CCD and CMOS sensors

The charge-coupled device (CCD) has been developed primarily as a compact image sensor for consumer and industrial markets, but is now also the preeminent visible and ultraviolet wavelength image sensor in many fields of scientific research including space-science and both Earth and planetary remote sensing. Today’s scientific or science-grade CCD will strive to maximise pixel count, focal plane coverage, photon detection efficiency over the broadest spectral range and signal dynamic range whilst maintaining the lowest possible readout noise. The relatively recent emergence of complementary metal oxide semiconductor (CMOS) image sensor technology is arguably the most important development in solid-state imaging since the invention of the CCD. CMOS technology enables the integration on a single silicon chip of a large array of photodiode pixels alongside all of the ancillary electronics needed to address the array and digitise the resulting analogue video signal. Compared to the CCD, CMOS promises a more compact, lower mass, lower power and potentially more radiation tolerant camera. The charge-coupled device The concept of the charge-coupled device (CCD) emerged from the search for a silicon-based electrical equivalent of the magnetic bubble memory (Boyle and Smith 1970). In its simplest implementation, the CCD structure consists of a series of closely spaced electrodes separated from an underlying semiconductor substrate by a thin insulating oxide layer (Figure 23.1a). When a bias voltage is applied to an electrode, a depletion region is formed in the semiconductor immediately beneath it. The depletion region is in effect a potential well which can store an electrical charge packet. By pulsing the electrodes in an appropriate sequence the potential well, and hence its charge packet, can be transferred through the semiconductor (Figure 23.1b). A shift register can be formed by adding circuits for the insertion and detection of charge packets. Although the CCD was originally conceived as a device to store digital information, it was evident that because a potential well could store variable quantities of charge it could also convey analogue signals. As a result, the CCD concept has Space Science and Technology Department, Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot, UK