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 :
2.5 ns
A p-i-n photodiode's speed is set by how long a photogenerated carrier takes to drift across the intrinsic region. That transit time is
\(\tau_{t}=\dfrac{W}{v_{d}}\)
where W is the depletion (intrinsic) layer width and \(v_{d}\) the drift velocity. Substituting the printed values,
\(\tau_{t}=\dfrac{25\times10^{-6}}{3\times10^{4}}=8.33\times10^{-10}\ \text{s}=0.83\ \text{ns}\)
The offered answer of 2.5 ns is exactly three times this, which is what results if the velocity is taken as \(10^{4}\) m/s:
\(\dfrac{25\times10^{-6}}{10^{4}}=2.5\times10^{-9}=2.5\ \text{ns}\)
and 2.5 ns is the only option of the right order of magnitude — the others are tens of nanoseconds, which no combination of the given numbers produces. Option 2 is therefore the intended answer, and it is flagged for confirmation because the strict arithmetic gives 0.83 ns.
| Quantity | Value |
|---|---|
| Depletion width W | 25 μm |
| Drift velocity | 3 × 104 m/s (printed) |
| Strict transit time | 0.83 ns |
| Offered answer | 2.5 ns |
The design tension a p-i-n photodiode embodies is worth understanding, because the transit time is only half of it. A wide intrinsic layer is wanted so that most incident photons are absorbed within it, giving high quantum efficiency:
\(\eta=\left(1-R\right)\left(1-e^{-\alpha W}\right)\)
But a wide layer also means a long transit time and therefore a slow device. The two requirements pull in opposite directions, and the optimum is usually taken near \(W\approx1/\alpha\).
The other speed limit is the RC time constant, \(\tau_{RC}=R_{L}C_{j}\), and here the wide intrinsic layer helps: junction capacitance \(C_{j}=\varepsilon A/W\) falls as W grows. The overall response is
\(\tau=\sqrt{\tau_{t}^{2}+\tau_{RC}^{2}}\)
and the bandwidth follows as \(f_{3dB}\approx\dfrac{0.35}{\tau}\).
Hence, the response time is 2.5 ns.
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.
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 ?
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 :
