Floating-gate Amplifier Research Articles

Achieving Single-Electron Sensitivity at Enhanced Speed in Fully Depleted CCDs with Double-Gate MOSFETs.

We introduce a new output amplifier for fully depleted thick p-channel CCDs based on double-gate MOSFETs. The charge amplifier is an n-type MOSFET specifically designed and operated to couple the fully depleted CCD with high charge-transfer efficiency. The junction coupling between the CCD and MOSFET channels has enabled high sensitivity, demonstrating readout noise of 0.74 e_{rms}^{-}/pix in one pixel charge measurement. We have also demonstrated the nondestructive readout capability of the device. Single-electron and single-photon per pixel counting in the entire CCD pixel array has been made possible through the averaging of ten samples, achieving 0.15 e_{rms}^{-}/pix in a pixel readout time of 2.74ms. We have demonstrated fully depleted CCD readout with better performance than the floating diffusion and floating gate amplifiers available today, in both single and multisampling regimes, boasting at least 6 times the speed of floating gate amplifiers.

Single-Electron and Single-Photon Sensitivity with a Silicon Skipper CCD.

We have developed ultralow-noise electronics in combination with repetitive, nondestructive readout of a thick, fully depleted charge-coupled device (CCD) to achieve an unprecedented noise level of 0.068 e^{-} rms/pixel. This is the first time that discrete subelectron readout noise has been achieved reproducible over millions of pixels on a stable, large-area detector. This enables the contemporaneous, discrete, and quantized measurement of charge in pixels, irrespective of whether they contain zero electrons or thousands of electrons. Thus, the resulting CCD detector is an ultra-sensitive calorimeter. It is also capable of counting single photons in the optical and near-infrared regime. Implementing this innovative non-destructive readout system has a negligible impact on CCD design and fabrication, and there are nearly immediate scientific applications. As a particle detector, this CCD will have unprecedented sensitivity to low-mass dark matter particles and coherent neutrino-nucleus scattering, while future astronomical applications may include direct imaging and spectroscopy of exoplanets.

Design and Modelling of a Bidirectional Front-End for Resonating Sensors Based on Pseudo Floating Gate Amplifier

In this paper, we characterize and model a bidirectional front-end based on pseudo-floating gate amplifier (PFGA) for actuation and read-out of resonating sensors. The basic idea consists of swapping the power supply of the PFGA in order to change the directionality of the front-end. A detailed description of the system has been discussed in this paper and supported by simulations and measurement results. A prototype has been fabricated using discrete components and tested with a real transducer (Murata MA40S4) and a Butterworth Van Dyke (BvD) load, which has proved to be proved to be a well approximated model for resonant sensors. The bidirectional amplifier has been implemented with the integrated circuit CD4007UB, which is a commercial discrete component containing low leakage MOSFET. The values chosen for the BvD load are R b = 330 Ω , L m = 60 mH, C s = 450 pF, C E = 2 . 2 nF, which are approximately the same values of the lumped parameters reported in the data-sheet of the real sensor. This transducer is characterized by a nominal resonant frequency of 40 kHz. Measurement results show good fitting with the models developed in this work and the possibility to predict the sensor response by using the BvD load.

Sub-electron readout noise in a Skipper CCD fabricated on high resistivity silicon

The readout noise for Charge-Coupled Devices (CCDs) has been the main limitation when using these detectors for measuring small amplitude signals. A scientific CCD fabricated on a high-resistivity silicon substrate utilizing a floating gate amplifier with the capability of multiple sampling of the charge signal is described in this paper. The Skipper CCD architecture and its advantages for low noise applications are discussed. A technique for obtaining sub-electron readout noise levels is presented, and its noise and signal characteristics are derived. We demonstrate that with this procedure a very low readout noise of 0.2e − RMS can be achieved. The contribution of other noise sources (output stage, vertical and horizontal charge transfer inefficiency, and dark current noise) are also considered. The optimum number of samples for achieving the total lowest possible noise level is obtained. This technique is applied to an X-rays experiment using a 55Fe source.

An autozeroing floating-gate amplifier

We have developed a bandpass floating-gate amplifier that uses tunneling and pFET hot-electron injection to set its dc operating point adaptively. Because the hot-electron injection is an inherent part of the pFET's behavior, we obtain this adaptation with no additional circuitry. Because the gate currents are small, the circuit exhibits a high-pass characteristic with a cutoff frequency less than 1 Hz. The high-frequency cutoff is controlled electronically, as is done in continuous-time filters. We have derived analytical models that completely characterize the amplifier and that are in good agreement with experimental data for a wide range of operating conditions and input waveforms. This autozeroing floating-gate amplifier demonstrates how to use continuous-time floating-gate adaptation in amplifier design.

A second-order section built from autozeroing floating-gate amplifiers

We introduce the autozeroing floating-gate (AFGA) second-order section. We built this second-order filter where the corner frequency and Q are electronically tunable based on a classic filter topology and principles of operational transconductance amplifiers. We built this second-order filter using three AFGAs-our floating-gate amplifier that sets its operating point by the interaction of hot-electron injection and electron tunneling.

