During this lecture, we will take a close look at the fundamentals of Photodiodes. We will explore how they work, their characteristics, and why they are used in reverse bias. Additionally, we will analyze the advantages, disadvantages, and various applications of Photodiodes.
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Photodiode
- The photodiode is a semiconductor p-n junction device whose region of operation is limited to the reverse bias region. The below figure shows the symbol of the photodiode and the working principle of the photodiode.
Working Principle of Photodiode
- The photodiode is connected in reverse bias condition as shown in the above figure. The depletion region width is large. Under normal conditions, it carries a small reverse current due to minority charge carriers. When light is incident through the glass window on the p-n junction, photons in the light bombard the p-n junction, and some energy is imparted to the valance electrons. Due to this, valance electrons are dislodged from the covalent bonds and become free electrons. Thus more electron-hole pairs are generated. Thus the total number of minority charge carriers increases and hence the reverse current increases. This is the basic principle of operation of the photodiode.
Characteristics of Photodiode
- The photodiode is designed such that it is sensitive to light.
- When there is no light, the reverse-biased photodiode carries a current that is very small and called a dark current. It is noted as I_{\lambda}. it is purely due to thermally generated minority carriers.
- When light is allowed to fall on the p-n junction through a small window, photons transfer energy to valance electrons to make them free. Hence reverse current increases. It is proportional to the light intensity.
- The below figures show the photodiode characteristics. One figure shows the relation between reverse current and light intensity while the second figure shows the 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 the light intensity.
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Why is a Photodiode Operated in Reverse Bias
- The reverse current without light in the 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 control of the current instead of the light. The change in forwarding current due to light is negligible and can not be noticed. The resistance of the forward-biased diode is not affected by the light. Hence to have a significant effect of light on the current and to operate the photodiode as a variable resistance device, it is always connected in reverse-biased conditions.
Small Signal Model of Photodiode
- The below figures show the small signal model for Photodiode. One figure shows a Photodiode represented by an ideal junction diode in parallel with a current source which is proportional to the light intensity.
- The model in the figure assumes that the diode is heavily reversed biased, and hence that the side may be replaced by its reverse resistance R. This model also includes the effect of barrier capacitance C and the ohmic resistance r. The typical values for barrier capacitance, reverse resistance, and ohmic resistance are of the order of,
C = 10 pF, R = 50 M\Omega, r = 100 \Omega
- In both, figures, the symbol L represents light flux in lumens, and K is a proportionality constant in the range of 10 to 50 mA/lumen.
Also Read: Tunnel Diode: Definition, Characteristics & Applications
Sensitivity of Photodiode as a Function of Illumination
- We know that current in a revere-biased semiconductor photodiode depends upon the diffusion of minority carriers to the junction. if the radiation is focused on 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.
Photodiode as a Voltage Cell
- When the photodiode is illuminated without any biasing, there is an increase in the number of holes on the p-side and the number of electrons on 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 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.
Advantages of Photodiode
The advantages of a photodiode are,
- can be used as a variable resistance device
- Highly sensitive to the light
- The speed of operation is very high.
Disadvantages of Photodiode
The various disadvantages of Photodiode are,
- The dark current I_{\lambda} is temperature dependent.
- The overall photodiode characteristics are temperature dependent and hence have poor temperature stability.
- The current and the change in current are in the range μA which may not be sufficient to drive other circuits. Hence amplification is necessary.
Applications of Photodiode
- The two commonly used systems using photodiodes are alarm systems and counting systems.
1. The photodiode in an alarm System
The below figure shows a photodiode employed in an alarm system.
- The reverse current I_{\lambda} continues to flow as long as the light beam is incident on the photodiode. When the light is interrupted, the current I_{\lambda} drops to the dark current level. This initiates the alarm system to sound the alarm.
2. Photodiode in the counter operation
- The below figures show a photodiode used to count the items on a conveyor belt. As each item passes, the light beam is broken. Thus reverse current I_{\lambda} drops to the dark current level. In this activity, the counting mechanism and the counter are increased by one.
