18 pin photodiodes pin
TRANSCRIPT
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Pin and Schottky photodetectors
I. Pin photodiodes
Electric field profiles in a regular p+ - n diode:
Slope ~ qND1/0
E
x
V2 > V1
V1
V3 > V2
p+ n Metal
The disadvantages:
The field and the electron velocity is not uniform.
The space charge region width (the responsivity and the capacitance ) depends
on the bias
High bias voltage needed for high speed operation.
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As the doping in the active layer decreases the field becomes more uniform
Slope ~ qND2/0
p+ n Metal
E
x
V2 > V1
V1
V3 > V2
ND2 V1
V1
V3
> V2
p+
n Metal
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p-i-n photodiode
The concept of p-i-n diode design is to:1) enlarge the drift region for photocarriers and
2) decrease the junction capacitance.
The p-i-n diode consists of p+
- n+
junction with low-doped n-
orp- region in between.
It can be considered as p+ - n- or n+ - p-junction.
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p-i-n photodiode
The device features:
Dark current is small (highly doped n- and
p- sides), large potential barrier between n-and p-sides;
Photocurrent is due to strong electric field in
the i-region - no carrier loss, high efficiency
High electric field - small drift time - fast
drift response
Large n- and p- side separation - low
capacitance - fast RC response
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Si pin photodiode
Si is not a direct bandgap material.
The absorption length is big(> 10 m for visible/IR light)
Low loss in p+ layer
Thick n-
-layer needed for full absorption. Low speed of response
AlGaN/GaN pin photodiode
Fast response
Visible-blind operation
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Quantum efficiency and frequency response
of the pin photodiode
In general, photogenerated carriers move by drift and diffusion
and therefore the total current density through the reverse-biased
depletion layer is
The drift component is due to carrier generation in the depletion region.
The diffusion component is due to carrier generation OUTSIDE the
depletion region.
The device can be optimized to have the diffusion component as small
as possible (for the fastest response).
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The drift current density, Jdrassuming that all the carriers
are swept out by the electric field in the depletion region:
In case the depletion region is thick enough, i.e. W >> 1,
Jdr max
= q0
Incident photon flux
(the number of photons per unit area, per second):
R is the reflection coefficient of the top surface, A is the device area, Pinc is theincident optical power.
( )0 1in c
R
P
A h
=
( )0 1 WdrJ q e =
1) The drift current density
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2) Diffusion current density, Jdiff
:
This component contributes to the total current due to the carriersgenerated OUTSIDE the depletion region (which are LOST for the
drift current)
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The diffusion current (the absolute value) for the pin-diode is given by:
The total photocurrent density, J = Jdr
+ Jdif
:
Note that pN0 is very small in the n-type region, therefore,
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The external quantum efficiency of pin-photodetector:
is given by the same expression as that for p-n junction detector
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Frequency response of p-i-n diode
depends on three major factors:
1) the drift time through the depletion region,
2) the diffusion time for the carriers generated outside the depletion region,
3) the RC constant of the device.
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Frequency response of p-i-n diode:
1) the carrier drift time:
ttr e,h
= W2/(n,p
V)
for the electrons and holes correspondingly.If the electric field in the depletion region is strong enough, both
electrons and holes move with thesaturation velocity,
vS 107
cm/s.In this case, t
tr= W/v
S
pin-photodiode is a very fast device.
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2) the effect of diffusion current on the frequency response:
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3) RC component of the frequency response
The equivalent circuit of the photodiode:
Cj
is the junction capacitance.
For the pin diode, Cj = 0A/W;
The RC time constant, RC is (ignoring the package capacitance):
RC= (RS+RL)Cj = 0A (RS+RL) / W;
When W increases, RC
decreases, however, ttr
increases.
Optimal design corresponds to RCttr