A programmable continuous-time floating-gate Fourier processor

We present a programmable continuous-time floating-gate Fourier processor that decomposes the incoming signal into frequency bands by analog bandpass filters, multiplies each channel by a nonvolatile weight, and then recombines the frequency channels. A digital signal processor would take a similar approach of computing a fast Fourier transform (FFT), multiplying the frequency components by a weight and then computing an inverse PFT. We decompose the frequency bands of the incoming signal using the transistor-only version of the autozeroing floating-gate amplifier (AFGA), also termed the capacitively coupled current conveyer (C/sup 4/). Each band decomposition is then fed through a floating-gate multiplier to perform the band weighting. Finally, the multiplier outputs are summed using Kirchoff current law to give a band-weighted output of the original signal. We examine many options to reduce second-order harmonic problems inherent in the single-sided C/sup 4/. We present a method for programming arrays of floating-gate devices that are used in the weighting of the bands. All of these pieces fit together to form an elegant and systematic Fourier processor.

Low-noise and high-speed charge detection in high-resolution CCD image sensors

This paper analyzes problems associated with low-noise and high-speed charge detection encountered in high-resolution image sensors. It is found that the conventional Floating Diffusion (FD) charge detection concept is inferior to the previously studied, but not frequently utilized, Floating Gate (FG) approach. A new output charge detection well reset technique and an improved biasing method allowed the design of the FG charge detection amplifier with a comparable conversion gain to FD structures but with much better noise performance at the data rates up to 40 MHz. The theoretical analysis of the FG amplifier performance, including the Correlated Pixel Clamp signal processing method, is confirmed by measurements performed on a high-resolution 1000/spl times/1000 pixel Frame Transfer CCD image sensor built using an Advanced Virtual Phase Technology. The described details of the sensor design include: (1) the over all device architecture, (2) the pixel cross section with the cross section of the lateral overflow drain antiblooming structure, (3) the dual serial register with a single output amplifier, and (4) the resistive gate reset structure for the output charge detection well. The developed image sensor does not need the conventional Correlated Double Sampling (CDS) circuit for the signal processing, since the FG detection node is sensing charge nondestructively without generation of kTC noise. The described progress in the FG charge detection approach thus opens up a possibility for future designs of distributed FG amplifiers that can theoretically reach the ultimate low-noise performance at virtually any clocking frequency.

A field-sensitive photoconductive probe for sampling through passivation layers

A field-sensitive photoconductive sampling technique has been demonstrated in measurements performed through an insulating layer without the need for conductive contact. Sampled signals are sensed by a virtual-ground, floating-gate amplifier without draining charge from the device under test or the photoconductive switch. The minimum detectable signal is 2.5 μV/Hz1/2 with a spatial resolution of 7 μm, while the sampling bandwidth is essentially that observed using photoconductive sampling with a conductive contact to the device under test. The photovoltaic and shot current noise are negligible in comparison with the lock-in amplifier noise since the current flowing in this high-impedance, floating-gate probe is negligible.

A Large Area TDI Image Sensor for Low Light Level Imaging

A 1030 X 128 element time delay and integration (TDI) CCD image sensor has been developed for low-light-level (L/sup 3/) imaging applications. For L/sup 3/ imaging, output is derived from a high-gain low-noise floating-gate amplifier (FGA). For larger input signal levels, a second, resettable floating-gate amplifier (RFGA) with lower gain and wider dynamic range provides output in parallel to the FGA. The device features four-phase buried-channel construction and a polysilicon gate design tailored to produce optimum broad-band responsivity. Input signal levels of 500 electrons have been successfully imaged and amplifier noise levels of approximately 20 electrons have been observed.

A large area TDI image sensor for low light level imaging

A 1030 × 128 element time delay and integration (TDI) CCD image sensor has been developed for low-light-level (L <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ) imaging applications. For L <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> imaging, output is derived from a high-gain low-noise floating-gate amplifier (FGA). For larger input signal levels, a second, resettable floating-gate amplifier (RFGA) with lower gain and wider dynamic range provides output in parallel to the FGA. The device features four-phase buried-channel construction and a polysilicon gate design tailored to produce optimum broad-band responsivity. Input signal levels of 500 electrons have been successfully imaged and amplifier noise levels of approximately 20 electrons have been observed.

Design and operation of a floating gate amplifier

A unique amplifier configuration is examined that fully exploits the intrinsically high signal-to-noise performance of charge-coupled devices (CCD's). In this amplifier, the signal charge is detected with a conducting `floating gate' embedded in the oxide between a bias electrode and the silicon substrate. The change of voltage on the floating gate produced by the signal charge in the CCD channel is then used to modulate the current flow in a metal-oxide-semiconductor (MOS) transistor. The signal charge remains isolated and can be moved downstream in the CCD channel; thus, it can be detected again by other similar structures. Computer analysis, test structure design, and experimental results of a floating gate amplifier (FGA) are presented.