Photodiode

       The photodiode converts optical energy to electric current.

       The photodiode is a semiconductor p-n junction device whose region of operation is limited to the reverse region. The Fig. 1(a) shows the symbol of photodiode while the Fig. 1(b) shows the working principle of photodiode.

       The photodiode is connected in reverse biased condition. The depletion region width is large. Under normal condition, it carries small reverse current due to minority charge carriers. When light is incident through glass window on the p-n junction, photons in the light bombard the p-n junction and some energy is imparted to the valence electrons. Due to this, valence electrons are dislodged from the covalent bonds and become free electrons. Thus more electron-hole pairs are generated. Thus total number of minority charge carriers increase and hence the reverse current increases. This is the basic principle of operation of photodiode.

1. photodiode Characteristics

       The photodiode is designed such that it is sensitive to the light.When there is no light, the reverse biased photodiode carries a current which is very small and called dark current. It is denoted as I_λ. It is purely due to thermally generated minority carriers. When light is allowed to fall on a p-n junction through a small window, photons transfer energy to valence electrons to make free. Hence reverse current increases. It is proportional to the light intensity. The Fig 2. shows the photodiode characteristics. The Fig. 2(a) shows the relation between reverse current and light intensity while the Fig. 2(b) shows relation between reverse voltage and reverse current at different light intensities. It can be seen that reverse current is not dependent on reverse voltage and totally depends on light intensity.

2. Use of Photodiode as Variable Resistance Device
       Consider a typical photodiode with dark current I_λ= 20 A    at V_R = - 2V

       If now photodiode is illuminated with 2500 lm/m² (Lumens per square metres) then current changes to 350 µA at same reverse voltage.

       This shows that the photodiode can be used as a variable resistance device controlled by light intensity. It is also called photoconductive device. The response of photodiode is very fast hence change in resistance from high to low or otherwise is also very fast. Hence it can be used in variety of applications.

3. Why to be used in Reverse Biased?

       The reverse current without light in diode is in the range of µA. the change in this current due to the light is also in the range of µA. Thus such a change can be significantly observed in the reverse current. If the photodiode is forward biased, the current flowing through it is in Ma. The applied forward biased voltage takes the control of the current instead of the light. The change in forward current due to light is negligible and can not be noticed. The resistance of forward biased diode is not affected by the light. Hence to have significant effect of light on the current and to operate photodiode as a variable resistance device, it is always connected in reverse biased condition.

4. Small Signal Model of Photodiode

       The Fig. 3 shows the small signal model for photodiode. In Fig. 3(a) a photodiode is represented by an ideal junction diode in parallel with a current source which is proportional to the light intensity. The model in the Fig. 3(b) assumes that the diode is heavily reversed biased, and hence that the diode may be replaced by its reverse resistance R. This model also includes the effect of barrier capacitance Cc and the ohmic resistance r. the typical values for barrier capacitance, reverse resistance and ohmic resistance are of the order of

       In both the figures, the symbol L represents light flux in lumens, and K is a proportionality constant in the range 10 to 50 Ma/lumen.

5. Sensitivity with Position of Illumination

       We know that the current in a reverse biased semiconductor photodiode depends upon the diffusion of minority carriers to the junction. If the radiation is focused into a small spot far away from the junction, the injected minority carriers may recombine before diffusing to the junction. This results in a much smaller current. On the other hand, if the radiation is focused from near the junction more current will result. Therefore, we can say that the photocurrent is a function of the distance from the junction at which the light spot is focused. This is illustrated in the graph shown in the Fig. 4.

6. Photodiode as a Voltage Cell

       When the photodiode is illuminated without any biasing, there is increase in the number of holes in the p-side and the number of electrons in the n-side. Due to this, minority carriers are swept across the junction. We know that, the barrier potential is negative on the p-side and positive on the n-side. This barrier potential tends to reduce because of the the flow of minority carriers. When an external circuit is connected across the diode terminals, the minority carriers will return to the original side via the external circuit. The electrons which crossed the junction from p to n will now flow out through the n terminal and into the p-terminal. This means that the device is behaving as a voltage cell with the n-side being the negative terminal and the p-side the positive terminal. Thus, the photodiode is a photovoltaic device as well as a photoconductive device.

7. Advantages

       The advantages of photodiode are,

1. Can be used as variable resistance device.

2. Highly sensitive to the light.

3. The speed of operation is very high. The switching of current and hence the resistance value from high to low otherwise is very fast.

8. Disadvantages

       The various disadvantages of photodiode are,

1. The dark current I_λ is temperature dependent.

2. The overall photodiode characteristics are temperature dependent hence have poor temperature stability.

3. The current and change in current is in the range of A which may not be sufficient to drive other circuits. Hence amplification is necessary.

9. Photodiode Applications

       The two commonly used systems using photodiode are alarm system and a counting system.

       The Fig. 5 shows a photodiode employed in an alarm system.

       The reverse current I_λ continues to flow as long as light beam is incident on the photodiode. When the light is interrupted, the current I_λ drops to the dark current level. This initiates the alarm system sounding the alarm.

       The Fig. 6 shows a photodiode used to count the items on a conveyor belt. As each item passes, the light beam is broken. Thus reverse current I_λ drops to the dark current level. This activates the counting mechanism and the counter is increased by one.

10. Photodiode Control Circuit

       The Fig. 7 shows the typical photodiode control circuit. When there is no light incident on the photodiode, the current through the photodiode is negligible, dark current.

       Part of this current is a current through R₂. Such a small current through R₂ keeps the voltage drop across R₂ low, making transistor and relay 'OFF'. When light is incident on the photodiode the current through diode and hence the current through R₂ is sufficient to forward bias both the junctions, making transistor and relay 'ON'.