While the storage locations are being read out, the active pixels can be accumulating charge for the next image, and this allows frame-transfer CCDs to achieve higher frame rates than full-frame CCDs. One of these sections is the normal light-sensitive imaging array, and the other is a storage array that is shielded from incident light.Īfter integration, charge packets for all of the pixels are rapidly transferred to the storage array, and then readout occurs in the storage array. This is accomplished by dividing an FF CCD into two equally sized sections. The additional energy required to actuate a physical object is also undesirable in battery-powered applications.Īn FT CCD allows us to maintain some of the benefits of the FF CCD while (almost) eliminating the need for a shutter. A shutter (like any other rapidly moving, high-precision mechanical device) makes the design more complex, the end product more expensive, and the overall system more subject to failure. In general, we prefer to control exposure electronically. This is the basic architecture of a full-frame CCD. Without a mechanical shutter that blocks incoming light after the exposure period is complete, the charge packets generated during (intentional) integration would be corrupted by light that arrives during readout.

The photoactive areas of the CCD don’t stop being photoactive simply because you have decided that it’s time to perform readout. However, a major limitation is the need for a mechanical shutter (or a synchronized, short-duration light source called a strobe). This maximizes the number of pixels that can be included in a given area of silicon, and it also maximizes the portion of each pixel that is actually capable of converting photons into electrons. Thus, gate electrodes are made from transparent polysilicon.įull-frame CCDs are (relatively) straightforward and (relatively) easy to fabricate, and they allow the entire CCD surface to be light sensitive. In a full-frame CCD, we need to be able to apply these control voltages to regions that also function as photodetectors. After integration, the charge packets are moved vertically through the pixel locations toward the horizontal shift register.Īs we know from the previous article, we obtain CCD pixel data by applying carefully timed clock signals that sequentially create potential wells and potential barriers in the charge-transferring structure of the device.

During integration, the pixel location accumulates electric charge in response to incident photons. I find it somehow counterintuitive that a semiconductor region could function as both a photoactive element and a charge-transfer device, but that’s exactly what happens in FF CCDs.

In this article, we’ll examine the full-frame (FF), frame-transfer (FT), and interline-transfer (IT) CCD architectures. But these elements can be organized and implemented in different ways, and consequently “CCD” is a general image-sensor category that consists of several subcategories. The two most fundamental elements of any CCD image sensor are the photoactive area (such as a pinned photodiode) and the charge-transferring structures (i.e., horizontal and vertical shift registers). In the previous article, we looked at the general structure of CCDs at the semiconductor level.

Full frame sensor