Surface-channel Charge-coupled Devices Research Articles

MCT heteroepitaxial 4 × 288 FPA

4 × 288 heteroepitaxial mercury–cadmium telluride (MCT) linear arrays for long wavelength infrared (LWIR) applications with 28 × 25 micron diodes and charge coupled devices (CCD) silicon readouts were designed, manufactured and tested. MCT heteroepitaxial layers were grown by MBE technology on (0 1 3) GaAs substrates with CdZnTe buffer layers and had cutoff wavelength λ co≈11.8±0.15 μm at T=78 K. To decrease the surface influence of carrier recombination processes the compositionally dependent layers with increase of Cd content both toward the surface and HgCdTe/CdZnTe boundary interface were grown. Silicon readouts with CCD multiplexers with input direct injection circuits were designed, manufactured and tested. The testing procedure, to qualify readout integrated circuits on wafer level at T=300 K, was worked out. The silicon readouts for 4 × 288 arrays, with skimming and partitioning functions included were manufactured by n-channel MOS technology with buried or surface channel CCD register. The HgCdTe arrays and Si CCD readouts were hybridized by cold welding indium bumps technology. With skimming mode used for 4 × 288 MCT n–p-junctions, the detectivity for several tested 4 × 288 arrays was within D λ ∗≅9×10 10–1.8×10 11 cm Hz 1/2/W for background temperature T b=295 K.

Low-temperature operation of silicon surface-channel charge-coupled devices

This paper presents results of the measurement and modeling of the temperature dependence of the charge-transfer inefficiency (CTI) on n- and p-channel surface-channel charge-coupled devices (CCD's) over the 25-300 K temperature range. The CTI was measured at clocking frequencies of 1, 10, and 40 kHz with minimum values of 0.00075 and 0.00018 reached near 50 K for the n- and p-channel devices, respectively. The CTI was modeled in terms of the interaction of the signal charge with an energy-dependent interface state density distribution. At temperatures above 200 K, thermally generated carriers or dark current modify the simple dependence on the interface state density distribution. The two-part model correctly simulates the dependence of the CTI on both the temperature and the frequency of operation. Fractional loss measurements were used to study surface state parameters. Other features of CCD low temperature operation measured and modeled include the input stage equilibration process.

Determination of surface- and bulk-generation parameters from dark-current measurements in surface-channel CCD's

A technique for determining surface-generation velocity and bulk minority-carrier generation lifetime from measurements in surface-channel charge-coupled devices (SCCD) is described. Depleted surface-generation velocity of 1.1 cm/s and bulk minority-carrier generation lifetime of 130 µs have been determined in the channel region for the devices used, and are in good agreement with data obtained by other techniques.

Experimental observation of avalanche multiplication in charge-coupled devices

Avalanche multiplication of signal charge in surface-channel charge-coupled devices is reported in this paper. Experimental observations show that avalanche multiplication takes place when the electrons are made to fall down a steep barrier of more than 8 V in an overlapping gate structure. For a 16-V fall, the gain in charge is about 3-percent per transfer. A simple model is developed which explains the experimental data reasonably well. The upper limit to the amplitude of clock voltages that can be applied to a CCD is likely to be determined by this avalanche multiplication mechanism rather than the oxide breakdown criterion.

Determination of transfer noise from transfer-loss measurements in SCCD's

A simple technique is described that allows the determination of surface-state-induced transfer noise from transfer-loss measurements in surface-channel charge-coupled devices (SCCD's). Transfer noise has been measured on two-phase overlapping polysilicon electrode long-channel SCCD's at temperatures between 77 and 325 K in a frequency range of 1 KHz to 1 MHz. The validity of the technique is supported by the close agreement of the experimentally obtained data and the theoretically calculated values based upon independent measurements of the surface-state density and capture cross section in SCCD's.

Control of boron diffusion in polysilicon for constructing overlapping polysilicon gate charge-coupled devices

The diffusion coefficient of boron having values significantly different in silicon and silicon dioxide has been used to control the doping of boron impurity in intrinsic polysilicon deposited over the gate oxide. The method reduces the possibility of doping gate oxide while diffusing boron in polysilicon. Using the method, silicon gate p-MOSFETS and twenty bit photo-sensor, four phase, double overlapping polysilicon gate surface channel charge-coupled devices have been constructed with a transfer efficiency of 0.9990. The measured values of the threshold voltage of MOSFETS are in close agreement with their corresponding calculated values.

