Andrea Cester

Radiation induced leakage current and stress induced leakage current in ultra-thin gate oxides

Low-field leakage current has been measured in thin oxides after exposure to ionising radiation. This Radiation Induced Leakage Current (RILC) can be described as an inelastic tunnelling process mediated by neutral traps in the oxide, with an energy loss of about 1 eV. The neutral trap distribution is influenced by the oxide field applied during irradiation, thus indicating that the precursors of the neutral defects are charged, likely to be defects associated with trapped holes. The maximum leakage current is found under zero-field condition during irradiation, and it rapidly decreases as the field is enhanced, due to a displacement of the defect distribution across the oxide towards the cathodic interface. The RILC kinetics are linear with the cumulative dose, in contrast with the power law found on electrically stressed devices.

Accelerated wear-out of ultra-thin gate oxides after irradiation

We have investigated how ultra-thin gate oxides subjected to heavy ion irradiation react to a subsequent electrical stress performed at low voltages. Even in devices exhibiting small (or even no) increase of the gate current after irradiation, the time-to-breakdown is substantially reduced in comparison with unirradiated samples due to the onset of a soft or hard breakdown, in contrast with previous results found on thicker oxides. In fact, we have demonstrated that the radiation damage acts as a seed for further oxide degradation by electrical stress during the device operating life. The accelerated oxide wear-out depends on the linear energy transfer (LET) coefficient of radiation source.

Drain current decrease in MOSFETs after heavy ion irradiation

In this work, we have focused our attention on MOSFETs, which are the real basic elements of all CMOS applications. We have studied the immediate and latent effects produced by heavy ion irradiation on MOSFETs with ultrathin gate oxide, even after electrical stresses subsequent to irradiation. We found that a single ion can generate a physically damaged region (PDR) localized in the Si-SiO/sub 2/ interface, which may hamper the surface channel formation. In order to generate a PDR the ion hit must be close enough to MOSFET borders, i.e., in correspondence with the STI or the LDD spacer. Consequently, if both MOSFET W and L are large enough only few ion hits may give place to a PDR, mitigating the radiation damage. Finally we have developed an original model to describe the impact of the PDR on channel conductance in the ohmic linear region. On the basis of this model, we predict a PDR size around 0.2-1 /spl mu/m.

Noise characteristics of radiation-induced soft breakdown current in ultrathin gate oxides

We have investigated new aspects of the gate leakage current due to radiation-induced soft breakdown (RSB) of thin oxides subjected to heavy-ion irradiation. Temperature and noise characteristics of RSB on MOS capacitors with 3- and 4- nm MOS oxides have been experimentally investigated. We have developed an empirical law to describe quantitatively the temperature dependence of the RSB current. A small activation energy has been found by using an Arrhenius relation, in agreement with the RSB tunneling conduction mechanism. The RSB variation at high temperature has been only estimated, as measurements of RSB oxides easily produced catastrophic breakdown. We have studied the RSB noise and identified different contributions to the characteristic random telegraph noise, correlated with the trapping and conduction characteristics of the RSB spots. An original model has been developed that successfully describes the different probability distributions of the current fluctuations that cannot be simulated by using previous models, such as those based on Levy or Gaussian distributions. Finally, a correlation was established between the shape of the fluctuation distribution and the degradation level of the oxide.

Trapping phenomena in AlGaN/GaN HEMTs: a study based on pulsed and transient measurements

Slow trapping phenomenon in AlGaN/GaN HEMTs has been extensively analyzed and described in this paper. Thanks to a detailed investigation, based on a combined pulsed and transient investigation of the current/voltage characteristics (carried out over on an 8-decade time scale), we report a detailed description of the properties of trap levels located in the gate–drain surface, and in the region under the gate of AlGaN/GaN HEMTs. More specifically, the following, relevant results have been identified: (i) the presence of surface trap states may determine a significant current collapse, and reduction of the peak transconductance. During a current transient measurement, the emission of electrons trapped at surface states proceeds through hopping, as demonstrated by means of temperature-dependent measurements. The activation energy of the de-trapping process is equal to 99 meV. (ii) The presence of a high density of defects under the gate may induce a significant shift in the threshold voltage, when devices are submitted to pulsed transconductance measurements. The traps responsible for this process have an activation energy of 0.63 eV, and are detected only on samples with high gate leakage, since gate current allows for a more effective charging/de-charging of the defects.

