Marco Silvestri

Intentionally Carbon-Doped AlGaN/GaN HEMTs: Necessity for Vertical Leakage Paths

Dynamic on-resistance (RON) in heavily carbon-doped AlGaN/GaN high electron mobility transistors is shown to be associated with the semi-insulating carbon-doped buffer region. Using transient substrate bias, differences in RON dispersion between transistors fabricated on nominally identical epilayer structures were found to be due to the band-to-band leakage resistance between the buffer and the 2-DEG. Contrary to normal expectations, suppression of dynamic RON dispersion in these devices requires a high density of active defects to increase reverse leakage current through the depletion region allowing the floating weakly p-type buffer to remain in equilibrium with the 2-DEG.

Iron-induced deep-level acceptor center in GaN/AlGaN high electron mobility transistors: Energy level and cross section

Dynamic transconductance dispersion measurements coupled with device physics simulations were used to study the deep level acceptor center in iron-doped AlGaN/GaN high electron mobility transistors (HEMT). From the extracted frequency dependent trap-conductance, an energy level 0.7 eV below the conduction band and a capture cross section of 10−13 cm2 were obtained. The approach presented in this work avoids the non-equilibrium electrical or optical techniques that have been used to date and extracts the device relevant trap characteristics in short channel AlGaN/GaN HEMTs. Quantitative prediction of the trap induced transconductance dispersion in HEMTs is demonstrated.

Gate Bias Dependence of Defect-Mediated Hot-Carrier Degradation in GaN HEMTs

Monte Carlo analysis of hot-electron degradation in AlGaN/GaN high-electron mobility transistors shows that, for gate voltages corresponding to semi-ON bias conditions, the average electron energy has a spatial peak with (EAVE) ~ 1.5 eV. The peak is located at the edge of the gate. At this location, the carrier versus energy distribution has a large tail extending over 3 eV. When transferred to the lattice, this energy can cause defect dehydrogenation and device degradation. These results are consistent with the experimental data indicating maximum degradation in the semi-ON bias condition.

Impact of Proton Irradiation-Induced Bulk Defects on Gate-Lag in GaN HEMTs

The relationship between proton-induced defects and gate-lag in GaN high-electron mobility transistors (HEMTs) is examined using simulations and experiments. Surface traps are primarily responsible for the pre-irradiation gate-lag. Experimental data and detailed two-dimensional device simulations demonstrate that bulk traps increase the amount of observed gate-lag after irradiation to high-proton fluences.

Dynamic Transconductance Dispersion Characterization of Channel Hot-Carrier Stressed 0.25-

Using the dynamic transconductance frequency dispersion technique, we characterize unstressed and hot-electron stressed short-channel AlGaN/GaN high-electron-mobility transistors. The results reported in this letter demonstrate that the stress-induced degradation in dc and pulsed characteristics is unlikely to be ascribable to sizable trap generation at the AlGaN/GaN interface.

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We present new experimental results about channel hot carrier degradation of enclosed layout transistors as a function of previous accumulated total ionizing dose, stress temperature, and transistor geometry. We show that the parametric degradation follows a power law, whose exponent is higher than in conventional open layout transistors, possibly due to a different diffusion geometry of hydrogen. Through physical simulation we attribute this effect to the electric field at the device corners, which leads to a non-uniform impact ionization. Previous irradiation reduces the channel hot carrier degradation in MOSFETs with 5.2-nm gate oxide, while having a minor influence with 2.2-nm gate dielectric.

AlGaN/GaN HEMTs

We investigated the combined effect of heavy-ion irradiation and large applied bias on the dielectric breakdown of ultra-thin gate oxides, analyzing the impact of border regions through dedicated test structures. We found that the irradiation bias polarity plays a fundamental role, with inversion being more detrimental than accumulation for the onset of gate rupture. Moreover, the average voltage to breakdown was, under certain conditions, lower in structures more closely resembling real MOSFETs as compared to those commonly used for the evaluation of Single Event Gate Rupture. These findings raise some important hardness assurance issues concerning the integrity of gate oxides in radiation environments.

Degradation Induced by X-Ray Irradiation and Channel Hot Carrier Stresses in 130-nm NMOSFETs With Enclosed Layout

We investigate how X-ray exposure impact the long term reliability of 130-nm NMOSFETs as a function of device geometry and irradiation bias conditions. This work focuses on electrical stresses on n-channel MOSFETs performed after irradiation with X-ray up to 136 Mrad(SiO2) in different bias conditions. Irradiation is shown to negatively affect the degradation during subsequent hot carrier injection. Increasing the bias during irradiation slightly reduces the impact on following electrical stress in core MOSFETs. Through device simulations, we attribute these effects to an enhanced impact ionization at the bulk-STI interfaces due to radiation-induced trapped charge and defects.

Gate Rupture in Ultra-Thin Gate Oxides Irradiated With Heavy Ions

The location of the time dependent degradation in OFF-state stressed AlGaN/GaN high electron mobility transistors is studied using low frequency 1/f noise measurements, with additional electroluminescence analysis. The gate bias dependence of the 1/f noise is shown to be a powerful tool to illustrate that in addition to the gate edge breakdown, progressive time-dependent trap generation occurs underneath the gate area, possibly extending in the gate-drain access region due to the electric field peak associated with the gate field plate.

Channel Hot Carrier Stress on Irradiated 130-nm NMOSFETs

Through the use of experimental data and Monte Carlo simulations we investigate the shielding properties of spacecraft-shell compositions exposed to 1 GeV/nucleon 56Fe ions, representative of the worst part of the Galactic Cosmic Ray (GCR) spectrum. Through the use of the Geant4 Radiation Analysis for Space (GRAS) tool, the dose reduction and the 56Fe-fragmentation induced by those structures currently used to protect part of the International Space Station (ISS) or designed for future inflatable habitats, are analyzed. The possible effects on spacecraft electronics are discussed.