Question

Given below are two statements ; one is labelled as Assertion $A$ and the other is labelled as Reason $R$

Assertion A: Photodiodes are used in forward bias usually for measuring the light intensity

Reason R: For a p-n junction diode at applied voltage $V$ the current in the forward bias is more than the current in the reverse bias for $\left|V_2\right|>\pm V \geq\left|V_0\right|$ where $V_0$ is the threshold voltage and $V _2$ is the breakdown voltage

In the light of the above statements, choose the correct answer from the options given below

• A. A is true but $R$ is false
• B. A is false but $R$ is true
• C. Both $A$ and $R$ are true but $R$ is NOT the correct explanation $A$
• D. Both $A$ and $R$ are true and $R$ is correct explanation $A$

Answer 1

The Correct Option is B

To determine the correct relationship between the given assertion and reason, it is important to understand the working principles of photodiodes and the operation of p-n junction diodes. 

1. Understanding Assertion ($A$):
   • A photodiode is a semiconductor device that converts light into an electrical current. It is typically used in reverse bias configuration rather than forward bias for measuring light intensity. In reverse bias, the width of the depletion region is maximized, which allows the photodiode to effectively capture and respond to incident photons.
   • Thus, the assertion that "Photodiodes are used in forward bias usually for measuring the light intensity" is incorrect.
2. Understanding Reason ($R$):
   • The statement describes the behavior of a p-n junction diode. In a p-n junction diode, the current in forward bias is indeed significantly larger than in reverse bias. This is because, in forward bias, the potential barrier is reduced, allowing charge carriers to flow through the junction, thereby increasing current. Conversely, in reverse bias, only a small leakage or reverse saturation current flows, as the potential barrier is increased.
   • The mathematical expression mentioned, $\left|V_2\right|>\pm V \geq\left|V_0\right|$, refers to the voltages involved with considered forward and reverse bias conditions up to breakdown.
   • The reason is accurate concerning diode operation up to the point of breakdown voltage ($V_2$) and threshold voltage ($V_0$), making it true.
3. Evaluating the Relationship Between $A$ and $R$:
   • Based on the above explanations, the assertion is false because photodiodes are not typically used in forward bias for measuring light intensity.
   • The reason is true as it correctly describes the current behavior in a p-n junction diode for the given voltage ranges.
   • Therefore, the correct choice is "A is false but $R$ is true".