Farhad Taghibakhsh

Characterization of Short-Wavelength-Selective a-Si:H MSM Photoconductors for Large-Area Digital-Imaging Applications

Photoconductor-type photodetectors are attractive as sensors due to their compatibility with thin-film-transistor (TFT) fabrication processes. Since they exhibit photogain, photoconductor detectors have better or comparable responsivity and quantum efficiency (QE) compared to p-i-n photodiodes. In this paper, the operation of metal-semiconductor-metal photoconductor-based photodetectors using aluminum electrodes and thin hydrogenated amorphous-silicon (a-Si) films is investigated. The experimental results of photocurrent measurements, as well as the responsivity and QE for different a-Si film thicknesses, bias voltages, and electrode gaps, are presented. Integration with TFT fabrication and its application in large-area digital imaging are discussed.

Two-Transistor Active Pixel Sensor Readout Circuits in Amorphous Silicon Technology for High-Resolution Digital Imaging Applications

Active pixel sensor (APS) architectures using two transistors per pixel are reported in this paper for high-resolution low-noise digital imaging applications. The fewer number of on-pixel elements and reduced pixel complexity result in a smaller pixel pitch and increased pixel gain, which makes the two-transistor (2T) APS architectures promising for high-resolution, low-noise, and high-speed digital imaging including emerging medical imaging modalities, such as mammography tomosynthesis and cone beam computed tomography. Measured results from in-house fabricated test pixels using amorphous silicon (a-Si) thin-film transistors, as well as driving schemes for minimizing the threshold voltage metastability problem and increasing frame rate, are presented. The results indicate that a pixel input referred noise value of down to 220 electrons is achievable with a 50- -pixel-pitch a-Si 2T APS.

UV-Enhanced a-Si:H Metal–Semiconductor–Metal Photodetector

Planar metal-semiconductor-metal (MSM) photodetectors with very thin hydrogenated amorphous silicon (a-Si) films were fabricated for the detection of ultraviolet (UV) radiation. Since DNA and proteins strongly absorb UV radiation, these detectors find application in DNA and protein detection. The performance of top and bottom electrode MSM structures with aluminum electrodes is compared. The measured results include a responsivity of 150 mA/W and an external quantum efficiency of 74% at a wavelength of 260 nm for the top electrode configuration at a bias of 2 V/mum and a 10-mum finger spacing.

Two-Transistor Active Pixel Sensor for High Resolution Large Area Digital X-ray Imaging

A novel architecture for active pixel sensor (APS) arrays with only two thin film transistors (TFTs) is presented, and a new driving scheme for minimizing the threshold voltage metastability problem and increasing frame rate is discussed. The fewer number of on-pixel elements, and reduced pixel complexity, results in a smaller pixel pitch and increased pixel gain, and makes the presented APS architecture promising for high resolution, high speed imaging modalities such as digital mammography tomosynthesis.

Contactless Heart Monitoring (CHM)

An apparatus for measuring mechanical heart movement, and heart rate monitoring without physically connecting electrodes or other sensors to the body was developed. Contactless heart monitoring (CHM) based on Doppler radar with frequency of 2.4 GHz at a distance of up to 30 cm is capable of detecting the mechanical vibrations of the heart, without any contact with the persons body, and it can operate and record the heart rate through a mattress, wall or other barriers. In this paper we present the theory, and preliminary results of our basic CHM setup.

High dynamic range 2-TFT amplified pixel sensor architecture for digital mammography tomosynthesis

On-pixel amplifiers in amorphous silicon (a-Si) technology are an attractive replacement for industry standard on-pixel switch architectures in active matrix flat panel imagers in order to meet the low noise requirements of low-dose digital imaging modalities such as x-ray fluoroscopy and, more recently, 3D mammography tomosynthesis. However, implementing a-Si pixel amplifiers requires high-performance thin film transistors (TFTs) that are relatively large in size. In this research, a novel high dynamic range amplified pixel architecture using only two TFTs is introduced that is capable of amplifying the sensor value with a user controllable gain over a wide input range. Circuit operation and driving circuits required for on-pixel amplifier arrays are investigated, and simulation results are presented that indicate the feasibility of this pixel architecture for high resolution, low noise and x-ray tomosynthesis applications.

