3D Time-of-flight distance measurement with custom - Universität ...

70 CHAPTER 3
Flicker noise is caused by so-called semiconductor traps, which randomly capture
and release charge carriers, leading to statistical variations of the mobility and
concentration of free charge carriers in the transistor channel of the source follower
transistor [PS2]. Referred back to an input equivalent amount of charge, one
obtains the flicker noise contribution ∆Qflicker:
4 ⋅ k ⋅ T ⋅ f
∆ Q C
l
flic ker = ⋅
⋅ B
Equation 3.16
gm
⋅f
s
where fl is the corner frequency, a process and layout dependent parameter, fs is
the sampling/ readout frequency, gm the transistor transconductance and B is the
bandwidth.
The noise equivalent input charge of the resistor noise in the channel of the source
follower transistor (Amplifier Johnson noise) is:
4 k T
Qamp
C
⋅ B
gm
α ⋅ ⋅ ⋅
∆ = ⋅
Equation 3.17
(α: 0.7..1, CMOS transistor parameter.)
Neglecting the amplification of the source follower output stage (usually about 0.8-
0.9) we can transform the above mentioned input noise out of the charge domain
into the equivalent output noise in the voltage domain:
∆Vreset
=
∆Vflic
ker =
∆Vamp
=
k ⋅ T
C
4 ⋅ k ⋅ T ⋅ fl
⋅ B
gm
⋅f
s
4 ⋅ k ⋅ T ⋅ α
gm
⋅ B
Equation 3.18
Dark current corresponds to the thermal generation of free electron hole pairs,
which competes with the optical generation and adds an offset charge to the
optically generated signal. Since thermally generated charge carriers are also
quantized, dark current adds an additional Poisson-distributed shot noise
contribution to the overall noise.
The sum of photocharge conversion noise can be drastically suppressed if the
sensor is operated at low temperatures (cooled imagers). With decreasing
minimum feature size of current and future semiconductor processes, lower