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SOLID-STATE IMAGE SENSING 49
3. Solid-state image sensing
Solid-state imaging has experienced a rapid growth, especially in the last ten years.
This development has benefited from the steady improvements and miniaturization
in the semiconductor industry and is also part of today’s multimedia revolution. The
first semiconductor solid-state imagers were photodiode arrays, where each diode
was directly connected to the output by a metal wire. The large capacitance of
these metal wires, resulting in a relatively slow signal readout speed and poor noise
performance was, however, the killing factor that prevented a wide propagation of
photodiode arrays, at least at that time.
In 1970 Boyle introduced a new, amazingly simple device for semiconductor image
sensing [BOY, AME], the so-called charge coupled device: CCD. These elements
collect optically generated charge carriers under locally separated photogates.
Bundled to charge packets in the single pixels, the optically generated charge
carriers can then, after a certain integration time, be moved through the
semiconductor into certain storage areas by making use of the charge-coupling
principle. This method is explained in more detail below. From these storage areas
they are then sequentially shifted to one output stage, where the charge is
converted into a voltage (Figure 3.1 c.) The use of only one low-capacitance output
stage is still one of the essential advantages of CCDs since it means both low noise
and very high homogeneity between the single pixels. Additionally, with the chargecoupling
principle, it is possible to perform certain sorts of signal processing [SP4].
The essentially noise-free charge addition of single charge carriers is only one
example of CCD’s amazing charge domain signal processing capabilities. However,
to operate CCDs, voltages between 10 and 20V are necessary, very often requiring
large currents of up to some amperes, since capacitances of some nanofarads
have to be charged and discharged within nanoseconds. This results in relatively
large power dissipation, a major drawback of CCDs compared to some of today’s
CMOS APS sensors. Other weak points of CCDs are the facts that (1) the complete
sensor always has to be read out in order to reset the pixel values and to start a
new image acquisition and (2) the image is after-exposed during readout, known as