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 :
5.2 ns
The official answer key records option (D), 5.2 ns. The printed data do not lead to any of the four figures, so the physics is worked through below and the mismatch is stated rather than glossed over.
The governing relation. The response of a p-i-n photodiode is limited by the time a photogenerated carrier takes to drift across the depletion (intrinsic) layer. That transit time is simply width divided by drift velocity:
\(\tau_{tr}=\dfrac{W}{v_{d}}\)
Substituting the printed values. With \(W=25\ \mu\text{m}=25\times10^{-6}\ \text{m}\) and \(v_{d}=3\times10^{4}\ \text{m/s}\):
\(\tau_{tr}=\dfrac{25\times10^{-6}}{3\times10^{4}}=8.33\times10^{-10}\ \text{s}\approx 0.83\ \text{ns}\)
That is roughly 0.83 ns, which matches none of 52 ns, 2.5 ns, 62 ns or 5.2 ns. Working backwards, each option implies a different drift velocity:
| Option | Implied drift velocity |
|---|---|
| (A) 52 ns | about \(4.8\times10^{2}\ \text{m/s}\) |
| (B) 2.5 ns | \(1.0\times10^{4}\ \text{m/s}\) |
| (C) 62 ns | about \(4.0\times10^{2}\ \text{m/s}\) |
| (D) 5.2 ns | about \(4.8\times10^{3}\ \text{m/s}\) |
None of these is the \(3\times10^{4}\ \text{m/s}\) the question states, so the item’s numbers are internally inconsistent with its options.
What the method is, since that is what is actually examinable. The speed of a p-i-n diode is set by three effects, and the slowest one wins :
| Limit | Expression | How to improve it |
|---|---|---|
| Transit time | \(\tau_{tr}=W/v_{d}\) | Make the intrinsic layer thinner |
| RC time constant | \(\tau_{RC}=R_{L}C_{j}\), with \(C_{j}=\varepsilon A/W\) | Make it thicker, or reduce area and load |
| Diffusion of carriers generated outside the depletion region | A slow tail on the response | Confine absorption to the depletion region |
The design tension is the point worth carrying away: transit time falls as the layer is thinned, but junction capacitance — and hence the RC limit — rises as it is thinned. The optimum width balances the two, and a wide intrinsic layer is still wanted for good quantum efficiency, since light must be absorbed inside the depletion region to be collected quickly. The corresponding bandwidth is \(f_{3dB}\approx\dfrac{0.35}{\tau}\).
Note on the recorded answer. The answer stored here follows the official key, as required. On the printed data the transit time is about 0.83 ns, and no option corresponds to it.
Similar Questions
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 :
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 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 :
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
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.
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 ?
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 :
(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 :
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 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 :
