3. Charge-Coupled Devices

A charge-coupled device (CCD) is an array of metal-oxide-semiconductor (MOS) capacitors which can accumulate and store charge due to their capacitance. A cross-section of a single MOS capacitor is shown in Figure 2.3 (Gendreau (1995)). When the device is properly biased electrons see a potential well in which they can be trapped. The active region of this device is the depletion region. Free charge resulting from X-ray absorption here is accumulated in the potential well of the MOS capacitor.

The array is read out by transferring the charge from one MOS capacitor to its neighbor on one side. This is done by clocking the gate voltages. In a three-phase CCD array each pixel has three gate electrodes. In each row of pixels, the corresponding electrodes on each pixel are connected. Figure 2.3 shows how charge is transferred. To avoid smearing of the X-ray image the readout time is a small fraction of the integration time. The charge from the imaging CCD is transferred to another CCD array called the frame storage array. While the imaging array is integrating an image again the charge in the frame store array is transferred to a serial register and then to the gate of a JFET or a similar charge-sensitive amplifier. This results in a train of voltage pulses. The spatial position of the original pixel can be deduced from the pulse's position in the train. The size of the pulse is proportional to the collected charge and thus to the energy deposited by the incident X-ray.

Figure 2: Cross-section of a MOS capacitor showing the potential function. From Gendreau (1995).

Figure 3: Charge transfer in a three-phase CCD array. From Gendreau (1995). At the top are the electrodes. The curves under these represent voltage profiles at different times and the shaded regions in the curves represent the trapped charge.