Given below are two statements: one is labelled as Assertion A and the other is labelled as Reason R Assertion A: In a photodetector, it is desired that the width of the depletion region W be large enough so that most of the photons are absorbed within W rather than in the neutral n and p regions. Reason R: A wider width W results in small junction capacitance. In the light of the above statements, choose the most appropriate answer from the options given below
Assertion A Analysis: Photodetector Depletion Region
Assertion A states that a large depletion region width (W) is desired in a photodetector.
- Purpose: Maximizing photon absorption within the depletion region is crucial.
- Mechanism: Electron-hole pairs generated by absorbed photons are efficiently separated by the electric field in the depletion region. Absorption in neutral regions yields fewer useful carriers.
- Verdict: Assertion A is correct.
Reason R Analysis: Depletion Width and Junction Capacitance
Reason R states that a wider depletion region width (W) leads to small junction capacitance.
- Capacitance Formula: The junction capacitance ($C_J$) is related to the depletion width ($W$) by the approximate relation $C_J \propto \frac{\epsilon A}{W}$, where $\epsilon$ is the permittivity and $A$ is the junction area.
- Relationship: As $W$ increases, $C_J$ decreases.
- Verdict: Reason R is correct.
Evaluating the Connection
Both Assertion A and Reason R are independently correct statements:
- Assertion A highlights the benefit of a wide depletion region for photon absorption.
- Reason R explains that wider depletion regions lead to lower junction capacitance, which benefits detector speed.
- However, the primary goal stated in A (efficient photon absorption) is achieved by having a wide depletion region, not *because* the capacitance is reduced. Lower capacitance is a related physical characteristic, but not the direct explanation for why absorption efficiency necessitates a large $W$.
Conclusion
Since both statements are true but Reason R does not explain the primary reason behind Assertion A, the correct option is that both are correct, but R is not the correct explanation for A.
Important Questions from Photodetectors - Teaching
Consider the following statements for photo diode (PD) and Avalanche Photo Diode (APD)
A. The response time of APD is larger than PD
B. Photo current is a diffusion current for PD and APD
C. APD can handle greater amount of power in comparison to PD
D. Photo current is a minority carrier current for PD and APD
E. compare to ordinary pn diode, power handling capacity of PD is higher.
Choose the correct answer from the options given below :
In a p-i-n photodiode the depletion region thickness can be tailored to
(a) optimize the quantum efficiency
(b) optimize the frequency response
(c) optimize the quantum efficiency but not the frequency response.
(d) optimize the frequency response but not the quantum efficiency.
Out of these
In case a reverse biased photodiode is kept in dark condition, the current flowing through the device corresponds to:
Given below are two statements :
Statement I : In case a reverse biased photodiode is kept in a open light, the current flowing through the device corresponds to value of reverse saturation current.
Statement II : Photovoltaic emf of semiconductor photodiode is that voltage at which the resultant current flowing the device becomes zero.
In the light of the above statement, choose the most appropriate answer from the options given below :
Assertion (A) : Those semiconductor materials that have practically no thermally generated current carriers at room temperature are used as photoconductors.
Reason (R) : These materials are essentially insulators in the dark and carriers generated by the absorption of light cause maximum possible change in the resistivity of the cell.
Select your answer using the codes given below.
