Konstantin D. Stefanov

A numerical model has been developed to simulate mid-wave infrared Ga-free InAs/InAsSb type 2 superlattice (T2SL) unipolar barrier (XBn) technology-based photodetectors. The model has been extensively studied to evaluate the effects of doping levels and the thickness of both the absorber and barrier layers on key metrics such as dark current (DC) and quantum efficiency (QE). The model utilizes the nextnano 8-band k·p theory and optics module to solve the Schrödinger equation and compute the band dispersion and the absorption coefficient. The band dispersion of the structure demonstrates the formation of minibands, indicating the carrier’s delocalization. This crucial condition allows us to consider the T2SL as a bulk material. Material parameters such as band gap energy, electron and hole effective masses, intrinsic concentration, and extinction coefficient are extracted from the quantum simulation results and input into the Technology Computer-Aided Design software by Silvaco. The software solves the Poisson drift–diffusion continuity equations to compute the DC and QE. The model has been validated using experimental results from existing literature, demonstrating good agreement with experimental findings, making it an effective tool for designing and predicting the performance of Ga-free T2SL XBn technology-based structures.

CIS221-X is a prototype monolithic complementary metal-oxide-semiconductor (CMOS) image sensor, optimized for soft X-ray astronomy and developed for the proposed European Space Agency Transient High Energy Sky and Early Universe Surveyor (THESEUS) mission. One significant advantage of CMOS technology is its resistance to radiation damage. To assess this resistance, three backside-illuminated CIS221-X detectors have been irradiated with 10 MeV protons using the MC40 Cyclotron Facility at the University of Birmingham, United Kingdom. Each detector received 1/2, 1, and 2 THESEUS end-of-life proton fluences (6.65×108 p+/cm2). One had already been exposed to ionizing radiation [up to 59.04 krad total ionizing dose (TID)] during a previous radiation campaign. Using unirradiated readout electronics, the electro-optical performance of each device has been measured before and after proton irradiation. No significant change was observed in the readout noise and image lag. An increase in mean dark current was recorded, as was an increase in the number of hot pixels. The degradation of CIS221-X performance due to non-ionizing radiation effects is similar to that of comparable CMOS image sensors and has been attributed to an increase in the number of bulk silicon defects.

CIS221-X is a prototype complementary metal-oxide-semiconductor (CMOS) image sensor, optimized for soft x-ray astronomy and developed for the proposed ESA Transient High Energy Sky and Early Universe Surveyor (THESEUS) mission. The sensor features 40 μm pitch square pixels built on a 35 μm thick, high-resistivity epitaxial silicon that is fully depleted by reverse substrate bias. Backside illumination processing has been used to achieve high x-ray quantum efficiency, and an optical light-blocking filter has been applied to mitigate the influence of stray light. A comprehensive electro-optical characterization of CIS221-X has been completed. The median readout noise is 3.3 e − RMS with 90% of pixels reporting a value <3.6 e − RMS. At −40 ° C, the dark current is 12.4 ± 0.06 e − / pixel / s. The pixel photo-response is linear to within 1% for 0.3 to 5 keV photons (82 to 1370 e − ) with 80 % quantum efficiency. With the exception of dark current, these results either meet or outperform the requirements for the THESEUS mission, strongly supporting the consideration of CMOS technology for soft x-ray astronomy.

Even though the recent progress made in complementary metal–oxide–semiconductor (CMOS) image sensors (CIS) has enabled numerous applications affecting our daily lives, the technology still relies on conventional methods such as antireflective coatings and ion‐implanted back‐surface field to reduce optical and electrical losses resulting in limited device performance. In this work, these methods are replaced with nanostructured surfaces and atomic layer deposited surface passivation. The results show that such surface nanoengineering applied to a commercial backside illuminated CIS significantly extends its spectral range and enhances its photosensitivity as demonstrated by >90% quantum efficiency in the 300–700 nm wavelength range. The surface nanoengineering also reduces the dark current by a factor of three. While the photoresponse uniformity of the sensor is seen to be slightly better, possible scattering from the nanostructures can lead to increased optical crosstalk between the pixels. The results demonstrate the vast potential of surface nanoengineering in improving the performance of CIS for a wide range of applications.

We present an imaging pixel featuring dual conversion gain in a single exposure based on the pinned photodiode (PPD). The signal charge is first converted to voltage nondestructively using a floating gate, and a second conversion is done at a p-n junction-based sense node (SN). Higher signal dynamic range (DR) is achieved due to the sensing of the same charge with two different conversion gains. The results from a prototype 10- $\mu \text{m}$ -pitch pixel manufactured in a 180-nm CMOS image sensor process demonstrate a gain ratio of 3, DR of 90 dB, 3.6 e− rms readout noise, and negligible image lag.

We present a readout scheme for CMOS image sensors that can be used to achieve arbitrarily high dynamic range (HDR) in principle. The linear full well capacity (LFWC) in high signal regions was extended 50 times from 20 to 984 ke $^{-}$ via an interlaced row-wise readout order, while the noise floor remained unchanged in low signal regions, resulting in a 34-dB increase in DR. The peak signal-to-noise ratio (PSNR) is increased in a continuous fashion from 43 to 60 dB. This was achieved by summing user-selected rows that were read out multiple times. Centroiding uncertainties were lowered when template-fitting a projected pattern, compared to the standard readout scheme. Example applications are aimed at scientific imaging due to the linearity and PSNR increase.