Transfer Gate Research Articles

Oxo–Mo(IV)(dithiolene)thiolato Complexes: Analogue of Reduced Sulfite Oxidase

A series of [Mo(IV)O(mnt)(SR)(N-N)](-) (mnt = maleonitriledithiolate; R = Ph, nap, p-Cl-Ph, p-CO2H-Ph, and p-NO2-Ph; N-N = 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen)) complexes analogous to the reduced active site of enzymes of the sulfite oxidase family has been synthesized and their participation in electron transfer reactions studied. Equatorial thiolate and dithiolene ligations have been used to closely simulate the three sulfur coordinations present in the native molybdenum active site. These synthetic analogues have been shown to participate in electron transfer via a pentavalent EPR-active Mo(V) intermediate with minimal structural change as observed electrochemically by reversible oxidative responses. The role of the redox-active dithiolene ligand as an electron transfer gate between external oxidants and the molybdenum center could be envisaged in one of the analogue systems where the initial transient EPR signal with <g> = 2.008 is replaced by the appearance of a typical Mo(V)-centered EPR (<g> = 1.976) signal. The appearance of such a ligand-based transient radical at the initial stage has been supported by the ligand-centered frontier orbital from DFT calculation. A stepwise rationale has been provided by computational study to show that the coupled effects of the diimine bite angle and the thiolato dihedral angle determine the metal- or ligand-based frontier orbital occupancy. DFT calculation has further supported the similarity between the reduced, semireduced, and oxidized resting state of the molybdenum center in Moco of SO with the synthesized complexes and their corresponding one-electron and fully oxidized species.

Feedforward Effect in Standard CMOS Pinned Photodiodes

The charge handling capacity or the full well of the photodiodes used in CMOS image sensors is a very important characteristic because it affects the saturation level and the dynamic range of the image sensor. The scaling of the pixel size to increase the spatial resolution is also reducing the barrier separating the photon detection and the collection node in a standard pinned photodiode (PPD). The barrier reduction and the thermionic emission of the electrons allow some of the charges from the photodiode well to feed into the collection node, resulting in a feedforward voltage. In conventional readout of the pixels, this feedforward voltage is neglected and lost when the collection node is reset. The barrier height of the transfer gate (TG) determines the quantity of electrons held back in the photodiode well. Thus, the knowledge of this barrier height is very important in determining the true charge handling capacity of the photodiode potential well. Experiments with standard PPDs showed that a barrier height of around 0.5 V is needed to hold the electrons in the photodiode potential well. This is analogous to the barrier potential for charge-coupled devices reported in the literature. Furthermore, the barrier height dependence on the charge storing time in the photodiode well and the structural dimensions of the TG and photodiode length are also explored in this paper.

Memoryless Wide-Dynamic-Range CMOS Image Sensor Using Nonfully Depleted PPD-Storage Dual Capture

A new low-cost high-performance dual-capture image sensor for wide-dynamic-range (WDR) applications is proposed. The proposed pixel uses a pinned photodiode (PPD) as a charge storage to achieve a WDR operation capability. By controlling the applied voltage at a transfer gate (TG) of the pixel to operate the PPD in a nonfull-depletion operation mode, the fixed pattern noise (FPN) that originated from the threshold characteristic variation of the TG is efficiently calibrated without additional circuitry or image processing steps. The prototype sensor was fabricated by using a 0.13-μm CMOS image sensor process. The chip includes a 320 × 240 pixel array with a 2.25-μm pixel pitch. A WDR of 91 dB has been achieved from the proposed operation mode while maintaining an FPN less than 0.7% over the entire dynamic range.

Charge transfer efficiency improvement of a 4-T pixel by the optimization of electrical potential distribution under the transfer gate

The charge transfer efficiency improvement method is introduced by optimizing the electrical potential distribution under the transfer gate along the charge transfer path. A non-uniform doped transfer transistor channel is introduced to provide an ascending electrical potential gradient in the transfer transistor channel. With the adjustments to the overlap length between the R1 region and the transfer gate, the doping dose of the R1 region, and the overlap length between the anti-punch-through (APT) implantations and transfer gate, the potential barrier and potential pocket in the connecting region of transfer transistor channel and the pinned photodiode (PPD) are reduced to improve the electrical potential connection. The simulation results show that the percentage of residual charges to total charges drops from 1/104 to 1/107, and the transfer time is reduced from 500 to 110 ns. This means the charge transfer efficiency is improved.