Fabrication of surface-channel charge-coupled devices with ultralow density of interface states

Surface-channel charge-coupled devices (CCD’s) have been fabricated with ultralow density of (fast) interface states in the range 1–3×108/cm2 eV. This low interface state density is achieved by hydrogen implantation into the metal-nitride-oxide-silicon (MNOS) insulator structure of the CCD as the final fabrication step after aluminum interconnect metallization. The CCD’s are shown to have excellent operating characteristics including high transfer efficiency (∼0.99995 without bias charge), low dark current (0.25–0.50 nA/cm2 at 20°C), and high signal charge capacity (1.55×1012 e/cm2 for 10-V clock swing).

Numerical studies of the charge capacity of surface-channel charge-coupled devices

The signal charge distribution and electrostatic potential of a three-phase charge-coupled device are found in a self-consistent manner. The length of the center gate is varied to simulate processing variations and their effect on charge capacity. Potential profiles are shown for a variety of charge levels and gate dimensions. The full-well charge capacity scales with the geometric gate length (down to at least 1 μm) in our example, although the partly filled charge level does not scale. A simple one-dimensional estimate of the full-well capacity is shown to be accurate.

A simple distributed circuit model for charge-coupled devices

A simple distributed circuit model suitable for use in computer-aided design (CAD) has been developed for surface-channel chargecoupled devices (CCD). Both uniformly doped and ion-implanted devices are considered. The model is conceptually simple and numerically efficient.

Charge-transfer noise theory for surface-channel charge-coupled devices

The detailed expression of charge-transfer noise for surface-channel CCD's is derived from the surface generation-recombination rate equation. Not only the charge fluctuation due to occupied interface states but also that due to unoccupied interface states are taken into account. The derived expression can explicitly predict the spectrum intensity dependences on the signal charge concentration and the clock frequency for various amounts of the fat-zero charge. The comparison between theory and experimental results shows good agreement over a wide range of signal charge concentration and clock frequency. It is shown that charge-transfer noise depends heavily on the ratio of the signal charge concentration to the clock frequency and the capture cross section in the absence of fat-zero charge. For the small ratio, the charge fluctuation is ruled by the transfer-process noise which is determined from the amount of unoccupied interface states. For the very large ratio, the storage-process noise which is determined from the interface state distribution over the band gap rules the charge fluctuation. Fat-zero charge hardly reduces the transfer-process noise for the small ratio. For the large ratio, the fat-zero charge enhances the total transfer noise.

Nonoverlapping dual-gate bucket-brigade devices

A novel bucket-brigade charge-transfer device concept is presented and experimentally verified. The dual-gate BBD exhibits operational performances comparable to surface-channel charge-coupled devices, while still enjoying fabrication simplicity and high yield. The concept is implemented using conventional p-channel aluminum-gate technology with only an additional shadowed-gap lift-off process which produces self-aligned bias-gate electrodes separated from the adjacent clock buses by submicron (typically 0.5 μm) spacings. Transfer inefficiencies below 10−4 at 100 kHz and around 5×10−4 at 1 MHz have been currently observed, despite the low carrier mobility (140 cm2/V s).

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.

Green's function calculation of electric fields in surface-channel charge-coupled devices

The tangential component of the electrostatic field at the insulator-semiconductor interface of an MIS structure is studied by means of Green's function techniques. The two-dimensional problem is solved by taking into account the presence of the metallic gate, the dielectric discontinuity at the interface, the influence of the fixed ionized impurity atoms in the depletion region, and that of the mobile surface inversion charge. The theory derived is used to calculate the tangential field at the Si-SiO2 interface in a standard surface-channel charge-coupled device with co-planar electrodes. Special emphasis is placed upon the physical interpretation of the mathematical analysis.

Green's function calculating of electric fields in surface-channel charge-coupled devices : R. M. Barsan. Int. J. Electronics44, (2) 177 (1978)

With the advanced development of fiber-optics, the large bandwidth-expansion required by spread spectrum code-division multiple-access (CDMA) can now be accomodated by using a fiber-optic channel for transmission and incoherent optical signal processing for code generation and correlation. Optical synchronous CDMA (S/CDMA), a synchronous transmission format, is introduced and compared with optical (asynchronous) CDMA. Using modified prime sequence codes, S/CDMA is shown to accomodate a larger number of subscribers and more simultaneous users than CDMA. A two-user experiment demonstrating the feasibility of optical S/CDMA is performed at 10 Mbit/s (500 Mchip/s), using optical signal processing. (A chip is a bit-representation component; please see Section 2.) An environment in which S/CDMA would be particularly suited is also discussed. In addition, recent research shows that the feasibility of optical CDMA is also determined by the architecture of the optical encoders/decoders. The coding architecture affects, for example, the amount of power loss and length of optical delays associated with code sequence generation and correlation, which, in turn, affect the power budget, size, and cost of an optical CDMA system. Therefore, a new, modified 2n coding architecture is proposed and compared with prime and 2n coding architectures. Study shows that the modified 2n architecture performs the best and is particularly suitable for ultrafast all-optical processing and waveguides implementation for the future all-optical CDMA networks. A 100 Mbit/s optical CDMA experiment in free space demonstrating the feasibility of the modified 2n coding architecture, using a serial combination of 50/50 beam splitters and retroreflectors, at 10 Tchip/s (i.e., 100,000 chip/bit) with 100 fs laser pulses is also reported.

