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

SOLID-STATE IMAGE SENSING 57
3.1.2 MOS photogate
The physical basis for a CCD device is the MOS photogate, also known as MOS
capacitor or MOS-diode [SZ2]. This gate is biased in deep depletion mode, which
means that a volume of depth xd in the semiconductor underneath the photogate is
totally depleted. In this depletion zone no free charge carriers are available. (Here
electrons correspond to minority carriers and holes to majority carriers, since a psubstrate
process is used for this work.) If free minority carriers were available, the
MOS gate would be in strong inversion, causing an inversion layer of electrons
below the gate. For a MOS transistor this assumption is usually valid because free
charge carriers come from the source and the drain. This is, however, not the case
if we look at a photogate or CCD gate. In the latter case free charge carriers are
only available if either generated optically (optical signal) or thermally (dark
current). We therefore call this operation mode deep depletion (a sort of nonequilibrium
mode of strong inversion).
(a)
(b)
(c)
(d)
(I) (II) (III)
Figure 3.7 MOS gate on p-type silicon (I) in deep depletion, (II) in weak
inversion after integration of optically generated photoelectrons, (III)
in strong inversion: saturation of photogate. (a) principle figure, (b)
electrostatic potential and equivalent potential bucket, (c) electric
field and (d) charge distribution diagram. [TEU].