Dominique Carisetti

Analysis of current collapse effect in AlGaN/GaN HEMT: Experiments and numerical simulations

In this work, current collapse effects in AlGaN/GaN HEMTs are investigated by means of measurements and two-dimensional physical simulations. According to pulsed measurements, the used devices exhibit a significant gate-lag and a less pronounced drain-lag ascribed to the presence of surface/barrier and buffer traps, respectively. As a matter of fact, two trap levels (0.45 eV and 0.78 eV) were extracted by trapping analysis based on isothermal current transient. On the other hand, 2D physical simulations suggest that the kink effect can be explained by electron trapping into barrier traps and a consequent electron emission after a certain electric-field is reached.

Analysis of Schottky Gate degradation evolution in AlGaN/GaN HEMTs during HTRB stress

GaN based technologies are promising in terms of electrical performances for power and high frequencies applications and their reliability assessment remains a burning issue. Thus, a good understanding of their degradation mechanisms is required to warranty their reliability. In this paper, an electrical parasitic effect has been observed on the gate-source diode forward characteristics of a set of devices under HTRB stress carried out at 175 °C up to 4000 h. This parasitic effect has been attributed to lateral surface conduction and correlated with EL signature under diode forward biasing conditions but not under transistor pinch-off biasing conditions. Then, physical analyses have pointed out the formation and growing over time of pits and cracks at the gate edge on the drain side

Thermal Characterization Using Optical Methods of AlGaN/GaN HEMTs on SiC Substrate in RF Operating Conditions

Performance and reliability of wide bandgap high-power amplifiers are correlated with their thermal behavior. Thermal model development and suitable temperature measurement systems are necessary to quantify the channel temperature of devices in real operating conditions. As a direct temperature measurement within a channel is most of the time not achievable, the common approach is to measure the device temperature at different locations close to the hotspot and then to use simulations to estimate the channel temperature. This paper describes a complete thermal characterization of AlGaN/gallium nitride (GaN) on silicon carbide high electron-mobility transistors (HEMTs) when devices are operating in dc bias, pulsed, and continuous wave. Infrared thermography, charge-coupled device-based thermoreflectance microscopy, and micro-Raman spectroscopy have been performed to extract the thermal resistance of the components. Results have been compared with simulations using a 3-D finite-element model to estimate the operating channel temperature. Measurements have shown that the RF-biased thermal resistance and the dc-biased thermal resistance of GaN HEMTs are similar.

Influence of gate leakage current on AlGaN/GaN HEMTs evidenced by low frequency noise and pulsed electrical measurements

The study of the pulsed drain current or noise characteristics in AlGaN/GaN HEMTs is the key of knowledge for designing the power amplifiers, the low noise amplifiers and the oscillators or mixers, but it is well accepted today that this study is not fully accomplished without pointing on the effect of the gate leakage current; It is obvious that the transistors leakage current may disturb its operation at high power and high frequency. Leakage currents studies are also an area of great importance in optimization of safe operating area and reliability of HEMTs. Therefore, room temperature pulsed I-V and low frequency noise measurements of gate and drain currents of AlGaN/GaN HEMTs have been investigated under different bias conditions on two devices showing identical drain current and different gate current levels. The results show a correlation between two non-destructive measurement techniques applied on devices under test.

Evidence of relationship between mechanical stress and leakage current in AlGaN/GaN transistor after storage test

In this paper, leakage current signatures in AlGaN HEMT are studied after storage at 300 °C. Electrical characterization of the gate to source diode as a function of the temperature has been performed on HEMT with two different gate pad topologies, but it has not allowed identifying significant difference in the electron transport mechanisms. In forward and low reverse bias, the preeminent conduction mechanism can be attributed to thermionic field emission (TFE). By localized FIB cuts, Optical Beam Induced Resistance Change (OBIRCh) analysis was used to localize current path. Results tend to indicate that mechanical stresses in the gate structure strongly influences the leakage current of the transistor. The OBIRCh analysis technique, widely used in silicon technology, appears to be a very efficient tool to localize leakage paths, in particular for HEMT topology with source terminated field plate.

Gate defects in AlGaN/GaN HEMTs revealed by low frequency noise measurements

From the last decade, Nitride-based High Electron Mobility Transistors (HEMTs) have demonstrated excellent electrical and noise performances to address transceivers modules. Within this paper, a discussion on the low frequency noise on the gate access (i.e. gate current spectral density SIG) in the frequency range of 1Hz-100kHz is presented. SIG spectra reveal different signatures according to the configuration of biasing of the transistor, and to the dimensions of the transistor: the noise of the Schottky diode is studied alone (under open drain configuration), and compared with SIG of the transistor biased in the saturated region (at VDs=8V). Two designs of devices are tested: research-level devices featuring single gate finger are compared with commercial devices featuring four gate fingers, where each finger is four times larger (i.e. 16 times larger gate width than research level devices). The two sets of devices followed the same fabrication process. It is found that whatever the sizing and the gate pad configuration of the devices, the LFN spectra observed on each set of transistors feature identical signatures. The leakage carriers are following the same path between the gate and source accesses. LFN can be used as an accurate tool to discriminate between conduction mechanisms of devices, and to help to understand what are the underlying mechanisms leading to the conduction of the Schottky diode (and thus to its degradation). Moreover, it is shown that the SIG measurements under transistor biasing conditions can be correlated to the gate current spectral density in diode mode: the leakage current zone can also be tracked under this biasing operating mode.

Thermoelectric characterization of nanostructured selenium doped bismuth telluride obtained by a solution route

Thermoelectric devices offer a reliable all solid state solution to active temperature control and are more and more considered as a promising way for thermal management of Integrated Circuits. However, low material performances still represent a hurdle to the spreading of such devices for high power electronics cooling. Enhanced performances bulk materials are needed to access high performance board level cooling, and at the same time very stable processes are needed to ensure repeatability of the materials performances for large scale production. Assembly of nanoparticles obtained by a solution route is an attractive bulk fabrication method in this regard. However, to be effective this solution must give access to fine control of the composition of the material as well. We studied the synthesis of selenium substituted bismuth telluride nanoparticles using a scalable process and characterized the electronic and thermoelectric transport properties of the materials obtained by sintering of these particles.