George R. Kunnen

Optimizing diode thickness for thin-film solid state thermal neutron detectors

In this work, we investigate the optimal thickness of a semiconductor diode for thin-film solid state thermal neutron detectors. We evaluate several diode materials, Si, CdTe, GaAs, C (diamond), and ZnO, and two neutron converter materials, 10B and 6LiF. Investigating a coplanar diode/converter geometry, we determine the minimum semiconductor thickness needed to achieve maximum neutron detection efficiency. By keeping the semiconductor thickness to a minimum, gamma rejection is kept as high as possible. In this way, we optimize detector performance for different thin-film semiconductor materials.

Thin film cadmium telluride charged particle sensors for large area neutron detectors

Thin film semiconductor neutron detectors are an attractive candidate to replace 3He neutron detectors, due to the possibility of low cost manufacturing and the potential for large areas. Polycrystalline CdTe is found to be an excellent material for thin film charged particle detectors—an integral component of a thin film neutron detector. The devices presented here are characterized in terms of their response to alpha and gamma radiation. Individual alpha particles are detected with an intrinsic efficiency of >80%, while the devices are largely insensitive to gamma rays, which is desirable so that the detector does not give false positive counts from gamma rays. The capacitance-voltage behavior of the devices is studied and correlated to the response due to alpha radiation. When coupled with a boron-based neutron converting material, the CdTe detectors are capable of detecting thermal neutrons.

Stability and Low-Frequency Noise in InAs NW Parallel-Array Thin-Film Transistors

In this letter, we present the dc characteristics, stability, and low-frequency noise (LFN) measurements, for n-type indium arsenide nanowire (NW) parallel-array thin-film transistors (TFTs) with a global back gate. These devices perform with mobilities ranging from 200-1200 cm2V-1s-1 and produce a threshold voltage shift less than 0.25 V after 10 000 s of stress. The resulting LFN measurements indicate that the 1/f noise can be modeled by the number fluctuation model, at low drain currents, which can provide an essential guideline for the device design considerations of NW TFTs.

Microelectromechanical Systems (MEMS) Based-Ultrasonic Electrostatic Actuators on a Flexible Substrate

We present a microelectromechanical systems (MEMS)-based electrostatic actuator on a flexible substrate, made of polyethylene naphthalate, which emits acoustic waves at ultrasonic frequencies. The actuator has a suspended diaphragm, made of parylene, of 2-6-mm diameter and a 6-μm gap between the diaphragm and substrate. The actuator is driven by hydrogenated amorphous silicon integrated circuits. The driving circuits consist of voltage-controlled oscillator and buffer chain, which can tune the output voltage from 3 to 32 V with 35-V supply. A single actuator emits ultrasonic waves at 25 kHz and pressure of 27-dB sound pressure level (SPL), and a 1 × 2 array emits up to 34.6-dB SPL at 1-cm distance.

Development of a testbed for flexible a-Si:H photodiode sensing arrays

Large area, flexible sensing arrays for imaging, biochemical sensing and radiation detection are now possible with the development of flexible active matrix display technology. In particular, large-area flexible imaging arrays can provide considerable advancement in defense and security industries because of their inherent low manufacturing costs and physical plasticity that allows for increased adaptability to non-planar mounting surfaces. For example, a flexible array of photodetectors and lenslets formed into a cylinder could image simultaneously with a 360 degree view without the need for expensive bulky optics or a gimbaled mount. Here we report the design and development of a scalable 16x16 pixel testbed for flexible sensor arrays using commercial-off-the-shelf (COTS) parts and demonstrate the capture of a shadow image with an array of photodiodes and active pixel sensors on a plastic substrate. The image capture system makes use of an array of low-noise, InGaZnO active pixel amplifiers to detect changes in current in 2.4 μm-thick reverse-biased a-Si:H PIN diodes. A thorough characterization of the responsivity, detectivity, and optical gain of an a- Si:H photodiode is also provided. At the back end, analog capture circuitry progressively scans the array and constructs an image based on the electrical activity in each pixel. The use of correlated-double-sampling to remove fixed pattern noise is shown to significantly improve spatial resolution due to process variations. The testbed can be readily adapted for the development of neutron, alpha-particle, or X-ray detection arrays given an appropriate conversion layer.

