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

SOLID-STATE IMAGE SENSING 53
immediately separated and captured, extends up to d0 + wsc, where d0 is the
position of the pn-junction.
VDD VSS V SS
V DD
n+ diode
p - (substrate) n - (n-well) p + n +
V SS V DD
n-well diode
V SS V DD
Diffusion Depth of
junction
Doping
concentration
n+ diffusion 200 nm 2⋅10 20 cm -3
n-well 3700 nm 5⋅10 16 cm -3
p-substrate 5⋅10 15 cm -3
Photodiode wsc for Vr=5 V Active depth
(wsc + junct.
depth)
n+ diode 1.2 µm 1.4 µm
n-well diode 1.2 µm 4.9 µm
Figure 3.2 Realizations of photodiodes in a CMOS process. The tables give
typical values of doping concentration and depth of the diffusion
regions for 0.5µm CMOS process. Also the width of the space
charge region is calculated.
Penetration depth of light
The absorption of photons entering the semiconductor material is a statistical
process. The sites of photon-absorption are statistically distributed with an
exponential dependence of distance from the semiconductor surface and
1
wavelength of the incoming light. The distance where an amount of = 37%
of the
e
total photon flux Φ(x) is already absorbed is called the penetration depth. That is
the inverse of the absorption constant α [Sze].
⎛ −x
⎞
⎜
⎟
( −α⋅x
Φ = Φ ⋅
)
= Φ ⋅ ⎝ penetration
depth
( x)
⎠
o e
o e
Equation 3.2
Figure 3.3 illustrates the exponential attenuation of the photon flux in silicon.