Microelectronic solutions for digital photography
Microelectronic solutions for digital photography

This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course

Microelectronic solutions for digital photography

3 Microelectronic solutions for digital photgraphy

3.1 Charge-coupled device (CCD) detectors

The first digital imagers to be developed were called charge-coupled devices, because of the way in which accumulated charges are passed along rows in order to read the contents of each element of an array.

A CCD array has at each pixel an MOS photocapacitor. Potential on a gate electrode holds the accumulating, photo-generated charges in place during the exposure interval. The first thing to appreciate is that the photons that generate the electron–hole pairs have to penetrate the polysilicon gate and its underlying silicon dioxide. For visible wavelengths, the absorption coefficient of silicon changes from a little over 105 m−1 for red light to a little under 107 m−1 for blue light. That means for red light passing through 1 µm of silicon the intensity would fall to about 90% [= exp(−105 × 10−6)] of its initial value. The same path would extinguish blue light altogether. The specification for gate thickness is therefore critical and is considerably less than 1 µm; some designs also have small holes etched in the gates of the blue sensors. Across the entire array, the thickness of gate material and the distribution of holes in the gates of blue sensors must be highly uniform, otherwise different parts of the array will see similar things somewhat differently.

The next point to note is that not all the silicon surface of an imaging chip is available for image capture. This is due to two more important strategies. The first is the electronic read-out from the pixels. The second is an electronic shutter that effectively freezes an image before reading out.

T356_2

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to university level study, find out more about the types of qualifications we offer, including our entry level Access courses and Certificates.

Not ready for University study then browse over 900 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus