This document discusses avalanche photodiodes (APDs) and their bandwidth. It begins by introducing APDs and explaining that they operate under reverse bias and use the photoelectric effect to convert light to electrical signals. It then explains that APDs use avalanche multiplication to provide gain but also increase noise. The document discusses how APD structure, doping levels, and applied voltages impact depletion width, capacitance, transit time, and ultimately bandwidth. It notes that while APDs provide high sensitivity and speed, they also require high operating voltages and exhibit higher noise levels than PIN photodiodes. The document concludes by listing some applications that benefit from APDs' gain, such as laser range finders, data communication receivers,

1. PRESENTED BY:-
BHUPESH SHARMA
ROLL NO : 15S/EL017
AVALANCHE PHOTODIODE &
THERE BANDWIDTH

2. INTRODUCTION
 The performance of Avalanche photo diode depends upon
the efficiency.
 It convert light energy in to electrical signal.

3. PRINCIPLE
 Designed to operate in reverse bias condition.
 Photo electric effect:

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

6. APD Detectors
Signal Current


 
  
 
s
q
i M P
h
APD Structure and field distribution (Albrecht 1986)

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

8. Carrier transit time
Transit time is a function of depletion width and
carrier drift velocity
td= w/vd

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 = 1/2RC

11. MERITS
 APD
 Large gain
 Greater level of sensitivity
 High speed

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