Architectures of image sensors

For the acquisition of 1-D and 2-D distributions of incident light, arrays of pixel are required. Such arrays can be realized as an arrangement of CCD columns or as suitably placed and interconnected individual photodiodes as described in Section 5.3.1. Depending on the choice of arrangement and interconnection, different types of image sensors result.

5.6.1 Frame-transfer charge-coupled-devices

The simplest type of CCD image sensor is theframe-transfer(FT) CCD.

As illustrated in Fig.5.16, it consists of three CCD sections. One CCD area (A register) is used for the conversion of photons into photocharge during the exposure time and for the storage of this photocharge in the pixels. This 2-D photocharge distribution is subsequently shifted down into another CCD area (B register), which is covered with an opaque metal shield. From the B register, an image row at a time is shifted down into a CCD line (C register), with which the photocharges are transported laterally to the output amplifier, so that the content of this image row can be accessed sequentially.

The disadvantage of the FT-CCD principle is the afterexposure of bright areas that can occur when the photocharge pattern is trans-ported from the A register into the light-shielded B register. This oc-curs because the A register remains light-sensitive during the vertical photocharge transportation time. The afterexposure effect in FT-CCDs can create saturated (“bright”) columns without any contrast

informa-5.6 Architectures of image sensors 135 a

output amplifier A register

B register

C register

b

output amplifier C register

c

output amplifier C register

d

row-addressingcircuit

column addressing circuit column select

column line

output amplifier

Figure 5.16:The four most important architectures of solid-state image sensors:

aframe-transfer (FT) CCD with its three registers;binterline-transfer (IT) CCD with column light shields for vertical charge transfer;cfield-interline-transfer (FIT) CCD, combining FT-CCD and IT-CCD principles for studio and broadcast applications;dtraditional photodiode array image sensor with one photodiode and one selection transistor per pixel.

tion. For this reason, high-quality FT-CCD cameras employ a mechani-cal shutter, shielding the A register from incident light during the ver-tical photocharge transportation time.

The big advantage of the FT-CCD is that the whole A register area is photosensitive; one speaks of anoptical fill factor of 100 %. Because the A register is covered with polysilicon CCD electrodes that tend to absorb in the blue and UV, an FT-CCD is not very sensitive in the blue spectral region. For special applications this can be remedied by thin-ning down an FT-CCD to about 10µm thickness and by illuminating it from the back. Such back-side illuminated FT-CCDs offer 100 % fill factor, an excellent response over the whole visible spectrum, and they are the image sensors of choice for scientific and astronomical appli-cations.

5.6.2 Interline-transfer charge-coupled-devices

In consumer applications, a mechanical shutter is impractical to use, and for this reason FT-CCDs are rarely used in video and surveillance cameras. Rather, theinterline-transfer (IT) CCD principle is employed, as illustrated in Fig.5.16b. Photocharge is collected in the individual pixels, and after the exposure time the photocharge is transferred via the pixels’ transfer register into a corresponding vertical CCD column.

These CCD columns are shielded from light with an opaque metal layer.

A 2-D photocharge distribution can therefore be shifted downwards, one row at a time, into the horizontal output register, from where the photocharge packets are read out sequentially. As the vertical CCD columns are shielded, the afterexposure problem is much less severe than in FT-CCDs. One pays for this with a reduced fill factor, because the column light shields reduce the available photosensitive area on the image sensor’s surface. The typical fill factor of an IT-CCD is about 30%, reducing the total sensitivity to about a third of that observed in FT-CCDs.

With the IT-CCD principle a very useful functionality becomes avail-able: Because there is essentially no time-constraint in exposing the pix-els and transferring their accumulated photocharge under the shielded columns, one can implement anelectronic shutter. The exposure time can be as short as a few 10µs, extending up to several seconds in cam-eras not conforming to a video standard. The exposure time is essen-tially bounded by the dark current, which depends strongly on tem-perature, as described in Section5.5.2. The desirable properties of the IT-CCD make it the image sensor of choice for most of today’s video and surveillance cameras, especially for consumer applications. In or-der to increase the optical fill factor of IT-CCDs, some manufacturers supply each pixel with its ownmicrolens, so that more light can be di-rected to the IT-CCD’s photosensitive surface. An even more efficient, albeit more expensive improvement is the coverage of an IT-CCD with amorphous silicon, with which the optical fill factor can be increased further, close to 100 %.

5.6.3 Field-interline-transfer charged-coupled-devices

Although the column light shield in the IT-CCD is an efficient light blocker, there is always some residual photocharge seeping into the columns from the sides. For this reason, an IT-CCD can still show some afterexposure effects. For professional applications such as video broadcasting, this is considered not acceptable, and a combination FT-and IT-CCD principle has been invented to overcome this problem, the field-interline-transfer (FIT) CCD, illustrated in Fig. 5.16c. The upper part of a FIT-CCD really consists of an IT-CCD. The lower part, however,

5.6 Architectures of image sensors 137 is realized like the B and C registers of an FT-CCD. The FIT-CCD is oper-ated by acquiring an image conventionally, making use of the IT-CCD’s variable exposure time functionality. The resulting 2-D photocharge distribution is then shifted quickly under the shielded vertical columns, from where it is transported very fast under the completely shielded in-termediate storage register. The sequential row-by-row readout is then effectuated from the B and C registers, exactly as in FT-CCDs.