Radiation Effects in Pinned Photodiode CMOS Image Sensors: Pixel Performance Degradation Due to Total Ionizing Dose

Several Pinned Photodiode (PPD) CMOS Image Sensors (CIS) are designed, manufactured, characterized and exposed biased to ionizing radiation up to 10 kGy(SiO2 ). In addition to the usually reported dark current increase and quantum efficiency drop at short wavelengths, several original radiation effects are shown: an increase of the pinning voltage, a decrease of the buried photodiode full well capacity, a large change in charge transfer efficiency, the creation of a large number of Total Ionizing Dose (TID) induced Dark Current Random Telegraph Signal (DC-RTS) centers active in the photodiode (even when the Transfer Gate (TG) is accumulated) and the complete depletion of the Pre-Metal Dielectric (PMD) interface at the highest TID leading to a large dark current and the loss of control of the TG on the dark current. The proposed mechanisms at the origin of these degradations are discussed. It is also demonstrated that biasing (i.e., operating) the PPD CIS during irradiation does not enhance the degradations compared to sensors grounded during irradiation.

A Wide Dynamic Range CMOS Image Sensor Based on a Pseudo 3-Transistor Active Pixel Sensor Using Feedback Structure

A dynamic range extension technique is proposed based on a 3-transistor active pixel sensor (APS) with gate/body-tied p-channel metal oxide semiconductor field effect transistor (PMOSFET)-type photodetector using a feedback structure. The new APS consists of a pseudo 3-transistor APS and an additional gate/body-tied PMOSFET-type photodetector, and to extend the dynamic range, an NMOSFET switch is proposed. An additional detector and an NMOSFET switch are integrated into the APS to provide negative feedback. The proposed APS and pseudo 3-transistor APS were designed and fabricated using a 2-poly 4-metal standard complementary metal oxide semiconductor (CMOS) process. Afterwards, their optical responses were measured and characterized. Although the proposed pixel size increased in comparison with the pseudo 3-transistor APS, the proposed pixel had a significantly extended dynamic range of 98 dB compared to a pseudo 3-transistor APS, which had a dynamic range of 28 dB. We present a proposed pixel that can be switched between two operating modes depending on the transfer gate voltage. The proposed pixel can be switched between two operating modes depending on the transfer gate voltage: normal mode and WDR mode. We also present an imaging system using the proposed APS.

A CMOS Image Sensor Based on Unified Pixel Architecture With Time-Division Multiplexing Scheme for Color and Depth Image Acquisition

We propose a CMOS image sensor with time-division multiplexing pixel architecture using standard pinned-photodiode for capturing 2-D color image as well as extracting 3-D depth information of a target object. The proposed pixel can alternately provide both color and depth images in each frame. Two split photodiode and four transfer gates in each pixel improve the transfer speed of generated electrons to be capable of demodulating a high-frequency time-of-flight signal. In addition, four-shared pixel architecture acquires a color image with high spatial resolution and generates a reliable depth map by inherent binning operation in charge domain. A 712 496 pixel array has been fabricated using a 0.11μm standard CMOS imaging process and fully characterized. A 6m pixel with 34.5% aperture ratio can be operated at 10-MHz modulation frequency with 70% demodulation contrast. We have successfully captured both images of exactly same scene from the fabricated test chip. It shows a depth uncertainty of less than 60 mm and a linearity error of about 2% between 1 and 3 m distance with 50-ms integration time. Moreover, high-gain readout operation enables to improve the performance, achieving about 43-mm depth uncertainty at 3 m.

