Noise in buried channel charge-coupled devices

In this paper we discuss the noise measured at the output of a buried channel charge-coupled device (BCCD) linear shift register. The measured noise arises from four sources; the electrical insertion of signal charge, the output amplifier, dark current, and bulk state trapping. In making these measurements the concept of correlated double sampling was used in an output circuit which had a noise level which was equivalent to less than 3O noise electrons. A critical component in this output was a low noise MOSFET which was achieved by use of the buried channel technology. A low noise input structure for electrical insertion of signal charge was used which introduced a signal which had a noise level which ranged from less than 10 e^-to as high as 60 e^-depending on the size of the signal charge. The dark current noise was found to be well characterized as a shot noise and levels on the order of 20 e^-were measured. The above low noise levels made possible direct measurement of the noise due to bulk state trapping, and depending on the signal size and clock rate noise levels were measured which ranged from less than 10 to over 100 noise electrons. One of the most important bulk traps was found to be due to gold impurities which had a density of ∼ 2 × 10¹¹cm^-3.     

Noise and distortion considerations in charge-coupled devices

Noise and distortion considerations for charge-coupled devices used in imaging applications are presented. Distortion occurs because of transfer inefficiency and leakage current. Noise contributions due to thermal noise during the formation of the potential wells, shot noise in the leakage current and interface-state noise are discussed.     

Interlacing in charge-coupled imaging devices

Solid-state imaging devices to be used in commercial broadcast TV or the Picturephone® system have to supply the video information in a 2:1 interlaced timing format. An efficient way to achieve such a readout from charge-coupled area imaging devices of the frame transfer type is described. The resolution cells of the device are elongated in the vertical dimension to extend across the distance corresponding to two scanning lines in the display. For the two fields forming a full frame the charge generated by the incident light is integrated alternately underneath different sets of electrodes. Experimental results obtained on a 64 × 106 element frame transfer array are presented and compared to theory. They show that in a three-phase structure, by alternately integrating the charge underneath electrodes number 1 and numbers 2+3 jointly, the vertical resolution can be improved by about a factor of 2. This results in a limiting resolution in the vertical direction of about 75 percent of that given by the spatial pitch of the transfer cells. The value 75 percent is characteristic of all raster scanned imaging devices and is a result of the line scanned display. The usefulness of this scheme for line imaging devices is also discussed but is dismissed as being inferior to a bilinear approach with separate interdigitated integration sites.     

Overlapping-gate buried-channel charge-coupled devices

In this letter the advantages of the overlapping-gate buried-channel charged-coupled devices over the 3-phase metal-gate and resistive-gate buried-channel c.c.d. are discussed and pertinent design considerations for the overlapping-gate c.c.d.s are presented.     

Moat-etched two-phase charge-coupled devices

A novel technique for fabricating two-phase charge-coupled devices is described. The structure requires only thermally grown SiO₂ and makes use of moats etched into the silicon which in conjunction with a single layer metallization achieve small interelectrode spacings and directionality of charge transport. The feasibility of the technique is demonstrated experimentally. The devices fabricated were successfully operated as both digital and analog shift registers. The method described offers certain advantages in ease of fabrication and reliability along with the capability for high speed operation.     

Introduction to charge-coupled devices

The operating principles of charge-coupled devices are described in terms of the basic properties of semiconductors. The overall aim is for those unfamiliar with CCD's to understand how they work and thus to appreciate their capabilities as well as their limitations. The topics discussed include charge storage and transfer, electrical and optical inputting of charge and charge sensing, factors governing maximum and minimum operating frequencies, transfer efficiency, and basic CCD fabrication techniques.     

Hot electrons in short-gate charge-coupled devices

Calculations are presented which show that the heating of electrons by high electric fields in short-gate surface-channel charge-coupled devices slows down the charge transfer process because of mobility reduction and hot carrier diffusion. Other consequences of carrier heating are a reduction of the interface-trapping noise and transfer inefficiency at short transfer cycles.     

Thermal carrier generation in charge-coupled devices

The build-up of thermally generated carriers in a charge-coupled device shift register is characterized by constructing a model for the generation inside a single shift-register bit. Using the model, theoretical response curves are constructed for two practical modes of operation where the contribution from the generation of carriers can be substantial. Experiments are presented which confirm all aspects of the theoretical response curves, including the presence of an initial period of reduced generation in one of the two modes. Procedures for determining generation parameters directly from observed CCD characteristics are presented and implemented. One generation parameter, the minority carrier lifetime τ, is determined by employing the CCD connected in a gate-controlled diode configuration; two others, the depleted surface generation velocity s0, and the general shape of the depletion layer, are determined utilizing a curve fitting procedure. The spatial variation in generation rates is also investigated and found to possess a distribution which is skewed positively and not Gaussian.     