Large area sensing arrays for detection of thermal neutrons

Developments of flexible detection arrays suggest that portable robust detectors are indeed possible. A large area flexible array promises a large capture cross section in a light weight rugged format suitable for deployment at ports of entry. The approach for this detector uses a high neutron-capture cross-section layer, such as 10B, which captures incident thermal neutrons, and emits energetic ionizing charged particles. These ionizing particles are sensed using an integrated diode. The resulting charge is then amplified via a low-noise thin film transistor amplifier. We present a low-noise optimized active pixel sensor (APS) design which can be implemented in either a low temperature InGaZnO or an a-Si:H thin film transistor (TFT) process compatible with plastic substrates. Here, we also present a detectable alpha particle response with our dual stage APS design in combination with an externally connected commercial PIN diode. Furthermore, we discuss detector and array modeling which will further aid in future designs.

Ultrasonic electrostatic actuators on a flexible substrate

We present a MEMS-based electrostatic actuator on a flexible substrate, made of Polyethylene Naphthalate (PEN), which emits acoustic waves at ultrasonic frequencies, aiming for an ultrasound microdevice for equipment inspection. The actuator has a suspended diaphragm, made of parylene, of 2-6 mm diameter and a 6 μm gap between the diaphragm and substrate. A single actuator emits ultrasonic waves at 25 kHz, and pressure of 27 dB SPL (sound pressure level) and a 1 × 2 array emits up to 34.6 dB SPL at 1 cm distance.

A Low-Noise Dual-Stage a-Si:H Active Pixel Sensor

This paper presents a low-noise and low-current autozeroed (AZ) dual-stage active-pixel-sensor circuit with a noise figure of 830 input-referred rms electrons and compares its performance with the conventional pixel with a figure of 1950 electrons. Through the analysis, we show how the dual stage can increase the photoinduced signal transconductance gain while keeping the relative output-referred noise low by using lower bias currents. From the analysis and verified-via-noise measurements, we report a 55% reduction in the input-referred noise using the optimized readout. The new design performs reset autozeroing, which stabilizes the gain every reset period. Using a subthreshold-mode a-Si:H TFT photodetector, the designed AZ dual stage provided the maximum gain with transient light. The dual stage is also less sensitive to electrical degradation induced by stressing but increases the overall pixel size.

High-performance logic circuits using solution-based, low-temperature semiconductors for flexible electronics

In this work we demonstrate high performance and low-power n-type inverters using solution-based CdS as the semiconductor in thin film transistors. Our fabrication process consists of five mask levels and a maximum temperature of 150 °C. The CdS is deposited using chemical bath deposition at 70 °C to provide full compatibility with flexible substrates. Isolated TFTs showed mobilities up to 10 cm2/V-s and threshold voltages of approximately 0.5V. Inverters were biased at 1, 3 and 5 V, resulting in maximum gains in the range of 60 at VDD = 3V. The devices and circuits are fully patterned using standard photolithographic techniques that can be used to design more complex circuitry for flexible and large area electronic applications. In addition we used an extraction parameter method for our TFTs that allows the use of regular SPICE simulation software to design and test the circuits. Our simulations are in good agreement with the experimental data for isolated devices and inverters. Other circuits such as NAND gates are also demonstrated.

Sol gel ZnO films doped with Mg and Li evaluated for charged particle detectors

In this work we assess the feasibility of ZnO films deposited from a sol gel precursor as a material for thin film charged particle detectors. There are many reports of polycrystalline ZnO thin film transistors (TFTs) in the literature, deposited by sputtering, pulsed laser deposition, and sol gel. There are also reports of sol gel derived ZnO doped with Li or Mg to increase the resistivity, however, these works only measure resistivity of the films, without determining the effect of doping on the carrier concentration. We study the effects of doping the ZnO with Mg and Li as well as the effects of thickness on the films’ resistivity, mobility, and carrier concentration, since these material parameters are critical for a charged particle sensor. Carrier concentration is particularly important because it must be kept low in order for the intrinsic region of a p-i-n diode to be depleted. In order to accomplish this we fabricate and electrically characterize test structures for resistivity, test structures for hall measurement, common back-gate TFTs, and metal-insulator-semiconductor (MIS) capacitors. We also conduct physical characterization techniques such as x-ray diffraction (XRD), atomic force microscopy (AFM), electron microscopy, UV-Vis spectroscopy, and ellipsometry to determine the effect of doping and film thickness on the microstructure and optical properties of the ZnO.