A Time-Resolved CMOS Image Sensor With Draining-Only Modulation Pixels for Fluorescence Lifetime Imaging

This paper presents a time-resolved CMOS image sensor with draining-only modulation (DOM) pixels, for time-domain fluorescence lifetime imaging. In the DOM pixels using a pinned photodiode (PPD) technology, a time-windowed signal charge transfer from a PPD to a pinned storage diode (PSD) is controlled by a draining gate only, without a transfer gate between the two diodes. This structure allows a potential barrierless and trapless charge transfer from the PPD to the PSD. A 256 × 256 pixel time-resolved CMOS imager with 7.5 × 7.5 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> DOM pixels has been implemented using 0.18-μm CMOS image sensor process technology with PPD option. The prototype demonstrates high sensitivity for weak signal of less than one electron per light pulse and accurate measurement of fluorescence decay process with subnanosecond time resolution.

Identification of Radiation Induced Dark Current Sources in Pinned Photodiode CMOS Image Sensors

This paper presents an investigation of Total Ionizing Dose (TID) induced dark current sources in Pinned PhotoDiodes (PPD) CMOS Image Sensors based on pixel design variations. The influence of several layout parameters is studied. Only one parameter is changed at a time enabling the direct evaluation of its contribution to the observed device degradation. By this approach, the origin of radiation induced dark current in PPD is localized on the pixel layout. The PPD peripheral shallow trench isolation does not seem to play a role in the degradation. The PPD area and a transfer gate contribution independent of the pixel dimensions appear to be the main sources of the TID induced dark current increase. This study also demonstrates that applying a negative voltage on the transfer gate during integration strongly reduces the radiation induced dark current.

Analyzing the Radiation Degradation of 4-Transistor Deep Submicron Technology CMOS Image Sensors

This paper presents a radiation degradation study on 4-Transistor (4T) complementary metal-oxide-semiconductor (CMOS) image sensors designed in standard 0.18-μm technology. The significant contribution of this paper is a systematic evaluation of the X-ray radiation effects on image sensors from the individual device level, to the pixel level and to the level of the entire sensor. The major degradation parameters of the sensor have been analyzed. This paper also includes test structures of varying geometries of in-pixel MOSFETs, pinned photodiodes (PPD), and transfer gates (TG). Characterization was performed during different X-ray doses up to 109 krad. The major degradation-an increase in the dark signal-is analyzed by modifying the TG charge transfer time and integration time. The PPD and the TG are the elements most sensitive to the dark signal of the sensor. The radiation-related dimensional effects on the sensors are also evaluated, which show different results compared to 3T pixels. The transfer-gate length influences the dark signal due to not only the electric field variation in the TG channel but also the local defect generations. In-pixel MOSFETs are used to identify the origin of increases in radiation-induced dark signal. Shallow trench isolation (STI) oxides are responsible for the radiation degradation of the sensor. A slight degradation of the quantum efficiency was observed after radiation in the short-wavelength region. Basic hardening-by-design techniques are also presented. The discussion results of the radiation-related dimensional effects on the sensors together with the STI effect can be used as a guideline for future layout designs of radiation-tolerant sensors. Identifying the pixel dark current origin can help to determine where and how to suppress the pixel dark current generation more effectively.

Highly Sensitive Gate/Body-Tied P-Channel Metal Oxide Semiconductor Field Effect Transistor-Type Photodetector with an Overlapping Control Gate

In this paper, a highly sensitive gate/body-tied p-channel metal oxide semiconductor field effect transistor (PMOSFET)-type photodetector with an overlapping control gate is proposed. The proposed photodetector has an overlapping control gate that makes it possible to control the sensitivity of the proposed photodetector. This sensitivity controllability extends the dynamic range and provides a high sensitivity in a low-light environment. The body of the proposed gate/body-tied PMOSFET-type photodetector is connected to the floating gate, and the control gate is placed on top of the floating gate. The proposed device was fabricated using a 0.35 µm standard complementary metal oxide semiconductor (CMOS) process. The amplified photocurrent of the proposed device was more than 100 times larger than that of a conventional n+/p-sub photodiode with the same area. The area of the proposed photodetector is 3.8×4.4 µm2, which is 24% smaller than that of a conventional gate/body-tied PMOSFET-type photodetector with a transfer gate. Therefore, the proposed photodetector can be suitable for high-sensitivity active pixel sensors (APSs) because of its much higher responsivity than that of a conventional n+/p-sub photodiode.

