Characterization of charge trapping mechanisms in GaN vertical Fin FETs under positive gate bias

In this paper, we present a comprehensive analysis of the charge trapping mechanisms that affect the GaN based vertical Fin FETs when the devices are submitted to positive gate bias. Devices with higher channel width show lower threshold voltage: with 2D simulations of the electron density we are able to explain the phenomenon and propose a trade-off to improve the technology. By using double pulse measurements and threshold voltage transients, two trapping/detrapping mechanisms under positive gate bias can be identified according to two voltage ranges. At low positive gate bias, electrons (previously trapped inside the oxide during the fabrication process) are detrapped towards the gate metal (mechanism 1). At higher gate bias, electrons are trapped at the GaN/oxide interface, moving the threshold towards positive values (mechanism 2). The second mechanism is observable at higher time of stress and it is predominant for higher voltages. Moreover, mechanism 2 is found to be recoverable only when the device is exposed to UV-light and electrons trapped in a specific level in the oxide acquire the energy necessary to escape and reach the n-type GaN and/or the UV-generated holes accumulate at the interface may reduce the trapped electron density. We demonstrate our hypothesis by calculating the interface state density in trapping/detrapping conditions by using photo-assisted Capacitance-Voltage measurements

Hydrodynamics of local excitations after an interaction quench in 1D cold atomic gases

We discuss the hydrodynamic approach to the study of the time evolution -induced by a quench- of local excitations in one dimension. We focus on interaction quenches: the considered protocol consists in creating a stable localized excitation propagating through the system, and then operating a sudden change of the interaction between the particles. To highlight the effect of the quench, we take the initial excitation to be a soliton. The quench splits the excitation into two packets moving in opposite directions, whose characteristics can be expressed in a universal way. Our treatment allows to describe the internal dynamics of these two packets in terms of the different velocities of their components. We confirm our analytical predictions through numerical simulations performed with the Gross-Pitaevskii equation and with the Calogero model (as an example of long range interactions and solvable with a parabolic confinement). Through the Calogero model we also discuss the effect of an external trapping on the protocol. The hydrodynamic approach shows that there is a difference between the bulk velocities of the propagating packets and the velocities of their peaks: it is possible to discriminate the two quantities, as we show through the comparison between numerical simulations and analytical estimates. In the realizations of the discussed quench protocol in a cold atom experiment, these different velocities are accessible through different measurement procedures. ArXI

Universal late-time dynamics in isolated one-dimensional statistical systems with topological excitations

We investigate the nonequilibrium dynamics of a class of isolated one-dimensional systems possessing two degenerate ground states, initialized in a low-energy symmetric phase. We report the emergence of a timescale separation between fast (radiation) and slow (kink or domain wall) degrees of freedom. We find a universal long-time dynamics, largely independent of the microscopic details of the system, in which the kinks control the relaxation of relevant observables and correlations. The resulting late-time dynamics can be described by a set of phenomenological equations, which yield results in excellent agreement with the numerical tests

Universal late-time dynamics in isolated one-dimensional statistical systems with topological excitations

We investigate the non-equilibrium dynamics of a class of isolated one-dimensional systems possessing two degenerate ground states, initialized in a low-energy symmetric phase. We report the emergence of a time-scale separation between fast (radiation) and slow (kink or domain wall) degrees of freedom. We find a universal long-time dynamics, largely independent of the microscopic details of the system, in which the kinks control the relaxation of relevant observables and correlations. The resulting late-time dynamics can be described by a set of phenomenological equations, which yield results in excellent agreement with the numerical tests

Charge trapping in 0.1 μm AlGaN/GaN RF HEMTs: Dependence on barrier properties, voltage and temperature

The goal of this paper is to describe and understand the de-trapping dynamics in AlGaN/GaN transistors for radiofrequency (RF) applications, as a function of three parameters: the aluminium content in the barrier (22.6%, 24.6%, and 26.6%), the stress voltage applied to the drain and temperature. The analysis is based on threshold voltage transient measurements, carried out under at different stress conditions. The original results of this analysis show that: (i) the analysed devices show the presence of a dominant charge-trapping process E1 that, at room temperature, has de-trapping time constants in the range of 10 ms, and activation energy around 0.6 eV. (ii) trap E1 (that is responsible for ~0.5 V threshold voltage shift) is located in the semiconductor material, and its effect increases with increasing aluminium content in the barrier. (iii) leakage measurements indicate that devices with higher Al content in the barrier have a higher gate leakage. Our hypothesis is that under off-state stress, traps in the semiconductor material are charged by electrons injected from the gate metal through defects located in the AlGaN barrier. (iv) a second trap E2, whose signal is independent on the properties of the barrier, was also detected. This trap is supposed to originate from surface defects, is responsible for a minor (~0.1 V) threshold voltage shift and is weakly thermally activated (0.25–0.4 eV). (v) finally, a detailed analysis of the de-trapping kinetics indicated that the signal associated to trap E1 increases linearly with the drain stress voltage, and the related kinetics are ideal exponentials (consistent with semiconductor traps), while the amplitude of trap E2 does not significantly depend on drain voltage, and its kinetics are described by a stretched exponential behaviour (consistent with surface traps)

