AIIMS 2006 Physics Photo-diodes Assertion Reason Question

To tackle the given question, we will first analyze the assertion and the reason provided, and then clarify why the correct answer is A, while also addressing why other options are incorrect.

Assertion:

A p-n junction with reverse bias can be used as a photo-diode to measure light intensity.

This statement is true. A p-n junction diode, when reverse-biased, allows a small amount of reverse current to flow. This current is primarily due to thermally generated carriers. When light (photons) hits the diode, it can generate electron-hole pairs, increasing the current flowing through the diode. This property allows the diode to act as a photodetector, making it suitable for measuring light intensity.

Reason:

In a reverse bias condition, the current is small but is more sensitive to changes in incident light intensity.

This statement is also true. Under reverse bias, the majority carriers are pulled away from the junction, which results in a very small leakage current. However, when light is incident on the diode, the generation of additional electron-hole pairs increases the current. The reverse-biased diode is more sensitive to these small changes in light intensity due to the low dark current (the current flowing in the absence of light). This sensitivity is crucial for applications in optical sensors and photodiodes.

Why the Correct Answer is A:

Both the assertion and the reason are true, and the reason correctly explains the assertion. The reverse bias condition not only allows the diode to function as a photodetector but also enhances its sensitivity to variations in light intensity. Therefore, option A is correct.

Why Other Options are Incorrect:

• Option B states that both Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion. This is incorrect because the Reason directly explains why the Assertion is true—higher sensitivity in reverse bias leads to effective light measurement.

• Option C states that the Assertion is true but the Reason is false. This is incorrect because both statements are true.

• Option D claims that both Assertion and Reason are false. This is clearly incorrect since we have established that both are indeed true.

Relevant Concepts and Formulas:

To further solidify our understanding, let's discuss relevant concepts:

1. Reverse Bias Operation: In a reverse-biased p-n junction:

   • The depletion region widens, and the electric field increases, which reduces the flow of current.
   • The current can be approximated as the sum of the dark current and the photocurrent generated due to light exposure.

2. Photocurrent: The increase in current due to light can be expressed as: $I_{\text{total}} = I_{\text{dark}} + I_{\text{photo}}$ where $I_{\text{dark}}$ is the reverse saturation current (a small constant in reverse bias), and $I_{\text{photo}}$ is the current generated by the light.

3. Sensitivity: The sensitivity of the photodiode can be defined as the change in photocurrent per change in light intensity, which reflects how responsive the device is to varying light conditions.

Conclusion:

The correct answer is A because both the assertion and the reason are true and the reason provides a valid explanation for the assertion. Understanding the operation of p-n junctions in reverse bias and their application as photodiodes is essential for students preparing for NEET/JEE examinations.