Novel method for identifying the cause of inherent ageing in Electron Multiplying Charge Coupled Devices

The charge multiplication process used in the Electron Multiplying CCD (EMCCD) is subject to an ageing effect in which the gain achieved at particular avalanche potentials, gradually decreases during operation. To utilise these devices for both space and terrestrial applications where recalibration of the gain is not feasible, a comprehensive understanding of the ageing process is required.A custom automated test equipment (ATE) has been produced and used to develop the techniques required for investigating the ageing process alongside the verification of simulation work on the subject. Simulation work carried out by e2v technologies has suggested hole build-up at the Si/Si02 interface below one of the transfer gates. This build up of holes has now been linked with a reduction in avalanche potential in the device. A novel experimental technique has therefore been developed to determine the actual potentials within the device and thereby determine the validity of this prediction.The initial results tend to support simulation as an increase in the potential beneath one of the phases is observed.

PD-storage dual-capture variable wide dynamic range CMOS image sensor

Proposed is a wide dynamic range CMOS image sensor (CIS) using the PD-storage dual capture (PDS-DC) method that can be applied to a 4-T active pixel sensor without any pixel modification. Dynamic range is increased by controlling the transfer gate during the integration time that is divided into two phases: long-exposure and short-exposure. The photon generated charges for both phases are stored in the photodiode (PD) while the excess charge is drained to VDD during the long-exposure. This feature allows variable dynamic range adjustment by controlling the exposure time ratio and bias voltage that determines the well-capacity during the long-exposure. The prototype sensor is fabricated using 0.13 µm CIS process. The major advantage of the proposed PDS-DC is that it provides complete correlated double sampling. Measurement results demonstrate variable wide dynamic range feature. The effects of process variation are also presented.

Evaluation of the full operational cycle of a CMOS transfer-gated photodiode active pixel

In this paper we evaluate the full operational cycle of the active-pixel circuit for CMOS image sensors in which four field-effect transistors are suitably combined with an ordinary photodiode; meaning a vertical p–n junction, as opposed to photogates, and with no custom pinned layer. Although this circuit topology itself is not novel, this evaluation intends to shine new light on the use of regular photodiodes. They became largely in disuse in four-transistor image chips under earlier process and design circumstances because the fourth transfer-gate FET did not hold a constant signal long enough and not for a fixed time. Our aim is to investigate the full underlying operational regimen of the transfer gate to propose that a regular photodiode might again be a choice to consider in the four-transistor pixel configuration, not only in conventional imaging but also in dedicated optical applications. The use of an ordinary p–n junction photodiode is advantageous as it offers full compatibility with even elementary mainstream CMOS processes. This investigation resorts to experiments and models both with fabricated integrated pixels and with a macro-pixel circuit implementation.

NMR basis for interprotein electron transfer gating between cytochrome c and cytochrome c oxidase

The final interprotein electron transfer (ET) in the mammalian respiratory chain, from cytochrome c (Cyt c) to cytochrome c oxidase (CcO) is investigated by (1)H-(15)N heteronuclear single quantum coherence spectral analysis. The chemical shift perturbation in isotope-labeled Cyt c induced by addition of unlabeled CcO indicates that the hydrophobic heme periphery and adjacent hydrophobic amino acid residues of Cyt c dominantly contribute to the complex formation, whereas charged residues near the hydrophobic core refine the orientation of Cyt c to provide well controlled ET. Upon oxidation of Cyt c, the specific line broadening of N-H signals disappeared and high field (1)H chemical shifts of the N-terminal helix were observed, suggesting that the interactions of the N-terminal helix with CcO are reduced by steric constraint in oxidized Cyt c, while the chemical shift perturbations in the C-terminal helix indicate notable interactions of oxidized Cyt c with CcO. These results suggest that the overall affinity of oxidized Cyt c for CcO is significantly, but not very much weaker than that of reduced Cyt c. Thus, electron transfer is gated by dissociation of oxidized Cyt c from CcO, the rate of which is controlled by the affinity of oxidized Cyt c to CcO for providing an appropriate electron transfer rate for the most effective energy coupling. The conformational changes in Lys13 upon CcO binding to oxidized Cyt c, shown by (1)H- and (1)H, (15)N-chemical shifts, are also expected to gate intraprotein ET by a polarity control of heme c environment.

