Physics and Technology of CCD

A charge-coupled device (CCD) is a device for the movement of electrical charge from one capacitor to another one developed by W. Boyle and G. Smith in Bell Laboratories from Bucket-Brigade Device (BBD) which is a device basically transfers charge packets from one transistor to another [1].

The CCD can perform a wide range of the electronic functions, including image sensing and signal processing. Boyle and Smith share one half of the 2009 Nobel Prize in Physics for their invention of the CCD used to generate high quality images in electronic form [2].

1. The developments of CCD

In 1969 F. Sangster and K. Teer of the Philips Research Labs invented the Bucket-Brigade Device or BBD. This device basically transfers charge packets from one transistor to another. One year later, W. Boyle and G. Smith of the Bell Laboratories extended this concept by inventing a transport mechanism from one capacitor to another one, which is CCD
[3]. The first working CCD was an 8-bit shift register [4], as a memory device which could only "inject" charge into the device at an input register. However, it was soon clear that the CCD could also accumulate charge via the photoelectric effect and electronic images could be created. By 1971, Bell researchers Michael F. Tompsett et al.[5] were able to capture images with simple linear devices, thus the CCD imager was born. The first commercial device which had a linear 500-element device and a 2-D 100 x 100 pixel device was developed in 1974 by ex-Bell researcher Gil Amelio, and since then, CCD become programs for many companies as the CCD had a significantly higher sensitivity (100 times greater than film) and it displaced other sensors within a few years [1,3].

2. Basic theory of CCD

2.1 Theory of MOS

As mentioned above, the fundamental building block of the CCD is the MOS capacitor, the backbone behind the charge collection and charge transfer. A p-MOS capacitor consists of p-type (e.g. boron doped) silicon substrate, a gate dielectric (e.g. silicon dioxide) and a conductive gate that is deposited on the Insulator, as showed in Fig. 1 [6]. In ideal MOS diode, the only charge exist in MOS are those in the semiconductor and those with equal but opposite sign on the metal surface adjacent to the oxide. There is no carrier transport through the oxide under direct current as the resistivity of the oxide is infinite.
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Fig. 1 Perspective view of a MOS

When the MOS diode is biased with positive or negative voltage, three cases may exist at the semiconductor surface: accumulation, depletion and inversion, as the energy band diagram showed in Fig. 2.

Fig. 2 Energy band diagrams of ideal MOS diode in (a) zero voltage,  (b) accumulation, (c) depletion and (d) inversion case [6].

2.2 Structure of CCD

The basic device consists of a closely spaced array of metal-oxide-semiconductor (MOS) diodes on a continuous insulator layer (oxide) that covers the semiconductor substrate, as showed in Fig. 3[6]. There are three gate electrodes on the top of Silicon dioxide, and the voltage applied can control the actions of charge storage and transfer of CCD, as the surface depletion can be controlled: a slight higher bias applied on the center electrode will induce the center of MOS a greater depletion and formed a potential well, while a higher bias on the side electrode will cause the transfer of minority carriers in n-type semiconductor.  Thus the quiescent storage site of MOS can be adjusted by the potential on the electrodes.

Each MOS can be seemed as a pixel, and the charges can be shifted from one pixel to another pixel by digital pulses applied to the top plates (gates). In this way the charges can be transferred row by row to a serial output register.

Since electrons can be optical generated or more precisely excited from the valence in the conduction band, the CCD can be used as a light sensor for cameras. Cameras where the light penetrates through the gate structure to reach the region where electrons are collected, are called front-illuminated. More sophisticated in the production, but with a higher sensitivity are cameras where the CCD chip is exposed from the opposite side. These cameras are called back-illuminated. To insure charge transport from the back to the front side where the electrons are collected, the silicon bulk is thinned.

Fig. 3 Cross section of a three-phase charge-coupled device [6].

2.3 Performance functions

Technically speaking, to generate an image the CCD must perform four primary tasks: (1) charge generation, (2) charge collection, (3) charge transfer and (4) charge measurement [7]. Firstly, the CCD should generate an electron in the silicon ship by intercepting an incoming photon. Then, the CCD should accurately reproduce an image from the electrons generated. This process related to the number of pixels contained on the chip, signal electrons that a pixel can hold and the ‘target pixel’ to efficiently collect electrons when they generated.  The third operation, charge transfer, is accomplished by manipulating the voltage on a parallel sequence of gates that form a CCD register, or conveyor belt in our bucket brigade analogy. It is important to lose as little of the charge as possible during the transfer process, which has been progressed from 99% of the first CCD to 99.9999% during the latest 25 years, which means only one in every million electrons is lost during a typical transfer. The last major operation to occur during CCD imaging is the detection and measurement of the charge collected in each pixel.  This is accomplished by dumping the charge onto a small capacitor connected to an output MOSFET amplifier, the only active element that requires power.

