4. Avalanche Photodiodes
High gain due to
avalanche
multiplication effect
Increased noise
Silicon has high gain
but low noise
Si-InGaAs APD often
used(diagram on
right)
n
+
p
+
pi
Electricfield
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
7. Detector Equivalent Circuits
Iph
Rd
Id Cd
PIN
Iph
Rd
Id Cd
APD
In
Iph=Photocurrent generated by detector
Cd=Detector Capacitance
Id=Dark Current
In=Multiplied noise current in APD
Rd=Bulk and contact resistance
9. Detector Capacitance
p-n junction
xp 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
w xp xn
C
A
2
2q
Vo Vbi
Nd
1/ 2
W
2(Vo Vbi)
qNd
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