S. Karim

Drain-bias dependence of threshold voltage stability of amorphous silicon TFTs

Amorphous silicon (a-Si:H) thin-film transistors (TFTs) used in emerging, nonswitch applications such as analog amplifiers or active loads, often have a bias at the drain terminal in addition to the gate that can alter their threshold voltage (V/sub T/) stability performance. At small gate stress voltages (0/spl les/V/sub ST//spl les/15 V) where the defect state creation instability mechanism is dominant, the presence of a bias at the TFT drain decreases the overall shift in V/sub T/(/spl Delta/V/sub T/) compared to the /spl Delta/V/sub T/ in the absence of a drain bias. The measured shift in V/sub T/ appears to agree with the defect pool model that the /spl Delta/V/sub T/ is proportional to the number of induced carriers in the a-Si:H channel.

Amorphous and Polycrystalline Photoconductors for Direct Conversion Flat Panel X-Ray Image Sensors

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI2 and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples.

Optical Properties of Crystalline−Amorphous Core−Shell Silicon Nanowires

The optical absorption in a nanowire heterostructure consisting of a crystalline silicon core surrounded by a conformal shell of amorphous silicon is studied. We show that they exhibit extremely high absorption of 95% at short wavelengths (λ < 550 nm) and a concomitant very low absorption of down to less than 2% at long wavelengths (λ > 780 nm). These results indicate that our nanowires do not have optically active energy levels in the band gap. The absorption edge of silicon nanowires arrays is observed to shift to longer wavelengths as a function of the overall nanowire diameter. The near-infrared absorption of the nanowire array is significantly better than that of thin film amorphous silicon. These properties indicate potential use in large area optoelectronic and photovoltaic applications.

Core-shell silicon nanowire solar cells

Silicon nanowires can enhance broadband optical absorption and reduce radial carrier collection distances in solar cell devices. Arrays of disordered nanowires grown by vapor-liquid-solid method are attractive because they can be grown on low-cost substrates such as glass, and are large area compatible. Here, we experimentally demonstrate that an array of disordered silicon nanowires surrounded by a thin transparent conductive oxide has both low diffuse and specular reflection with total values as low as < 4% over a broad wavelength range of 400 nm < λ < 650 nm. These anti-reflective properties together with enhanced infrared absorption in the core-shell nanowire facilitates enhancement in external quantum efficiency using two different active shell materials: amorphous silicon and nanocrystalline silicon. As a result, the core-shell nanowire device exhibits a short-circuit current enhancement of 15% with an amorphous Si shell and 26% with a nanocrystalline Si shell compared to their corresponding planar devices.

Thin film imaging technology on glass and plastic

Hydrogenated amorphous silicon (a-Si:H) technology offers a viable technological alternative for improved imaging of optical signals and high energy radiation. This paper reviews X-ray imaging technology in terms of detector operating principles, including optoelectronic characteristics, and fabrication process issues related to pixel (Schottky diode detector plus thin film transistor) integration. Recent results which describe the extension of the current fabrication processes to low (/spl sim/120/spl deg/C) temperature are also presented. The low temperature processing enables fabrication of thin electronics on flexible (polymer) substrates.

Static characteristics of a-Si:H dual-gate TFTs

This paper examines the effect of the top gate on the static characteristics of dual-gate hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs). Both forward and reverse regimes of operation are considered. The top gate has a distinct effect on the threshold voltage, subthreshold slope, drive-current capability, and the leakage current of the TFT. In particular, the threshold voltage is found to linearly decrease with increasing top-gate bias. Specific bias configurations of the dual gate TFT critical to vertical integration of on-pixel electronics for imaging and display applications are also presented.

Measurement of UV from a Microplasma by a Microfabricated Amorphous Selenium Detector

We spectrally demonstrate for the first time that an amorphous selenium metal-semiconductor-metal detector can be used for the measurement of ultraviolet photons (200-400 nm) generated from a portable battery-operated microplasma that is used as a light source. An advantage of this low-cost detector is that the device structure allows photons to strike the light-sensitive layer directly rather than through electrodes or blocking layers. Another advantage is that despite operation at high electric fields of up to 43 V/μm, the dark current of the detector at room temperature is 3 pA/mm2. Therefore, detector cooling is not required, and this facilitates portability for measurements on-site (i.e., in the field and away from a laboratory). Spectral response was monitored using a scanning monochromator, and it was compared with that obtained by a portable spectrometer fitted with a linear charge-coupled device detector. To demonstrate detector responsivity, emission signals with an appreciable signal-to-noise ratio were obtained by introducing nanogram amounts of the sample into the microplasma.

Lateral metal-semiconductor-metal photodetectors based on amorphous selenium

We report a lateral metal-semiconductor-metal (MSM) photodetector (PD) based on an amorphous selenium (a-Se). The PD exhibits a dark current below 200 fA under electric fields ranging from 6 to 12 V/μm, a responsivity of up to 0.45 A/W, a photogain of 1.2 near short wavelengths of 468 nm, and a high-speed photoresponse with a rise time of 50 μs, fall time of 60 μs, and time constant of 30 μs, respectively. The lateral MSM PD based on a-Se has great potential for use in digital x-ray imaging applications.

Amorphous silicon back-plane electronics for OLED displays

This paper reviews design considerations along with measurement results pertinent to amorphous silicon (a-Si:H) thin-film transistor (TFT) drive circuits for active matrix organic light-emitting diode displays, and follows from work presented earlier (A. Nathan et al., 2002), (A. Nathan et al., 2003). We describe both pixel architectures and TFT circuit topologies that are amenable for vertically integrated, high aperture ratio pixels. Here, the organic light-emitting diode layer is integrated directly above the TFT circuit layer to provide an active pixel area that is at least 90% of the total pixel area with an aperture ratio that remains virtually independent of scaling. Both voltage-programmed and current-programmed drive circuits are considered. The latter provides compensation for shifts in device characteristics due to metastable shifts in the threshold voltage of the TFT. Integration of on-panel gate drivers is also discussed, where we present the architecture of an a-Si:H-based gate demultiplexer that is threshold voltage shift invariant. In addition, a programmable current mirror with good linearity and stability is presented. Programmable current sources are an essential requirement in the design of source driver output stages.

Alternate pixel architectures for large-area medical imaging

Amorphous silicon active matrix flat-panel imagers have gained considerable significant in digital diagnostic medical imaging applications in view of their large area readout capability. The pixel, forming the fundamental unit of the active matrix, consists of a detector and readout circuit. The most widely used architecture is a passive pixel sensor (PPS) where the pixel consists of a detector and an a-Si:H thin-film transistor readout switch. While the PPS has the advantage of being compact and amenable towards high-resolution imaging, reading the low PPS output signal require external circuitry such as column charge amplifiers. More importantly, these amplifiers add a large noise component to the PPS that reduces the minimum readable sensor input signal. This work presents an alternate pixel architecture that can perform on-pixel input signal amplification, i.e. an active pixel sensor (APS). Two operating modes of the APS, voltage output (V-APS) and current output (C-APS) are introduced but the focus is on C- APS. Theoretical calculations indicating the feasibility of the C-APS for low-noise, real time imaging applications (e.g. fluoroscopy) are presented. Specifically, signal gain, dynamic range, readout rate and noise of the C-APS are examined. Lastly, initial experimental results of C-APS linearity and gain are presented in addition to a discussion on APS threshold voltage stability.