Jeff Hsin Chang

Two-dimensional a-Si:H n-i-p photodiode array for low-level light detection

This paper presents the design, fabrication process, and performance evaluation of a two-dimensional hydrogenated amorphous silicon (a-Si:H) n-i-p photodiode array, developed specifically for low-level light sensor applications. The design of the device is simpler than conventional active-matrix-arrays based on thin-film transistor (TFT) addressing electronics, owing to the utilization of the a-Si:H switching diodes for signal readout. The discussed technological developments are aimed to minimize the leakage current and to enhance the external quantum efficiency. The current-voltage characteristics of the sensing and switching diodes are analyzed to identify the sources of the excess leakage current. The optical losses in the photodiodes with an ITO/a-SiN/sub x/:H antireflection coating have been minimized using numerical modeling. Description of the peripheral electronics and associated timing diagrams along with the results of the detector characterization, including the linearity and response time measurements, are presented and discussed.

Two-dimensional a-Si:H based n-i-p sensor array

This article presents the design and fabrication process of a two-dimensional hydrogenated amorphous silicon (a-Si:H) n-i-p photodiode array, developed for low light level sensor applications. Utilization of the single a-Si:H switching diode for signal readout enables simpler device design and fabrication. Since the sensing and switching diodes are formed simultaneously by dry etching the a-Si:H layers deposited over the whole area, the number of masks required in lithography was reduced to six. A significant reduction of the leakage current has been achieved by tailoring defects at the i-p interface and optimizing the plasma processing conditions. Details of device fabrication along with results of the sensor array performance are presented.

High Performance Hydrogenated Amorphous Silicon n-i-p Photo-diodes on Glass and Plastic Substrates by Low-temperature Fabrication Process

We report on the fabrication and characterization of hydrogenated amorphous silicon (a-Si:H) films and n-i-p photodiodes on glass and PEN plastic substrates using low-temperature (150°C) plasma-enhanced chemical vapor deposition. Process conditions were optimized for the i-a-Si:H material which had a band gap of ~1.73 eV and low density of states (of the order 10 15 cm -3 ). Diodes with 0.5 μm i-layer demonstrate quantum efficiency ~70%. The reverse dark current of the diodes on glass and PEN plastic substrate is ~10-11 and below 10 -10 A/cm 2 , respectively. We discuss the difference in electrical characteristics of n-i-p diodes on glass and PEN in terms of bulk- and interface-state generation currents.

Segmented Amorphous Silicon n-i-p Photodiodes on Stainless-Steel Foils for Flexible Imaging Arrays

This paper reports the first successful attempt to fabricate amorphous silicon (a-Si:H) n-i-p photodiodes on a thin stainless-steel foil substrate for medical X-ray imaging applications. Two architectures of the n-i-p-photosensor, where the top electrode is based on amorphous or polycrystalline ITO, have been developed and characterized. The impact of critical fabrication steps including the deposition of semiconductor layers, dry etch of the NIP stack, diode passivation and encapsulation, as well as a contact formation on the device performance is presented and discussed. The test structures comprising segmented photodiodes with an active area ranged from 0.126 × 0.126 to 1 × 1 mm 2 have been fabricated on stainless-steel foils and on glass substrates for the purposes of process characterization. The fabricated samples are evaluated in terms of current-voltage, capacitance-voltage, and spectral response characteristics.

Modeling and Characterization of the Hydrogenated Amorphous Silicon Metal Insulator Semiconductor Photosensors for Digital Radiography

Because of the inherent desired material and technological attributes such as low temperature deposition and high uniformity over large area, the amorphous silicon (a-Si:H) technology has been extended to digital X-ray diagnostic imaging applications. This paper reports on design, fabrication, and characterization of a MIS-type photosensor that is fully process-compatible with the active matrix a-Si:H TFT backplane. We discuss the device operating principles, along with measurement results of the transient dark current, linearity and spectral response.

Temperature dependence of leakage current in segmented a-Si:H n-i-p photodiodes

Hydrogenated amorphous silicon (a-Si:H) n-i-p photodiodes may be used as the pixel sensor element in large-area array imagers for medical diagnostics applications. The dark current level is an important parameter that dictates the performances of these types of pixelated imaging devices. Through measurements performed at different ambient temperatures, the leakage current components of segmented a-Si:H n-i-p photodiodes were extracted and analyzed. It was found that the central component of the reverse current depends exponentially on bias and temperature. The activation energy of this component is independent of bias. The peripheral component of reverse current exhibits linear bias dependence at temperatures up to 50°C, while the contribution of this component diminishes at high temperatures. The dependence of dark current components on bias and temperature could be described by compact analytical equations. The model of forward and reverse dark current characteristics in temperature range was implemented in Verilog-A hardware description language.

Two-dimensional sensor array for low-level light detection

This paper presents the design, fabrication process, and performance evaluation of a two-dimensional hydrogenated amorphous silicon (a-Si:H) n-i-p photodiode array, developed specifically for low-level light sensor applications. The design of the device is simpler than conventional active-matrix-arrays with thin-film transistor (TFT) addressing electronics, owing to the utilization of the a-Si:H switching diodes for signal readout. Since both sensor diodes and switching diodes are formed simultaneously through dry etching the entire wafer area, the mask count required for lithography is reduced to six. The most challenging problem associated with the fabrication of these devices is the excess leakage current, which inherently limits the output signal-to-noise ratio and dynamic range. The reverse dark current in the photodiodes were minimized by tailoring the defects at the i-p interface. The junction-edge leakage current in the switching diodes is associated with damage caused by ion bombardment, and can be reduced by optimizing of the plasma processing conditions. A 3×4 pixels array prototype was tested using a specially designed test board which performs pre-amplification, double sampling and final amplification for each column of the array. Details of the signal readout process along with detector performances are presented and discussed.