3. Optical Receiver
The task of an optical receiver is to perform the Optical to
Electronic (OE) conversion of its input signal
In optical receivers, we will discuss:
1. The process of photodetection,
2. The parameters of a Photodetector (PD) and
3. The design of a commonly used PD termed as the PIN
photodiode
4. Photodetection
Photodetectors are made of semiconductor materials
When light falls on a semiconductor material, photons are
absorbed by the electrons in the valence band of the
semiconductor
These electrons in the valence band gain energy from the
absorbed photon and thus move to the conduction band of the
semiconductor material
Therefore an electron-hole pair is formed, which on the
application of an external potential, gives rise to the flow of
electric current, referred to as photocurrent
6. Photodetection
The process of photo-detection is shown in the stylized
illustration of the Figure
An electron absorbs a single photon to move into the
conduction band
In order for the electron transition to occur, the energy of the
incident photon should be equal to the energy difference
between the valence and conduction band
7. Photodetection
The simplest form of a PD is based on a combination of p-type
and n-type materials forming a p-n junction
When light falls on this type of a reverse-biased p-n junction,
electron-hole pairs are generated that flow in the opposite
direction due to the voltage applied
The width of the p-n junction determines the amount of current
flow
8. Photodetector Parameters
We now discuss some of the PD parameters, namely:
1. Responsivity,
2. Quantum efficiency and
3. Bandwidth
The responsivity R of a PD is defined as the ratio of the
photocurrent Ip generated, to the optical power Pin incident on
the PD, which is formulated as:
9. Photodetector Parameters
The quantum efficiency 𝜂 of a PD is a measure of how
efficiently the PD converts the incident light into electric current
It is the ratio of the rate of electron generation to the rate of
photon incidence on the PD
The rate of electron generation is given by the ratio of the
current flow to the charge of an electron
The rate of photon incidence is given by the ratio of the total
incident power to the energy absorbed when an electron jumps
from a lower energy level to a higher level
10. Photodetector Parameters
Hence the quantum efficiency can be expressed as:
Here q is the electron charge,
h is Plank’s constant and
f is the frequency of the incident photon
11. Photodetector Parameters
The bandwidth of a PD determines how promptly it responds to
changes in the incident light
The bandwidth in turn depends upon the rise time Tr
Tr is defined as the time it takes for the current to rise from 10%
to 90% of its final value for a step change in the power of the
incident light
For a p-n junction based PD, the rise time is dependent both
upon the RC time constant and on the transit time of the PD
12. Photodetector Parameters
The capacitance C of a p-n junction based PD is determined by
the length Ld of the depletion region which separates oppositely
charged electrons and holes
13. Photodetector Parameters
The transit time is the time required for an electron to travel
from one end of the PD to the other
Mathematically, the rise time Tr of a PD can be written as:
Where 𝜏𝑡𝑟 and 𝜏𝑅𝐶 denote the transit time and RC time
constant of the PD
The values of 𝜏𝑡𝑟 and 𝜏𝑅𝐶 are dependent upon the design of
the PD
14. Photodetector Parameters
Due to the analogy between the PD and RC circuit, the
bandwidth of the PD can be expressed in a way similar to that
of the RC circuit as:
Above Equation implies that the bandwidth of a PD can be
increased both by reducing the RC time constant and by the
transit time of the PD
15. Photodetector Parameters
The reduction in transit time can be achieved by reducing the
length LPD of the PD
The transit time is related to LPD by:
Here vd is the velocity of the electrons drifting across the length
Ld of the depletion region
16. Photodetector Parameters
A reduction in the length of the PD would reduce the surface
area on which the light falls
Therefore, the rate of electron generation will decrease
This in turn will reduce the responsivity and efficiency of the PD
This implies that there is a trade-off between the bandwidth and
responsivity of a PD
17. PIN Photodiode
We now discuss the most common type of PD called the PIN
photodiode
Apart from the drift of electrons across the depletion region, the
photocurrent can also be generated through diffusion of
electrons and holes
As mentioned earlier, the drift current is generated due to light
falling on the depletion region of the PD
Diffusion current is generated by the light falling on the n-type
and p-type materials in the PD
18. PIN Photodiode
The electrons and holes generated in p-type and n-type
materials respectively, have to diffuse towards the depletion
region boundary, before they can drift to the opposite sides
This diffusion of electrons and holes is a slow process and
hence delays the response of the PD to a sudden change in the
intensity of the incident light
In order to reduce the diffusion current, the length of the p-type
and n-type materials can be decreased, hence making the drift
current dominant over the diffusion current
19. PIN Photodiode
This can be achieved by inserting an intrinsic semiconductor
material between the n-type and p-type materials
20. PIN Photodiode
This will increase the length of the depletion region, while
decreasing the length of n-type and p-type regions
Due to the particular structure of the PD, it is generally called
PIN photodiode
Where P, I and N stands for the p-type, intrinsic and n-type
materials, respectively
