SISTEMAS HIDROPÓNICOS

easily handled by logarithmic pixels. In a parking garage, it is difficult to image dark corners and the interior of cars without being blinded by car headlights. Welding applications profit from the simultaneous imaging of the welding arc and its environment.

In contrast to other pixel types in which photocharge is integrated as discussed in Section5.3.1, the logarithmic pixel measures thevolt- ageat the drain of the MOSFET in series with the photodiode. For this reason, the dynamic behavior of such a logarithmic pixel depends on the photocurrent: the darker a scene (the lower a diode’s photocurrent), the longer it takes until this MOSFET is in equilibrium again. Therefore, logarithmic pixels react much more slowly at low than at high illumi- nation levels.

Besides their high dynamic range, logarithmic pixels have a property that should make them extremely interesting for image processing ap- plications: An object with a given local contrast, which is imaged with a logarithmic sensor, results in an image with local pixel differences that are independent of the scene illumination level. This property is easily explained with the observation that a (local) light intensity ratioI1/I2 results in a signal given by log(I1)−log(I2), and a proportional intensity change ofc×I results in a signal given by log(c)+log(I). The same object under brighter illumination looks the same in the logarithmic image, except for an additive shift of the background level.

5.4 Transportation of photosignals

The different types of image sensors described in the preceding pro- duce an electrical quantity as a measure for a certain property of the incident light. The electrical quantity can be an amount of charge (e. g., the integrated photocharge), a current (e. g., the photocurrent) or a volt- age level (e. g., the voltage difference of a discharged photodiode). This signal has to be transported as efficiently as possible to an output am- plifier, responsible for making this signal available to the off-chip elec- tronics.

5.4.1 Charge-coupled-device photocharge transportation

In the case of CCDs, the photocharge is stored under a precharged MOS capacitance. The basicCCD idea is to combine a linear array of such MOS capacitances, so that a stored photocharge can be moved later- ally under the influence of appropriate MOS electrode voltage patterns. This principle is illustrated in Fig.5.13, showing asurface-channel CCD (S-CCD). In the semiconductor, photocharge pairs are created under the influence of light. Moving by diffusion and by drift, the photoelectrons can find their way to positively biased MOS electrodes, also called gates, where they are stored at the interface between semiconductor and thin

Transfer direction One pixel _Polysilicon

electrodes Oxide layer Potential minimum for electrons φ φ φ 1 2 3 a b c

Figure 5.13: Illustration of the charge transport principle in CCDs. Different stages of the electrode clocking and charge shifting sequence are shown ina,

b, andc.

oxide. The photogenerated holes are repelled by the positive gate volt- age, and they move around by diffusion until they finally combine in the silicon substrate.

It is important to note that a CCD pixel is not represented only by the positively biased gate because this electrode can receive diffusing and drifting photoelectrons from its environment. A pixel’s geometry is therefore rather defined in terms of “effective photocharge collection area,” extending about halfway to the next positively biased electrode. This also shows that a pixel does not have sharply defined edges; the extent of the charge collection area representing a pixel depends on the wavelength, the electric field distribution, and the diffusion prop- erties of the semiconductor. Generally, longer wavelength light results in a lower contrast and offers reduced resolution, as discussed in Sec- tion5.2.2.

In Fig.5.13, the potential distribution under the electrodes right at the surface is indicated. Photocharge accumulates in the shown “po- tential wells.” By changing the gate voltage patterns, the potential wells can be widened, leading to a broadened distribution of photoelectrons. Using a suitable gate voltage pattern, one can also reduce the extent of the potential wells, and photoelectrons move again to regions with the lowest potential. As illustrated in Fig.5.13, it is physically possible to transport photocharge. This transport mechanism works rather well up to frequencies of a few MHz. In good S-CCDs, only about 0.01 % of the photocharge is lost on average in transporting a photoelectron packet from one gate to another, neighboring gate. Instead of this charge transport loss, one often uses thecharge transfer efficiency(CTE) con- cept, defined as the complement to 100 %. The CTE amounts to 99.99 % in the case of a good S-CCD.

