Types of Charge-Coupled Devices with their Working Principles

The scientists Williard Boyle and George E. Smith from AT&T Bell Labs, while working on semiconductor -bubble-memory designed a device, and termed it as ‘Charge Bubble Device’, which can be used as a Shift Register.

According to the fundamental nature of the device, it has the ability to transfer charge from one storage capacitor to the next, along the surface of the semiconductor, and this principle is similar to the Bucket-Brigade Device (BBD), which was invented in the 1960s at Phillips Research Labs. Eventually, from all such experimental research activites, the Charge Coupled Device (CCD) was invented in AT&T Bell Labs in 1969.

Charge Coupled Device (CCD)

Charge Coupled Devices can be defined in different ways according to the application for which they are used or based on the design of the device.

It is a device used for the movement of electrical charge within it for the charge manipulation, which is done by changing the signals through stages within the device one at a time.

It can be treated as CCD sensor, which is used in the digital and video cameras for taking images and recording videos through photoelectric effect. It is used for converting the captured light into digital data, which is recorded by the camera.

It can be defined as a light-sensitive integrated circuit imprinted on a silicon surface to form light-sensitive elements called pixels, and each pixel is converted into an electrical charge.

It is termed as a discrete-time device used for continuous or analog signal sampling at discrete times.

Types of CCD

There are different CCDs such as electron multiplying CCDs, intensified CCD, frame-transfer CCD and buried-channel CCD. A CCD can be simply defined as Charge Transfer Device. The inventors of the CCD, Smith and Boyle also discovered a CCD with greatly enriched performance than a general Surface Channel CCD and other CCDs; it is known as Buried channel CCD and is majorly used for practical applications.

Charge Coupled Device’s Working Principle

The silicon epitaxial layer acting as a photoactive region and a shift- register-transmission region are used for capturing images using a CCD.

Through the lens image is projected onto the photo active region consisting of capacitor array. Thus, the electric charge proportional to the light intensity of the image pixel color in the color spectrum at that location is  accumulated at each capacitor.

If the image gets detected by this capacitor array, then the electrical charge accumulated in each capacitor is transferred to its neighbor capacitor by performing as a shift register controlled by the control circuit.

In the above figure, from a, b and c, the transfer of charge packets is shown according to the voltage applied to the gate terminals. At last, in the array electrical charge of last capacitor is transferred into the charge amplifier in which the electric charge is converted into a voltage. Thus, from the continuous operation of these tasks, entire charges of the capacitor array in the semiconductor are converted into a sequence of voltages.

This sequence of voltages is sampled, digitized and then stored in memory in case of digital devices such as digital cameras. In case of analog devices such as analog video cameras, this sequence of voltages is fed to a low-pass filter to produce a continuous analog signal, and then the signal is processed for transmission, recording and for other purposes. To understand the charge coupled device principle and charge coupled device working in depth, primarily the following parameters need to be understood.

Charge Transfer Process

The charge packets can be moved from cell to cell by using many schemes in Bucket Brigade style. There are various techniques such as two phase, three phase, four phase, and so on. Every cell consists of n-wires passing through it in n-phase scheme. The height of the potential wells is controlled by using each wire connected to transfer clock. Charge packets can be pushed and pulled along the line of the CCD by varying the height of the potential well.

Consider a three-phase charge transfer, in the above figure, the three clocks (C1, C2 and C3) which are identical in shape but in different  phases are shown. If gate B goes high and gate A goes low, then the charge will move from space A to space B.

Architecture of CCD

The pixels can be transferred through the parallel vertical registers or vertical CCD (V-CCD) and parallel horizontal registers or horizontal CCD (H-CCD). The charge or image can be transferred using different scanning architectures such as full frame readout, frame transfer and interline transfer. The charge coupled device principle can be easily understandable with the following transfer schemes:

1. Full-Frame Readout

It is the simplest scanning architecture which requires a shutter in a number of applications to cut off the light input and to avoid smearing during the passage of charges  through parallel-vertical registers or vertical CCD and parallel-horizontal registers or horizontal CCD and then transferred to output in serial.

2. Frame Transfer

By using the bucket brigade process the image can be transferred from image array to opaque frame storage array. As it does not use any serial register, it is a fast process compared to other processes.

3. Interline transfer

Each pixel consists of a photodiode and opaque charge storage cell. As shown in the figure, the image charge is first transferred from light sensitive PD to the opaque V-CCD. This transfer, as the image is hidden, in one transfer cycle  produces a minimum image smear; hence,  the fastest optical shuttering can be achieved.

MOS Capacitor of CCD

Every CCD cell has metal oxide semiconductor, even though both surface channel and buried channel MOS capacitors are used in manufacturing the CCD. But frequently CCDs are fabricated on a    P-type substrate and manufactured by using buried channel MOS capacitors; for this a thin N-type region is formed on its surface. A silicon dioxide layer is grown as an insulator on the top of the N-region, and gates are formed by placing one or more electrodes on this insulating layer.

CCD Pixel

Free electrons are formed from photoelectric effect when the photons strike the silicon surface, and because of the vacuum, simultaneously, positive charge or the hole will be generated. Instead of choosing difficult process of counting the thermal fluctuations or heat formed by the recombining of hole and electron, it is preferred to collect and count electrons to produce an image. This can be achieved by attracting electrons generated by striking photons on silicon surface towards the positively biased distinct areas.

Full well capacity can be defined as the maximum number of electrons that can be held by each CCD pixel and, typically, a CCD pixel can hold 10ke to 500ke, but it depends on the size of the pixel (the bigger the size more electrons can be accumulated).

CCD Cooling

Generally CCDs work at low temperature, and thermal energy can be used for exciting inappropriate electrons into image pixels which cannot be differentiated from the real-image photoelectrons. It is called as a dark current process, which generates noise. The total dark current generation can be reduced by two times for every 6 to 70 of cooling with certain limits. The CCDs do not work below -1200 and the total noise generated from the dark current can be removed by cooling it around -1000, by thermally isolating it in an evacuated environment. CCDs are frequently cooled by using liquid nitrogen, thermo-electric coolers and mechanical pumps.

Quantum Efficiency of CCD

The rate of generation of photoelectrons depends on the light incident on the surface of the CCD. The conversion of the photons into electric charge is contributed by many factors and is termed as Quantum Efficiency. It is in the better range of 25% to 95% for CCDs compared to other light- detection technique.

The front- illuminated device generates a signal after the light passes through the gate structure by attenuating the incoming radiation.

The back-illuminated or back-thinned CCD consists of excess silicon on the underside of the device, which is imprinted in a way that unrestrictedly allows generation of photoelectrons.

This article thus concludes with the brief description of CCD and its working principle considering different parameters such as CCD scanning architectures, Charge transfer process, MOS capacitor of CCD, CCD pixel, cooling and quantum efficiency of CCD in brief. Do you know typical applications in which CCD sensor is being used frequently? Please post your comments below for detailed information regarding working and applications of CCDs.