4. Avalanche Photodiodes
⚫High gain due to
avalanche
multiplication effect
⚫Increased noise
⚫Silicon has high gain
but low noise
⚫Si-InGaAsAPD often
used(diagram on
right)
n+
p+
p
i
Electric
field
Depletion region
5. Avalanche Photodiodes (APDs)
⚫High resistivity p-doped layer increases electric
field across absorbing region
⚫High-energy electron-hole pairs ionize other sites
to multiply the current
⚫Leads to greater sensitivity
6. APD Detectors
s
Signal Current i M q P
h
APD Structure and field distribution (Albrecht 1986)
7. Iph
Detector Equivalent Circuits
Rd
Id Cd
PIN
Iph
Rd
Id Cd
In
APD
Iph=Photocurrent generated by detector
Cd=Detector Capacitance
Id=Dark Current
In=Multiplied noise current in APD
Rd=Bulk and contact resistance
9. Detector
x Capacitance
p xn
For a uniformly doped junction
Where: =permitivity q=electron charge
Nd=Active dopant density
Vo=Applied voltage V bi=Built in potential
A=Junction area
C
A
W
p-n junction
w xp xn
A 2q
C Nd
2 Vo Vbi
1/ 2
qNd
W
2(Vo Vbi)
1/2
P N
Capacitance must be minimized for high
sensitivity (low noise) and for high speed
operation
Minimize by using the smallest light collecting
area consistent with efficient collection of the
incident light
Minimize by putting low doped “I” region
between the P and N doped regions to
increase W, the depletion width
W can be increased until field required to fully
deplete causes excessive dark current, or
carrier transit time begins to limit speed.
10. Bandwidth limit
C=0K A/w
where K is dielectric constant, A is area, w is
depletion width, and 0 is the permittivity of free
space (8.85 pF/m)
B = 1/2RC
12. DEMERITS
⚫Much higher operating voltage required
⚫Much higher level of noise
⚫Output is not linear
⚫Requires high reverse bias for operation
⚫Not as widely used due to low reliability
13. APPLICATION
⚫Level of gain is of importance for high voltage
requirement.
⚫Laser range finders
⚫Fast receiver modules for data communications
⚫High speed laser scanner (2D bar code reader)
⚫Speed gun
