1. Responsivity (ℜ)
Quantum Efficiency (η) = number of e-h pairs
generated / number of incident photons
Avalanche PD’s have an internal gain M
0
/
/
pI q
P h
η
ν
=
M
p
I
M
I
= IM : average value of the total multiplied current
M = 1 for PIN diodes
mA/mW
APD PIN Mℜ = ℜ
24.1
ηλ
ν
η
===ℜ
h
q
P
I
o
p
2. Responsivity
When λ<< λc absorption is low
When λ > λc; no absorption
μm
)(
24.1
eVEE
hc
gg
c ==λ
24.1/ηλ=ℜ
3. Light Absorption Coefficient
• The upper wavelength
cutoff is determined by
the bandgap energy Eg
of the material.
• At the lower-
wavelength end, the
photo response cuts off
as a result of the very
large values of αs.
