In this article, a low-power, radiation-hard front-end circuit for monolithic pixel sensors, designed to meet the requirements of low noise and low pixel-to-pixel variability, the key features to achieve high detection efficiencies, is presented. The sensor features a small collection electrode to achieve a small capacitance (<5 fF) and allows full CMOS in-pixel circuitry. The circuit is implemented in the 180-nm CMOS imaging technology from the TowerJazz foundry and integrated into the MALTA2 chip, which is part of a development that targets the specifications of the outer pixel layer of the ATLAS Inner Tracker upgrade at the LHC. One of the main challenges for monolithic sensors is a radiation hardness up to 10 ¹⁵ 1-MeV $\text {n}_{\text {eq}}/\text {cm}^{{2}}$ non-ionizing energy loss (NIEL) and 80 Mrad total ionizing dose (TID) required for this application. Tests up to ${3} \cdot {10}^{15}$ 1-MeV $\text {n}_{\text {eq}}/\text {cm}^{{2}}$ and 100 Mrad were performed on the MALTA2 sensor and front-end circuit, which still show good performance even after these levels of irradiation, promising for even more demanding applications such as the future experiments at the high-luminosity large hadron collider (HL-LHC).

GaN-based heterostructures have been used in high-power radio frequency applications for a number of years due to the wide bandgap of GaN and high values of spontaneous and piezoelectric polarization, resulting in high breakdown voltages and high 2D carrier concentrations. However, the accurate modeling of low-field electron mobility within such structures remains a topic of interest. This paper presents a comprehensive numerical model for calculating the carrier mobility within a 2D electron gas in Al${}_x$ Ga${}_{1-x}$ N/GaN high electron mobility transistors. The model is based on solving the Schrödinger and Poisson equations self-consistently, taking into account the polarization charges at material interfaces and performing semi-classical numerical calculations of low-field electron mobility within the momentum relaxation time approximation, taking into account all relevant scattering mechanisms. Both intra- and inter-subband transitions are considered, and the differences in intra- and inter-subband scattering rates are analyzed for some of the key scattering mechanisms. The importance of including inter-subband transitions in the calculations is demonstrated by comparing the calculated results with experimentally measured mobilities.

In this article, we use a semiclassical low-field mobility modeling framework, which includes all relevant scattering mechanisms, to examine the dependence of electron mobility on 2-D electron gas (2DEG) density in previously characterized high Al-content AlGaN/gallium nitride (GaN) high electron mobility transistors (HEMTs) with and without an AlN spacer layer. Polar optical phonon (POP) scattering has previously been identified as the dominant scattering mechanism in GaN-based HEMTs at room temperature, so the analysis of POP scattering rates is of particular importance. We find that the POP-limited mobility in both structures decreases with increasing 2DEG density up to a critical density of around ${10}^{{13}}$ cm $^{-{2}}$ , above which we observe a significant improvement of up to ${46}\%$ , contrary to the behavior typically reported for GaN HEMT structures. The POP-limited mobility increase is explained by the interplay of ground-state energy and quasi-Fermi energy within the channel, by which intrasubband POP emission within the ground state is suppressed at high 2DEG densities. This provides a universal guideline for the design and operation of GaN-based HEMTs to achieve a minimum ON-resistance in switching applications at room temperature and above.

The difference in two-dimensional electron gas (2DEG) mobility between single-heterostructure (SH) and double-heterostructure (DH) AlGaN/GaN/(AlGaN) high electron mobility transistors (HEMTs) has been attributed to a variety of scattering mechanisms and physical effects within these devices. In this article, we aim to elucidate the origins of the change in mobility between SH and DH HEMTs, and identify the scattering mechanisms responsible for limiting the mobility in various device structures. To this end, we use a comprehensive numerical low-field mobility modeling framework to analyze the dependence of 2DEG mobility on temperature, Al content in the AlGaN buffer, and thickness of the GaN channel, also validating our simulation results with available experimental data. We show that reports of superior high-temperature mobility in DH HEMTs can be explained by a decrease in polar optical phonon (POP) scattering rates due to lower 2DEG density and in spite of the stronger quantum confinement. On the other hand, reports of lower mobility in DH HEMTs compared with their SH counterparts can be attributed to stronger alloy scattering in structures without an AlN spacer layer, or to an increase in POP scattering in structures with an AlN spacer having similar 2DEG densities, while the differences in interface roughness and dislocation scattering due to the change in crystal quality play only a limited role.

A novel structure of a single-photon avalanche diode (SPAD) fabricated in a commercial $\boldsymbol {180}$ nm high-voltage (HV) complementary metal-oxide-semiconductor (CMOS) process is presented. Device performance in terms of dark count rate (DCR) and photon detection probability (PDP) is compared to a previously published design fabricated as a reference on the same die. We find that the new structure exhibits a DCR of $\boldsymbol {0.11}$ Hz/ $\boldsymbol {\mu }$ m $\boldsymbol {^{2}}$ at an excess voltage of $\boldsymbol {5}$ V, which is among the lowest ever reported in a foundry CMOS process, and more than $\boldsymbol {3}$ times lower with respect to the reference device. Moreover, the new structure shows a PDP/DCR ratio higher by up to a factor of $\boldsymbol {4.2}$ compared to the reference device. The reasons underlying the differences in DCR and PDP between the two structures are explained using technology computer-aided design (TCAD) simulations. The novel device is demonstrated as promising for the design of low-cost SPAD array imagers, particularly in low-noise applications.