2. Content
• Physical Principles of Photodiodes
• pin, APD
• Photodetectors characteristics (Quantum efficiency,
Responsivity)
• Photodiode Response Time
3. Introduction
• Light detection is a process that converts incident light
into an electrical photocurrent.
• A light detection device is used at the front end of every
optical receiver to generate a photocurrent proportional
to the incident light intensity.
• Two main types (both semiconductors):
– Photoconductors – their conductivity increases when
the intensity of the incident light increases.
– Photodiodes – absorb photons and generate
photocurrent and are the primary type used in
communication systems for their high response
speeds. Two types – PIN diodes and APDs.
4. Absorption Coefficient
• One common characteristics of every light detection device is its
light absorption ability.
• Light absorption is mainly determined by the absorption coefficient α
of the detection device and the wave length of the light.
6. Cont’d
• For an absorption layer of thickness d and
absorption coefficient α, the fraction of
light power absorbed is (1 e d )
• When the energy of the incident photons is
smaller than the energy gap of the
material, i.e, hf < Eg or
λ>λc=(hc/Eg[ev])[μm], photons can not be
absorbed to excite EHPs. α drops sharply
when the wavelength exceeds λc.
7. Responsivity
• To quantify the photon absorption ability, a
parameter called the quantum efficiency (η) is
used.
• It is the ratio of the no of excited EHPs to the
total number of incident photons. It is given by
(1 R )(1 e d )
• Where R = r2 is the reflectivity at the front
material. The equation assumes that a larger
fraction of the incident power is absorbed. η can
be increased by having a smaller R, a larger
thickness or a larger α
8. Cont’d
• Once the quantum efficiency of a light detection
device is known, the corresponding responsivity
of the device is defined as
q
A /W
hf 1.24
• The responsivity of a light detection device is the
ratio of the output photocurrent to the incident
light power when the current gain is unity. It
increases with λ but it has a sudden drop due to
the wavelength cutoff condition.
9. Photoconductors
• Two main types of Photoconductions
• Intrinsic – is an intrinsic semiconductor.
• Extrinsic – a semiconductor with either N-type or P-type
doping. Its conductivity increases when electrons (or
holes) are excited from the N-type (or P-type) impurity
level.
• Because intrinsic photoconductors require photons of
much higher energies, they exhibit a strong long-
wavelength cutoff effect.
• Extrinsic semiconductors have free carriers, so they
have low resistance. This is undesirable from the thermal
noise consideration.
10. Photodiodes
• Two types – PIN and APDs.
• The structure of a typical PIN diode is shown below.
11. Cont’d
• A photon with sufficient energy (hf) can excite an
electron-hole pair. If the pair is in the presence
of a large electric field, the electron and hole will
be separated and move quickly in opposite
direction, resulting in a photocurrent.
• If the pair is in the presence of a small or zero
electric field, they move slowly and may even
recombine and generate heat.
• Therefore, a strong electric field in the depletion
region is essential.
12. Cont’d
• Because one absorbed photon generates
one EHP in PINs, the photocurrent is a
linear function of the input power Pin.
q
I ph Pin Pin Pin
hf 1.24
• At zero input power, the reverse bias
current is called the dark current. The total
current is thus
I tot I d I ph
13. Cont’d
• Unlike LEDs and LDs, photodiodes are generally
operated at reverse bias for detection in optical
communications. Some reasons:
– Photodiodes have large resistance at reverse bias.
Low resistance is undesirable from thermal noise
consideration.
– The electric field in the absorption layer is large at
reverse bias – carriers move quickly to the external
ckt – fast response – larger BW.
– The width of the depletion region is large at reverse
bias – results in small junction capacitance – small
RC time constant – fast response.
14. pin Photodetector
w
The high electric field present in the depletion region causes photo-generated carriers to
Separate and be collected across the reverse –biased junction. This give rise to a current
Flow in an external circuit, known as photocurrent.
16. Photocurrent
• Optical power absorbed,P(x)in the depletion region can be written in terms
of incident optical power, P0 :
s ( ) x
P( x) P0 (1 e )
• Absorption coefficient s ( ) strongly depends on wavelength. The upper
wavelength cutoff for any semiconductor can be determined by its energy
gap as follows:
1.24
c ( m )
E g (eV)
• Taking entrance face reflectivity into consideration, the absorbed power in
the width of depletion region, w, becomes:
(1 R f ) P( w) P0 (1 e s ( ) w )(1 R f )
17. Responsivity
• The primary photocurrent resulting from absorption is:
q
Ip P0 (1 e s ( ) w )(1 R f )
h
• Quantum Efficiency:
# of electron - hole photogener ated pairs
# of incident photons
IP / q
P0 / h
• Responsivity:
IP q
[A/W]
P0 h
19. Avalanche Photodiode (APD)
APDs internally multiply the
primary photocurrent before it
enters to following circuitry.
In order to carrier multiplication
take place, the photogenerated
carriers must traverse along a
high field region. In this region,
photogenerated electrons and
holes gain enough energy to
ionize bound electrons in VB
upon colliding with them. This
multiplication is known as Optical radiation
impact ionization. The newly
created carriers in the presence of Reach-Through APD structure (RAPD)
high electric field result in more showing the electric fields in depletion
ionization called avalanche region and multiplication region.
effect.
20. Responsivity of APD
• The multiplication factor (current gain) M for all carriers generated in the
photodiode is defined as:
IM
M
Ip
• Where I M is the average value of the total multiplied output current & I P
is the primary photocurrent.
• The responsivity of APD can be calculated by considering the current gain
as:
q
APD M 0 M
h
21. Photodetector Response Time
• The response time of a photodetector with its output circuit depends mainly
on the following three factors:
1- The transit time of the photocarriers in the depletion region. The transit
time t ddepends on the carrier drift velocity v d and the depletion layer
width w, and is given by:
w
td
vd
2- Diffusion time of photocarriers outside depletion region.
3- RC time constant of the circuit. The circuit after the photodetector acts
like RC low pass filter with a passband given by:
1
B
2RT CT
22. Photodiode response to optical pulse
Typical response time of the
photodiode that is not fully depleted