Short-channel effects on the input stage of surface-channel CCD's

A detailed study of transient signal charge injection into surface-channel charge-coupled devices using a two-dimensional computer model including the source diffusion and the self-induced and fringing field effects has been carried out. The total delay time required to inject a packet of charge into CCD's for a range of device structures was determined. It is found that the maximum clock pulse frequency of operation is determined by the input delay time and not by the speed of charge transfer which is normally assumed. The results of this study are compared with results obtained using a one-dimensional simulation model for charge injection into CCD's. Experimental justification of the one-dimensional model is provided. With the aid of this analysis a design expression for the intrinsic input delay (the delay associated with the fill portion) for short gate surface-channel CCD's is derived and presented in this paper. It is also shown that for short gate devices ( L µm) the input delay time due to scooping is about two to four times the intrinsic delay.

Experimental confirmation of an analytical model for charge transfer in charge-coupled devices

An analytical model for the charge loss as a function of transfer time under low charge levels in surface channel charge-coupled devices has been theoretically determined and experimentally confirmed. The model includes the effect of surface states. A new method of measuring charge transfer makes it possible to determine the role that surface states play in the degradation of signal charge transfer.

Computer model and charge transport studies in short gate charge-coupled devices

A computer model has been developed that simulates charge transport of carriers in a surface channel charge-coupled device. This model is based on the charge continuity and current transport equations with a time dependent surface field. The device structure of the model includes a source diffusion an input gate and transfer gate. The present model is the first real simulation of the input scheme of the surface-channel CCDs. The scooping and spilling techniques associated with the charge injection process are simulated by the input diffusion which is included in the model. As an application to a CCD practical problem the present model has been used to study the linearity of the electrical charge injection into surface channel charge-coupled devices. The generated harmonic components of a sinusoidal input are calculated using the transfer characteristics of the input stage obtained from the computer simulation. Using this model the spatial variations of the self-induced fringing field and total currents under the storage and transfer gates were computed. The charge transfer mechanisms for short-gate ( L ≤ 8 μm) CCDs was investigated. It was found that for short gates the charge transfer efficiency is governed mainly by the fringing field and self-induced current mechanisms. The results of this study help to clarify the mechanism by which the signal-charge level and gate length affect the charge transfer efficiency.

Signal transfer and degradation in surface-channel charge-coupled devices

The properties of surface-channel charge-coupled shift registers are analysed taking into account all four mechanisms involved in the transfer of charge, namely thermal diffusion, charge gradient-induced drift, fringing-field drift, and interface state trapping. A nonlinear equation for the simulation of the signal transfer and degradation in charge-coupled shift registers in the presence of interface states is derived. It is shown that under some realistic assumptions, the nonlinear properties of both 2-phase and 3-phase devices are governed by the same equation. The theoretical results are used to investigate, by computer simulation, the input/output properties of digital registers. Among the discussed results, special emphasis is placed on worst case output signals. Small-signal and large-signal degradation parameters are also discussed and numerical results are presented.

Electric potential and field in surface-channel charge-coupled devices : G. Baccarani, A. M. Mazzone and M. Rudan. Alta FrequenzaXLV, (12) 291E-735 (December 1976)

Electric potential and field in surface-channel charge-coupled devices : G. Baccarani, A. M. Mazzone and M. Rudan. Alta FrequenzaXLV, (12) 291E-735 (December 1976)

Charge transfer in charge-coupled devices in the presence of interface states

The transfer of charge from under a gate of a three-phase surface-channel charge-coupled device is analysed in terms of thermal diffusion, charge-gradient induced drift, fringing field drift, and interface state trapping. A method based on a piecewise approximation for the emission rate from interface states is proposed and used to derive the single-transfer characteristics in the presence of interface traps. It is shown that the emission rate exhibits a marked spatial dependence, which is a function of both fringing field profile and interface state density. It is also concluded that trapping effects are a strong limitation on the transfer efficiencies attainable in surface-channel charge-coupled devices at low and moderate frequencies.