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52 CHAPTER 3
3.1 Silicon properties for solid-state photo-sensing
Assuming that the energy of a photon penetrating the semiconductor is higher than
the bandgap of the semiconductor material, it will be absorbed (“band to band”
transition) and generate electron-hole pairs, a procedure known as
photoconversion. Once generated, these free charge carriers generally move within
the semiconductor by thermal diffusion and self-induced drift before they recombine
again after a certain lifetime τ [SZ1]. See also Figure 3.12. When they have
recombined, they can no longer be detected and the optical information carried by
the charge carriers is lost. In order to detect the carriers, the electron-hole pairs
have to be separated by an electrical field, either in the space charge region of a
(reverse biased) pn-junction, as is the case for photo diodes, or in the depletion
zone of a photogate, as is the case in CCD imagers.
3.1.1 Photodiodes in CMOS
For conventional CMOS processes, (we assume p-substrate processes in the
framework of this dissertation,) the designer has the freedom to realize different
types of photodiodes with the available process mask sets, e.g. using the n+ active
layer for an n+ in p-substrate diode (n+ diode) or the n-well layer for an n - in psubstrate
diode (n-well diode). Figure 3.2 schematically shows these diodes in a
cross sectional view. They are biased in reverse direction. Assuming that the
doping concentration of one side of the pn junction is much larger than the
concentration on the other side, nearly the complete space charge region of size
wsc of the pn-junction extends to the lower doped region. The following is an
approximation for an abrupt pn junction and complete depletion:
2 ⋅ ε0
⋅ ε
w
Si
sc = ⋅ ( Vbi
+ Vr
)
Equation 3.1
q ⋅ Nlower
(Vbi: built in potential ≈ 0.7..0.8 V; Vr: reverse bias voltage; Nlower: doping
concentration of the lower doped region). The table in Figure 3.2 gives typical
process characteristics of a 0.5µm standard CMOS process and calculates the
corresponding width of the space charge region for a reverse bias of 5 V. The
active region (active depth), in which optically generated charge carriers are