Thermal stress effects on Dye-Sensitized Solar Cells (DSSCs)

Abstract Since the DSSCs gain heat during exposure to sunlight increasing its own temperature, we have studied the role of temperature on the degradation of DSSCs. We have performed pure thermal stresses keeping the devices at a constant temperature inside a climatic chamber and monitoring the electrical parameters during stress. We found that temperature alone strongly impacts on the DSSC performances, enhancing the degradation of the sensitizer and then reducing the photo-generated current.

Collapse of MOSFET drain current after soft breakdown and its dependence on the transistor aspect ratio W/L

Gate-oxide soft breakdown (SB) can have a severe impact on MOSFET performance even when not producing any large increase of the gate leakage current. The SB effect on the MOSFET characteristics strongly depends on the channel width W: drain saturation current and MOSFET transconductance dramatically drop in transistors with small W after SB. As W increases, the SB effect on the drain current fades. The drain saturation current and transconductance collapse is due to the formation of an oxide defective region around the SB spot, whose area is much larger than the SB conductive path. Similar degradation can be observed even in heavy ion irradiated MOSFETs where localized damaged oxide regions are generated by the impinging ions without producing any increase of gate leakage current.Gate oxide soft breakdown (SB) can have a severe impact on MOSFET performance even when not producing any large increase of the gate leakage current. The soft breakdown effect on the MOSFET characteristics strongly depends on the aspect ratio W/L: drain saturation current and MOSFET transconductance dramatically drop in transistors with small W/L after soft breakdown. As W/L increases, the SB effect on the drain current fades. The drain saturation current and transconductance collapse are due to the formation of an oxide defective region around the SB spot, the area of which is much larger than the SB conductive path. Similar degradation can be observed even in heavy ion irradiated MOSFETs where localized damaged oxide regions are generated by the impinging ions without producing, any increase of gate leakage current.

Collapse of MOSFET drain current after soft breakdown

Gate-oxide soft breakdown (SB) can have a severe impact on MOSFET performance even when not producing any large increase of the gate leakage current. The SB effect on the MOSFET characteristics strongly depends on the channel width W: drain saturation current and MOSFET transconductance dramatically drop in transistors with small W after SB. As W increases, the SB effect on the drain current fades. The drain saturation current and transconductance collapse is due to the formation of an oxide defective region around the SB spot, whose area is much larger than the SB conductive path. Similar degradation can be observed even in heavy ion irradiated MOSFETs where localized damaged oxide regions are generated by the impinging ions without producing any increase of gate leakage current.

Effects of Positive and Negative Stresses on III–V MOSFETs With

We subjected III-V InGaAs MOSFETs to positive and negative gate stresses. The stress polarity strongly affects the degradation kinetics of the gate current. Positive stress features a remarkable increase of the gate current, a net negative trapped charge, a large telegraphic noise, and soft breakdown, before the occurrence of the final catastrophic breakdown. Negative stress features only positive trapped charge until the hard breakdown.

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In this work we studied the soft breakdown (SB) in ultra-thin gate oxides (<3 nm) subjected to constant current stress. SB current derives from the superposition of several random telegraph signals noises with different time constants and amplitudes. Such fluctuations derive from the conductance modulation of a damaged region inside the oxide layer, due to the electrical stress. We found that the current noise power density follows the 1/fα power law (with α between 1 and 2) over a wide range of frequency (1 Hz–100 kHz). Only at frequency smaller than 1–10 Hz a possible deviation from this low cannot be excluded. Moreover, the discrete fluctuations typical of SB are statistically independent events at least over time periods around hundreds of seconds, according to a Poisson process. This result suggest that electron trapping/detrapping in defect sites near or inside the SB conductive path can be claimed as responsible for such conductance modulation.