5.6.4 Conventional photodiode (MOS) arrays

A photodiode or MOS array image sensor consists of a 1-D or 2-D ar-rangement of PDs, each provided with its own selection transistor, as illustrated in Fig.5.16d. For a description of the PD image sensor’s operation, assume that all PDs are precharged to a certain reverse bias voltage, typically 5 V. Under the influence of the incident light, each pixel is discharged to a certain level. A pixel is read out by addressing the corresponding row and column transistors, providing a conducting line from the pixel to the output amplifier. Using this line the pixel is charged up again to the same reverse bias voltage as before. The am-plifier measures how much charge is required to do so, and this charge is identical to the photocharge (plus dark current charge) accumulated at the pixel site. In this way, each pixel can be read out individually, at random, and the exposure time is completely under the control of the external addressing electronics.

The random addressing freedom, however, comes at the price of a large capacitance of the conducting line between pixel and output amplifier of several pF. As is obvious from the inspection of Eq. (5.9), this leads to noise levels one or two orders of magnitude larger than in corresponding CCDs image sensors. For this reason, the usage of such traditional PD image sensors has been restricted to applications where the random pixel access is an absolute must. In video applications, CCD technology is used almost exclusively.

5.6.5 Active pixel sensor technology

As just discussed, the noise performance of PD array image sensors is much worse than that of a CCD because of the large effective capaci-tance the first MOSFET in the output amplifier sees. The logical conclu-sion is that it should be possible to realize CMOS-compatible PD array image sensors with a noise performance comparable to CCD imagers when this first MOSFET is placed in each pixel. It took surprisingly long until this seemingly trivial observation was made. As a consequence, it led directly to what is called today “active pixel sensor” (APS) imaging technology [33]. It is apparently not sufficient just to move the first MOSFET into the pixel, because its input requires a reset mechanism.

reset

r-sel VD

out

M_res M_sense

M_select

Figure 5.17:Schematic diagram of an APS pixel, consisting of a photodiode, a reset transistor, a sense transistor, and a row-select transistor. The active load transistor that completes the source-follower circuit is shared by all pixels in a column, and it is therefore needed only once per column.

For this reason, the simplest APS image sensor pixel consists of one photodiode and three MOSFETs as illustrated in Fig.5.17.

With the reset MOSFET the photodiode and the gate of the source follower MOSFET are precharged to a voltage of typically 3-5 V. The photocurrent produced by the photodiode (plus the dark current) dis-charges the capacitance of the reverse-biased PD. The resulting voltage can then be sensed efficiently with the source-follower MOSFET with a sensitivity that is comparable to that of CCD image sensors. As in the PD array, the third MOSFET is employed as a selection switch with which a row is selected. The active load MOSFET of this APS pixel can be shared by all the pixels in a column, and it does not need to be included in the pixel itself.

The APS technology is very attractive for several reasons: (1) APS image sensors can be produced in standard CMOS technology, opening the way to image sensors with integrated electronic functionality and even complete digital processors; (2) The pixels offer random access similar to PD arrays; (3) The pixel readout is nondestructive, and it can be carried out repeatedly for different exposure times; (4) The exposure times can be programmed electronically; (5) APS image sensors dissi-pate one or two magnitudes less electrical power than CCDs; (6) APS imagers show less blooming (spilling of electronic charge to adjacent pixels); And (7) APS pixels are more robust under x-ray radiation.

Disadvantages of APS image sensors include the reduced optical fill factor (comparable to that of IT-CCDs), the increased offset noise due to MOSFET threshold variations (see Section5.8), and the impossibility of performing correlated double sampling for noise reduction as dis-cussed in Section5.5.3. Fortunately, a combination of APS and CCD technology has been proposed, and the resulting photogate APS pixels offer this functionality [34].

5.7 Color vision and color imaging 139

400 450 500 550 600 650 700

0 0.2 0.4 0.6 0.8 1

λ[ ]nm λ[ ]nm

Figure 5.18: Estimates of the relative cone sensitivities of the human eye after DeMarco et al. [35].

Active pixel image sensors with up to 4k×4k pixels have been re-alized, with speeds of several thousand frames per second, with an input-referred charge noise of about 10electrons at room temperature and video speed, and with a DR of up to 84 dB. Many experts do not doubt, therefore, that CMOS imagers using APS techniques can replace CCD image sensors in many practical applications, and several con-sumer products in the electronic still and video camera market already contain CMOS imagers.

5.7 Color vision and color imaging