Effect of scintillator crystal geometry and surface finishing on depth of interaction resolution in PET detectors: Monte Carlo simulation and experimental results using silicon photomultipliers

Resolution of positron emission tomography (PET) systems benefits from information about depth of interaction (DOI) within scintillation crystals, particularly in small bore scanners or parallel plate detectors. In this investigation, the ability of the dual-ended readout detector module configuration to resolve DOI and crystal index was evaluated for a variety of detector pitches and light guide thicknesses to validate the dual-ended readout method. Experimental results with oneto- one coupling between saw-cut 2mm pitch LYSO scintillation crystals and silicon photomultipliers (SiPMs) achieved 2.1 mm DOI resolution. Monte Carlo simulations were used to investigate the effect of larger detector pitches and varied light guide thickness on the crystal index identification accuracy and DOI resolution for a pixilated crystal array in dual-ended readout configuration. It is reported that the accuracy in identifying a 2 mm scintillation crystal was >80% for detector pitches < 6 mm and that DOI resolution was < 2 mm for all detector pitches and light guide thicknesses.

Fabrication and Characterization of Nickel Amorphous Silicon Metal-Semiconductor-Metal Photoconductors

Fabrication and electrical characterization of metal-semiconductor-metal (MSM) photodetectors using nickel and amorphous silicon is reported. Fabricated detectors exhibit dark currents in the order of 10-12 A, a dynamic range higher than 106, and responsivity of 0.12 A/W. Detector current changes linearly with applied bias voltage in the range of 0 to 30 V, and incident optical power density between 0 and 107 mW/cm2. It was found that with channel length of less than 25 mum, external quantum efficiency of more than 30% is achievable. Such detectors are particularly suitable for imaging applications with very large area pixels.

Amplified pixel sensor architectures for low dose computed tomography using silicon thin film technology

Cone beam computed tomography (CBCT) has been recently reported using flat panel imagers (FPI). Here, detector technology capable of high speed imaging, high spatial resolution, large volume coverage, better contrast resolution and, in particular, lowered patient dose is required. Employing active matrix flat panel imagers (AMFPIs) as cone beam CT detectors has been proposed as a solution for improving volume coverage, contrast and resolution; however, clinical evaluations have shown that they suffer from low speed read out. Unlike passive pixel architecture which is currently the state-of-the-art technology for AMFPIs, our preliminary studies have shown that novel amplified pixel sensor (APS) architectures can overcome the low readout speed, and moreover, they provide gain which can be traded for higher frame rate and lower X-ray doses. Although APS architectures can meet the high dynamic range and low noise requirements of CT imaging, linearity and variations between pixel characteristics are major issues. In this study we will investigate novel APS architectures to address these concerns.

Low leakage a-Si:H thin film transistors deposited on glass substrates using hot-wire chemical vapor deposition

Stable, low leakage current hot-wire chemical vapor deposition (HWCVD) thin film transistors (TFTs) were fabricated using graphite filament on Corning 1737 glass substrates. TFTs were deposited using a bottom gate, inverted staggered, self-aligned, low temperature process with a molybdenum metal gate, a trilayer consisting of a HWCVD a-Si:Hi layer sandwiched between two plasma enhanced chemical vapor deposition (PECVD) a-SiNx:H layers, followed by a PECVD n+ a-Si:H Ohmic contact layer, and a final aluminum metal for source and drain contacts. Ion∕Ioff ratio exceeds 108 and the leakage current is less than 10−13A, while the field effect mobility is in the range of 0.13cm2∕Vs and the subthreshold slope is 0.6V∕decade. Time stress bias measurements show β of 0.156 for HW a-Si TFTs, showing superior stability compared to their fully PECVD counterparts.