18 pin photodiodes pin

8/11/2019 18 Pin Photodiodes PIN

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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

18 pin photodiodes pin

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