Image Processing Reference
In-Depth Information
2.2.2 npn Photodiode on p -Well
A photodiode that is formed in a p -well on an n -type substrate lowers the unnecessary
sensitivity of near-infrared light, as mentioned above. Figure 2.21b indicates an npn
photodiode 2 that is formed in a p -well on an n -type substrate. The p -well is grounded
and positive voltage is applied to the n -type substrate. Figure 2.21c shows the potential
distribution of an np photodiode and Figure 2.21d shows that of an npn photodiode. In
the case of the np photodiode, it is clear that the signal charges that are generated in
the deeper region of the substrate seem to be collected and in a photodiode, this can
contribute to the sensitivity. Meanwhile, in an npn photodiode, the charges that are
generated in the area deeper than the least potential point of the p -type region (which
is indicated by “Watershed” in the figure) cannot come up to the surface but are emit-
ted to the substrate and discharged, so they do not contribute to the sensitivity. The
spectral response of the npn photodiode is indicated by a dashed line in Figure 2.22. In
the case of the npn photodiode, the near-infrared sensitivity, which penetrates into the
depth of the substrate because of the low absorption coefficient, is shown to be greatly
reduced.
2.3 Circuit Components
The circuit components that are commonly used in image sensors will be described in this
section.
2.3.1 Floating Diffusion Amplifier
A floating diffusion amplifier (FDA) 3 is a component that measures the amount of electric
charge, which is explained in Figure 2.23.
As Figure 2.23a shows, an FDA consists of a capacitor, a reset transistor that resets the
potential of the capacitor to a power-supply voltage V d , and an amplifier that receives the
potential of the capacitor to produce voltage output. It operates as follows:
1. By applying a reset pulse to the gate electrode of the reset transistor to be on-state,
capacitor C is connected to V d , and by switching the reset of the transistor to off-
state, the potential of capacitor C is reset to V d , as Figure 2.23b shows.
2. The potential of capacitor C at this moment is received by the amplifier to produce
an output.
3. By transferring signal charges with quantity Q to capacitor C from the above state,
Q will make the potential of capacitor C shallower than V d by a signal voltage of
Q / C = V Q as Figure 2.23c shows.
4. The amplifier receives the potential of the capacitor at this state and produces an
output.
This means that the potential difference between (2) and (4) is the signal voltage that is
proportionate to the charge amount Q .
After completion of a measurement, a reset action is done again to accept the subse-
quent signal charges. From the principle of action, as Figure 2.23d shows, if the volume of
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