In long CCD lines, a CTE of 99.99 % is still not good enough. Charge is trapped at the surface, making it hard to improve the CTE. For this reason, another type of CCD has been invented, theburied-channel CCD (B-CCD), in which the transport takes place in the bulk of the semicon-

5.4 Transportation of photosignals 129

ductor, a few 100 nm away from the surface. In this way CTEs of up to 99.99995 % can be obtained in B-CCDs, and all commercially available CCD line- and image sensors are of this type.

Above a limiting clock frequency a CCD’s CTE starts to degrade rapidly. Nevertheless, CCDs have been operated successfully at very high clock frequencies. For silicon, 1 GHz has been achieved [24], while GaAs CCDs have reached 18 GHz clocking frequency [25]. Such high clock rates require special precautions in the CCD fabrication process, usually not available for standard video sensors. Today’s technology limits the analog bandwidth of CCDs to about 40MHz. This is suffi- cient for standard video imagers according to the European CCIR or the American RS-170black-and-white video standard. For HDTV sensors, however, the required pixel rate is around 75 MHz, making it necessary to operate two outputs in parallel in HDTV CCD imagers.

5.4.2 Photodiode photocharge signal transmission

The CCD technology provides a clean separation of the acquisition of photocharge and its electronic detection. This is achieved by transport- ing the photocharge with the almost perfect CCD transportation prin- ciple. Traditional photodiode arrays operate differently, by supplying each PD with its individual switch (see also Fig.5.17and Section5.6.4), and by connecting many switches to a common signal (“video”) line. This video line is most often realized using a well-conducting metal strip, leading to a common output amplifier structure. In a PD array, the image acquisition process proceeds in the following way: Assume that all PDs are initially precharged to a certain reverse bias, typically a few volts and that all switches are closed. Incident light generates pho- tocharge pairs in each pixel, leading to the flow of a photocurrent due to the separation of photocharge pairs in the electrical field region of the PDs. As a PD also represents a capacitance, this capacitance is dis- charged by the photocurrent. After a certain time (the exposure time), a pixel can be interrogated by connecting the PD via the appropriate switch to the video line. The output amplifier resets the photodiode to its initial voltage value through the conducting line, while measur- ing how much charge is necessary to do so. This charge is (apart from noise effects) the same as the accumulated photocharge in this pixel. This means that—in contrast to CCDs where the actual photocharge is transmitted and detected—a PD array works by charge equilibration in a usually long conducting line. As we will see in Section5.5.2, this charge equilibration process introduces noise in the signal detection process, which is proportional to the video line’s total capacitance: the larger the number of pixels, the larger the video line capacitance and the larger the image noise. It is this physical effect that made PD image sensors so unattractive compared to CCDs in the early 1980s and which led to their almost complete replacement by CCD image sensors.

5.4.3 Voltage signal transmission

Not all pixel types depend on the transmission of charge signals, as indicated by several examples of pixel functionality discussed in Sec- tion5.3. Voltage signals are sometimes generated in the individual pix- els and these voltage signals must be transmitted to an output amplifier structure. A similar architecture as described in the preceding is used for this, consisting of individual switches in each pixel that connect the local voltages to a common amplifier structure. In such an architecture the voltage signal transmission task is much easier to accomplish than the charge signal transmission just discussed here: Johnson noise in the conducting video line, filtered with the video line’s RC low-pass fil- ter characteristics results in voltage noise that is proportional to one over the square root of the video line’s capacitance [26]. The larger this capacitance, the lower the voltage noise. For this reason, voltage signals can be transmitted with much less noise and higher measurement pre- cision than (small) charge signals. This implies that image sensor types offering voltage transmission architectures, such as that provided by the logarithmic pixel described in Section5.3.5, have an inherent noise advantage over conventional PD architectures. This will be discussed in more detail in Section5.5.3.