Long term activity in a fitness centre preserve functional decrease in COPD patients

Physical exercise turns out to be recognised as a fundamental component of a correct life style also for persons suffering from a chronic pulmonary disease (COPD). Nevertheless it seems difficult to include exercise programs in the Health Care and COPD patients do not generally reached the requested level of daily PA. We tested the possibility to create a specific COPD exercise program to be implemented in an University Fitness Center (UFC) in order to evaluate the effective dissemination of this program on the field. 22 older COPD patients and 22 age matched healthy controls (CG), (mean\ub1SD: age 70.90\ub16.28 years, BMI 29.08\ub14.32 kg/m2; FEV1 62.67\ub112.59 % of predicted) have been involved in an exercise program: 2 times/week, 90 min per session in the UFC and home training twice/week low aerobic activity 30-40 min. The individual program was set up according to ACSM and American Thoracic Society (ATS) guidelines. Physical performances: 6' Walking Test, % HRRES at Treadmill Test, 1RM for Upper and Lower Arms, Sit&Reach, Back Scratch and Balance Test and standard pulmonary clinical test were assessed several times along a overall 36 month duration. COPD group (men) presents initial lower performances in 6'Walking, 1RM of lower limbs and Balance test vs. Control Group (CG) (p< 0.05). Attendance in UFC was 74.6%\ub13.24, while home exercise was very seldom performed: 13.0%\ub114.1, not significantly different from GC. Nonattendance in COPD was largely related to illness (43.0% vs. 27.0%, COPD and CG respectively.). Along the whole period of 36 months, COPD group show no change in FEV1. After 32 months, COPD group showed significant enhancement in 1RM of Upper Arms (p<0.05), a tendency to a slight improvements in 6'Walking, %HRres Treadmill, 1RM of Lower Arms test and a no changes in Flexibility and Balance test. In the same 32 months, CG shows improvement in 6'Walking and 1RM of Upper Arms (p< 0.05), and a tendency to improve all the others test. The pre-training assessment confirms the well known limitation of COPD patients in endurance and strength. The tailored exercise program in the fitness center appears to maintain or slightly improve physical performances in diseased group which also exhibited a substation stability in pulmonary condition along a 3 year period. Since the global amount of activity do not reach the required level it also seems necessary the identification of new strategies to involve subjects also in an autonomous Physical Activity, to meet ACSM and ATS Guidelines

Degradation mechanism of 0.15 \u3bcm AlGaN/GaN HEMTs: effects of hot electrons

The degradation mechanisms of AlGaN/GaN HEMTs adopting Fe and C co-doping, with high and low carbon doping concentration were investigated by means of hot-electron step stress and 24 h' stress tests. Firstly, DC and EL characterization at room temperature are summarized, then the parametric evolution during hot-electron step stress tests at the semi-on state was compared, the assumption for the degradation mechanism is that hot-electrons activated the pre-existing traps in the buffer, attenuate the electric field in the gate drain access region and damaging the gate contact, the parametric evolution during constant stresses is discussed

Thermally-activated failure mechanisms of 0.25 μm RF AlGaN/GaN HEMTs submitted to long-term life tests

Reliability and failure mechanism of 0.25 mu m AlGaN/GaN HEMTs under thermal storage tests and high temperature operating life (HTOL) tests have been evaluated. Results show that, during thermal storage tests, Schottky metal interdiffusion and gate sinking took place, possibly accompanied by thermo-mechanical degradation, with an activation energy of 1.8 eV. Failure modes consisted in carrier density decrease and sheet resistance increase, positive V-TH shift and I-DSS decrease. During HTOL tests, the degradation is mainly due to electrochemical oxidation of AlGaN, leading to on resistance increase, and I-DSS and g(m) decrease, with an activation energy of 1.0 eV

Failure mechanisms of GaN HEMTs for microwave and millimeter-wave applications: From interdiffusion effects to hot-electrons degradation

Gallium Nitride High Electron Mobility Transistor are the most prominent devices for application to high power density, high efficiency transmission system in the microwave and millimeter-wave frequency range. High bandwidth requirements of 5G and future telecommunication infrastructure require the adoption of higher frequency bands beyond 60 GHz, thus accelerating the scaling of devices gate lengths below 150 nm, possibly promoting failure mechanisms accelerated by electric field and hot-electron effects. This paper reviews main failure modes and mechanisms of GaN HEMTs for microwave and millimeter-wave applications11Work partially supported by EUGANIC project (EDA Contract B 1447 IAP1 GP), EC Horizon 2020 ECSEL project SG_GaN_2, ESA ESTEC project RELGAN, Italian MIUR PRIN project GANAPP, by the Office of Naval Research award N00014-20-1-2177, under the supervision of Paul Mak