3. Functional features

CCD can convert optical signals into digital signal directly to achieve the acquisition, storage, transmission and proceeding of images. The special characterizations are

1.   Small in size and light in weight
2.   Low power consumption, low working voltage
3.   Stable performance and long operational life, resistant of impact and vibration
4.   High sensitivity, low noise and large dynamic range
5.   Quick respond, with self-scanning function, small image distortion, non-residual image
6.   Applicable to ultra-large scale integrated circuit, with high integration of pixel, accurate size, and low cost

Consequently, CCD shows wide applications in varied fields.

4. Applications of CCD

CCD device and its application technology have been developed, and remarkable progress, especially in the mage sensor and non-contact measurement have been made in the past decades years. With the theory development, CCD becomes a high-sensitivity device and used in many regions. Some of them are listed here in this report:

4.1 CCD digital camera

CCD cameras contain light-sensitive silicon chips that detect electrons excited by incoming light, and the micro circuitry that transfers a detected signal along a row of discrete picture elements or pixels, scanning the image very rapidly [9]. Two-dimensional CCD arrays with many thousand of pixels are used in these CCD cameras, and they are often used in machine vision applications.

CCD cameras can operate in both monochrome (black, white, and grayscale) and color.  The range of colors is generated by varying combinations of different discrete colors, like red, green, and blue components (RGB), to create a wide spectrum of colors. 
Important performances of CCD cameras include horizontal resolution, maximum frame rate, shutter speed, sensitivity, and signal-to-noise ratio.  Other parameters to consider when specifying CCD cameras include specialty applications, performance features, physical features, lens mounting, shutter control, sensor specifications, dimensions, and operating environment parameters.

The CCD camera can be applied in astronomy, medicine, optical scanner, etc., as its high quantum efficiencies, linearity of outputs and ease of use [1].

4.2 CCD image sensor

CCD image sensors are electronic devices which are capable of transforming a light pattern (image) into an electric charge pattern (an electronic image). The CCD consists of several individual elements that have the capability of collecting, storing and transporting electrical charge from one element to another, as described in the theory part. Together with the photosensitive properties of silicon, CCD is used to design image sensors.

With semiconductor technologies and design rules, one or more output amplifiers at the edge of the chip collect the signals from the CCD, and electronic images can be obtained by applying series of pulses that transfer the charge of one pixel after another to the output amplifier, line after line. The output amplifier converts the charge into a voltage, while external electronics will transform this output signal into a form suitable for monitors or frame grabbers. Thus CCDs have extremely low noise figures.CCD image sensors can also be a color sensor or a monochrome sensor, as the CCD camera.

Important image sensor performances include spectral response, data rate, quantum efficiency, dynamic range, and number of outputs.  An important environmental parameter to consider is the operating temperature.

CCD image sensors have found important applications in many areas of society and science, like digital cameras, scanners, medical devices, satellite surveillance and in instrumentation for astronomy and astrophysics.

4.3 Optical scanner

CCD used in fax machines forms images on the surface of arrayed capacitor. The brightness of images produces each capacity with charges, which can be transferred to amplifier and forms voltage at the edge of circuit. With the information of the voltage, the images can be stored and print out.

5. Prospects of CCD

Solid-state image sensors and digital cameras have changed the role of images in our society, as they give electronic signals, digits, which can easily be transmitted and treated [2]. It is a real revolution that the images can be transferred and processed in scientist. Digital image processing is now a global commodity which enables the best international communication, through remote control and feedback through digital cameras. Furthermore, the evaluation of large amounts of data, like in mapping the universe, can be spread to many groups and even to volunteers from the general public.


[4] M. F. Tompsett, G. F. Amelio, and G. E. Smith., Charge coupled 8-bit shift register. Appl. Phys. Lett. 17, 111 (1970)
[5] Tompsett, M.F. Amelio, G.F. Bertram, W.J., Jr. Buckley, R.R. McNamara, W.J. Mikkelsen, J.C., Jr. Sealer, D.A. Charge-coupled imaging devices: Experimental results. IEEE Transactions on Electron Devices 18 (11): 992–996
[6] S.M. Sze. Semiconductor devices: Physicas and Technology (2nd ed.)Unite State of American. 2002
[7] James R. Janesick. Scientific charge-coupled devices. Unite state of American, 2001