Question

The photodiode is used to detect the optical signals. These diodes are preferably operated in reverse biased mode because:

• A. fractional change in majority carriers produce higher forward bias current.
• B. fractional change in majority carriers produce higher reverse bias current.
• C. fractional change in minority carriers produce higher forward bias current.
• D. fractional change in minority carriers produce higher reverse bias current.

Answer 1

The Correct Option is D

The question asks why photodiodes are preferably operated in reverse biased mode. Let's analyze the options and understand the functioning of a photodiode to provide a well-reasoned answer.

A photodiode is a semiconductor device that converts light into an electrical current. When photons are absorbed in the semiconductor material, electron-hole pairs are generated. These pairs are crucial for the functioning of the photodiode.

There are two types of charge carriers in a semiconductor: majority carriers and minority carriers. Majority carriers are those that are in abundance in a particular type of semiconductor (electrons in n-type and holes in p-type), whereas minority carriers are present in smaller numbers.

When a photodiode is reverse biased:

• The electric field within the depletion region of the photodiode increases.
• This field helps in quickly separating the electron-hole pairs generated by the absorbed photons.
• The movement of these minority carriers across the junction is what contributes to the photocurrent in reverse bias.

The reverse bias condition ensures that:

• The depletion region widens, reducing the recombination of electron-hole pairs.
• There is an increased sensitivity to light as even a small variation in light intensity results in significant changes in the reverse current due to the minority carriers.

Now, let's evaluate the options:

• Fractional change in majority carriers produce higher forward bias current: This option is incorrect because in forward bias, the depletion region narrows, and the role of majority carriers is highlighted, but the efficiency of photodetection decreases.
• Fractional change in majority carriers produce higher reverse bias current: Majority carriers do not significantly contribute to the reverse current in reverse bias.
• Fractional change in minority carriers produce higher forward bias current: Although minority carriers play a role in forward bias, the conditions do not enhance photodiode sensitivity as much as reverse bias does.
• Fractional change in minority carriers produce higher reverse bias current: [Correct Answer] This is correct as the generation of minority carriers significantly contributes to the rise in reverse current under illumination, enhancing the photodetector's sensitivity.

Therefore, the correct answer is that photodiodes are operated in reverse bias because "fractional change in minority carriers produce higher reverse bias current". This operation enhances the sensitivity and efficiency of the photodetection process.