These slides give an extensive knowledge about the photo diode. It covers the circuit diagram and its energy band diagram. Also includes important information about noise factors and resoponsitivities.

1. PIN DETECTOR
By Deekshitha S

2. PIN PHOTODETECTOR
The high electric field present in
the depletion region causes photo
generated carriers to separate and
be collected across the reverse-
biased junction. This gives rise to a
current flow in an external circuit,
known as photocurrent.

3. PHOTOCURRENT
Optical power absorbed, in the depletion region can be written in terms of incident optical power:
P(x) = P0(1-e-𝛂(λ)x)
Absorption coefficient strongly depends on wavelength. The upper wavelength cutoff for any
semiconductor can be determined by its energy gap as follows:
λc(µm) = 1.24/Eg(eV)
Taking entrance face reflectivity into consideration, the absorbed power in the width of depletion region
w, becomes:
(1-Rf)P(w) = P0(1-e- 𝛂(λ)x)(1-Rf)

4. RESPONSIVITY ( ℜ )
Quantum Efficiency (η) = number of e-h pairs generated / number of incident
photons
η = Ip/q
P0/h𝒗
Responsivity measures the input-output gain of a detector system. In the specific
case of a photodetector, responsivity measures the electrical output per optical
input.
ℜ = Ip/ P0 = ηq/h𝒗 mA/mW

5. When λ<<λc absorption is low
When λ>λc no absorption
λc = hc/Eg

6. LIGHT ABSORPTION COEFFICIENT
The upper cutoff wavelength is
determined by the band gap energy Eg
of the material.
At lower-wavelength end, the photo
response diminishes due to low
absorption (very large values of 𝛂s)

7. SPEED OF RESPONSE
1
The transit time of the photo-generated carriers through the depletion region
2
The electrical frequency response as determined by the RC time constant, which
depends on the diode’s capacitance
3 The slow diffusion of carriers generated outside the depletion region

8. Rise Time
Rise time is the time the output signal takes to rise
from 10% to 90% of the peak value after the input is
turned on instantaneously.
Fall Time
Fall time is the the time the output signal takes to
drop from 90% to 10% of the peak value after the
input is turned off abruptly.

9. NOISE SOURCES IN PHOTODETECTORS
THERMAL NOISE
This is the spontaneous
fluctuation due to the
thermal interaction
between the electrons
and the vibrating ions in a
conduction medium and
it is specially prevalent in
resistors in room
temperature.
QUANTUM NOISE
The quantum or shot
noise arises from the
random nature of the
production and collection
of photogenerated
electron when an optical
signal is incident on a
photodiode.
DARK CURRENT NOISE
It is the current that
continues to flow through
the bias circuit in the
absence of the light. This
is the combination of bulk
dark current, which is due
to thermally generated
electron and hole in the p-
n junction.

10. SIGNAL TO NOISE RATIO
In fiber optic communication systems, the photodiode is generally required to detect very weak optical
signals.
Detection of weak optical signals requires that the photodetector and it’s amplification circuitry be
optimized to maintain a given signal-to-noise ratio.
The power signal-to-noise ratio S/N at the output of an optical receiver is defined by:
SNR can’t be improved by amplification

11. NOISE-EQUIVALENT POWER
Noise-Equivalent Power (NEP) is the minimum input optical power to generate
photocurrent, equal to the rms noise current in a 1 hertz bandwidth.
This more directly measures the minimum detectable signal because it compares
noise directly to optical power.
NEP depends on the frequency of the modulated signal, the bandwidth over which
noise is measured, the detector area, and the operating temperature.

12. Thank You