantage of an optoisolator is the electrical isolation between the input and output circuits; the only contact between the input and output circuits is the stream of light.
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