All-Photonic Multifunctional Molecular Logic Device

Photochromes are photoswitchable, bistable chromophores which, like transistors, can implement binary logic operations. When several photochromes are combined in one molecule, interactions between them such as energy and electron transfer allow design of simple Boolean logic gates and more complex logic devices with all-photonic inputs and outputs. Selective isomerization of individual photochromes can be achieved using light of different wavelengths, and logic outputs can employ absorption and emission properties at different wavelengths, thus allowing a single molecular species to perform several different functions, even simultaneously. Here, we report a molecule consisting of three linked photochromes that can be configured as AND, XOR, INH, half-adder, half-subtractor, multiplexer, demultiplexer, encoder, decoder, keypad lock, and logically reversible transfer gate logic devices, all with a common initial state. The system demonstrates the advantages of light-responsive molecules as multifunctional, reconfigurable nanoscale logic devices that represent an approach to true molecular information processing units.

Distributed quantum computation with superconducting qubit via LC circuit using dressed states

A scheme is proposed where two superconducting qubits driven by a classical field interacting separately with two distant LC circuits connected by another LC circuit through mutual inductance, are used for implementing quantum gates. By using dressed states, quantum state transfer and quantum entangling gate can be implemented. With the help of the time-dependent electromagnetic field, any two dressed qubits can be selectively coupled to the data bus (the last LC circuit), then quantum state can be transferred from one dressed qubit to another and multi-mode entangled state can also be formed. As a result, the promising perspectives for quantum information processing of mesoscopic superconducting qubits are obtained and the distributed and scalable quantum computation can be implemented in this scheme.

Effective dynamics for two-atom entanglement and quantum information processing by coupled cavity QED systems

We propose a scheme for deterministic generation of entanglement embodied by two \(\Lambda \)-type atoms distributed in two coupled cavities. We study such a system in the dispersive atom-field interactions, where the dynamics of the system operates through the virtual population of both the atomic excited states and the photonic states in the cavities (plus the fiber). We verify the validity of the dynamics, and moreover, study the influences of the decoherence due to the spontaneous emission and photon leakage. We also apply the dynamics for realizing quantum state transfer and quantum phase gates.

A Low-Power Multi-Phase Oscillator with Transfer Gate Phase Coupler Enabling Even-Numbered Phase Output

We propose a transfer gate phase coupler for a low-power multi-phase oscillator (MPOSC). The phase coupler is an nMOS transfer gate, which does not waste charge to the ground and thus achieves low power. The proposed MPOSC can set the number of outputs to an arbitrary number. The test circuit in a 180-nm process and a 65-nm process exhibits 20 phases, including 90° different angles. The designs in a 180-nm CMOS process and a 65-nm CMOS process were fabricated to confirm its process scalability; in the respective designs, we observed 36.6% and 38.3% improvements in a power-delay products, compared with the conventional MPOSCs using inverters and nMOS latches. In a 65-nm process, the measured DNL and 3σ period jitter are, respectively, less than ±1.22° and 5.82ps. The power is 284µW at 1.85GHz.

A Three-Dimensional Time-of-Flight CMOS Image Sensor With Pinned-Photodiode Pixel Structure

A pixel architecture for providing not only normal 2-D images but also depth information by using a conventional pinned photodiode is presented. This pixel architecture allows the sensor to generate a real-time 3-D image of an arbitrary object. The operation of the pixel is based on the time-of-flight principle detecting the time delay between the emitted and reflected infrared light pulses in a depth image mode. The pixel contains five transistors. Compared to the conventional 4-T CMOS image sensor, the new pixel includes an extra optimized transfer gate for high-speed charge transfer. A fill factor of more than 60% is achieved with a 12 × 12 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> size for increasing the sensitivity. A fabricated prototype sensor successfully captures 64 × 16 depth images between 1 and 4 m at a 5-MHz modulation frequency. The depth inaccuracy is measured under 2% at 1 m and 4% at 4 m and is verified by noise analysis.