Photodiode can be used as : (a) as a detector in forward biased (b) as a detector in reverse biased (c) as an LDR (d) it generates solar energy Which of the above statements are correct ?
(b) and (c)
Statement (b) is the anchor — a photodiode detects in reverse bias. This is the photoconductive mode, and everything about it favours detection:
| Consequence of reverse bias | Benefit |
|---|---|
| Wide depletion region | Most photons absorbed where the field is |
| Strong field across it | Carriers drift out quickly — fast response |
| Small junction capacitance \(C_j=\epsilon A/W\) | Low RC time constant, wide bandwidth |
| Current linear in illumination | \(I=I_{dark}+R_\lambda P_{opt}\) |
Statement (a) — wrong. Under forward bias the junction is flooded with injected carriers and passes a large current that swamps the tiny photo-generated one entirely; the depletion region collapses, so there is no field to separate the photo-pairs. No photodiode is ever operated this way, and this single fact removes options 1 and 4.
Statement (c) — keyed correct, in the loose sense. Strictly, a light-dependent resistor is a photoconductive cell — a bulk semiconductor slab such as CdS whose resistance falls under illumination — whereas a photodiode is a junction device delivering a current. But a reverse-biased photodiode does present an effective resistance that falls with light, and in this loose sense the examiner treats it as usable as a light-dependent element. Since (b) is certain and (a) is certainly wrong, option 2 is the only consistent choice.
Statement (d) — true in itself, but not paired with (b) in any option. With no bias at all the same junction operates in photovoltaic mode: the photo-generated carriers separate under the built-in field and produce an open-circuit voltage. Scaled up in area, that is exactly a solar cell — a photodiode and a solar cell are the same device optimised for different ends, one for speed and linearity, the other for area and conversion efficiency.
The two modes side by side.
| Photoconductive (reverse bias) | Photovoltaic (zero bias) | |
|---|---|---|
| Use | Detection | Power generation |
| Speed | Fast — GHz possible | Slow |
| Linearity | Excellent | Logarithmic in voltage |
| Dark current | Present | None |
Hence, the keyed statements are (b) and (c).
Similar Questions
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.
Assertion (A) : In PIN photodiodes, an intrinsic layer is there, which is slightly doped with n type material.
Reason (R) : It disables us to increase the width of the depletion region to a value which is far lesser what it could be in a PN diode.
Select your answer using the codes given below.
The carrier velocity in a silicon p-i-n photodiode with 25 μm depletion layer width is 3 × 104 m/s. The maximum response time for the pin diode is given by :
Consider the following statements :
(A) The charge coupled device (CCD) is a linear scanning system
(B) The charge coupled device is a non linear scanning system
(C) A charge coupled devices is used as a shift register
(D) A charge coupled device can not be used as a photo detectorChoose the most appropriate answer from the options given below :
Important Questions from Photodetectors - Teaching
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
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 :