Free charge transfer in charge-coupled devices

The free charge-transfer characteristics of charge-coupled devices (CCD's) are analyzed in terms of the charge motion due to thermal diffusion, self-induced drift, and fringing field drift. The charge-coupled structures considered have separations between the gates equal to the thickness of the channel oxide. The effect of each of the above mechanisms on charge transfer is first considered separately, and a new method is presented for the calculation of the self-induced field. Then the results of a computer simulation of the charge-transfer process that simultaneously considers all three charge-motion mechanisms is presented for three-phase CCD's with gate lengths of 4 and 10 µ. The analysis shows that while the majority of the charge is transferred by means of the self-induced drift that follows a hyperbolic time dependence, the last few percent of the charge decays exponentially under the influence of the fringing field drift or thermal diffusion, depending on the design of the structure. The analysis shows that in CCD's made on relatively high resistivity substrates, the transfer by fringing-field drift can be very fast, such that transfer efficiencies of 99.99 percent are expected at 5- to 10- MHz bit rates for 10-µ gate lengths and at up to 100 MHz for 4-µ gate lengths.     

Drift-aiding fringing fields in charge-coupled devices

Transverse electric fields at the Si-SiO/SUB 2/ interface (fringing fields) can aid transfer of charge from one potential well to another in charge- coupled devices (CCD). The magnitudes of these drift-aiding fringing fields are evaluated by an approximate analysis and also by computer. The analytical approach assumes zero space charge in the silicon and agrees with the computer solutions in that limit. When the depletion width becomes less than the gate width the fringing field is reduced considerably. Transit times associated with these fringing fields are evaluated. Trapping of charge in interface states rather than transfer of free charge is expected to be the fundamental limitation on speed and transfer efficiency in appropriately designed CCDs.     

Theory of amorphous-silicon charge-coupled devices

The theory of operation of amorphous-silicon charge-coupled devices has been studied numerically and analytically under the assumption that the localized states in amorphous-silicon are distributed exponentially with respect to energy. The transfer inefficiency ε is found to depend not only on the localized state density but also on the transit time and initial density of signal electrons. The approximate analysis shows thatln(epsilon)is a linear function of logarithmic clock frequency, and that its coefficient is given by the characteristic temperature which represents the steepness of the localized state density distribution in amorphous-silicon.     

Transform coding using charge-coupled devices

A relatively simple discrete-time analogue technique is described by which short sections of waveforms can be reversibly transformed into a different domain by using a continuous, rather than block, mode of working. The chosen transform is set up as a matrix of resistors whose conductances correspond with the Fourier, Walsh, slant etc. basis vectors required.     

Simple model for charge-coupled devices

A theory for charge-coupled devices has been developed, based on the transient nature of charge flow in these devices. It has been shown that the limiting factor in charge-transport processes is that of diffusion, which gives a typical time of 10?8 s for 90% of charge transfer.     

Antialiasing inputs for charge-coupled devices

An input scheme for analog charge-coupled signal-processing devices has been developed that acts as a prefilter to suppress the aliasing frequencies produced by the sampled-data nature of the CCD. Each charge packet injected into the CCD is a sum of several weighted subsamples taken at a multiple of the CCD clock rate. Various approaches that differ in their rates of oversampling and in their time periods over which subsamples are combined have been compared with respect to their usefulness to produce a sharp cutoff prefilter. A particular implementation using only a factor of 2 oversampling but an averaging period that extends over two CCD clock periods results in a good prefilter characteristic and has been applied to a charge-coupled split-electrode transversal filter. Different implementations of the basic scheme and layout considerations are discussed.     

Interference-rejection filter using charge-coupled devices

An application of c.c.d. is described to realise a multiple-notch filter which can be made to track and remove a periodic interfering signal and its harmonics. It is shown that, to first order, the filter notches are not dependent on incomplete charge transfer.     

Video integration using charge-coupled devices

C.C.D. analogue shift registers are shown to promise a convenient method for video integration and bandwidth compression. The performance of an experimental prototype is described.     

Signal processing with charge-coupled devices

The purpose of this paper is to discuss applications of charge-coupled devices in signal processing systems. Both analog and digital CCD concepts are considered. Recent developments in high-speed (∼100 MHz) CCD's are discussed, and the uses of high-speed CCD and surface-acoustic wave (SAW) devices together are considered. Examples of the applications of CCD's in electro-optical systems, secure voice communication systems, sonar systems and radar systems are given. Finally, projections for future uses of CCD's in signal processing systems are presented.     

Charge transfer in overlapping gate charge-coupled devices

A detailed numerical simulation of the free charge transfer in overlapped gate charge-coupled devices is presented. The transport are analyzed in terms of thermal diffusion, self-induced fields, and fringing fields under all the relevant electrodes and interelectrode regions with time-varying gate potentials. The results of the charge transfer with different clocking schemes and clocking waveforms are presented. The dependence of the stages of the charge transfer on the device parameters are discussed in detail. A lumped-circuit model of CCD that could be used to obtain the charge-transfer characteristics with various clocking waveforms is also presented.     

Novel tapping technique for charge-coupled devices

The letter proposes a useful technique for the nondestructive tapping of charge-coupled devices, with a presentation of both theoretical and experimental results for device operation. The usefulness of the technique lies primarily in its simplicity of fabrication and operation.