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```ASSIGNMENT 7
5.7
InGaAs pin Photodiodes Consider a commercial InGaAs pin photodiode whose responsivity is
shown in Figure 5.49. Its dark current is 5 nA.
(a)
What optical power at a wavelength of 1.55 m would give a photocurrent that is twice the dark
current? What is the QE of the photodetector at 1.55 m?
(b)
What would be the photocurrent if the incident power in (a) was at 1.3 m? What is the QE at 1.3
m operation?
Figure 5.49 The responsivity of an InGaAs pin photodiode
5.11
Maximum QE Show that maximum QE occurs when
dR R

d 
that is, when the tangent at passes through the origin (R = 0, = 0). Hence determine the wavelengths
and the QEs where the QE is maximum for the InGAs pin, two Si pin and Ge photodiodes in Figure 5.49,
Figure 5.48 and Figure 5.47 respectively.
5.12 Si pin photodiode speed Consider Si pin photodiodes which has a p+ layer of thickness 0.75 m,
i-Si layer of width 10 m. It is reverse biased with a voltage of 20 V.
(a)
What is the speed of response due to bulk absorption? What wavelengths would lead to this type
of speed of response?
(b)
What is the speed of response due to absorption near the surface? What wavelengths would lead to
this type of speed of response?
Solutions Manual (Preliminary)
Chapter 3
3.2
8 March 2017
Figure 5Q12-1 Drift velocity vs. electric field in Si from Figure 5.10
5.18 Si APD The electron and hole ionization coefficients e and h in silicon are approximately given
in Example 5.6.3. Suppose we would like the avalanche to be achieved at a certain applied field in the
avalanche region (corresponding to a particular desirable voltage range). Suppose that we would like the
multiplication M to be 50 when E = 4.50×105 V cm-1. What should be the width w of the avalanche
region?
```
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