Nathalie Malbert

Evidence of traps creation in GaN/AlGaN/GaN HEMTs after a 3000 hour on-state and off-state hot-electron stress

A long-term 3000-hour test under on-state conditions (VDS =25V, 6W/mm constant dissipated power) and off-state conditions (V DS=46V, VGS=-6V) on GaN/AlGaN/GaN HEMTs is presented. Trapping presence and hot-electrons effect are characterized by means of low-frequency techniques (low-frequency noise measurements, transconductance frequency dispersion, gate-lag). The on-state stress shows the most important degradation. Since our measurements point out to the creation of traps in the gate-to-drain surface region during the stress, this degradation is ascribed to the effect of hot-electrons

A Scalable Electrothermal Model for Transient Self-Heating Effects in Trench-Isolated SiGe HBTs

This paper demonstrates a scalable electrothermal model for transient self-heating effects in trench-isolated SiGe heterojunction bipolar transistors (HBTs). The scalability of the thermal model has been investigated by considering pyramidal heat diffusion approximation between the heat source and the thermal ground. Three-dimensional thermal TCAD simulations have been carried out to obtain transient variations of the junction temperature and to extract the thermal impedance in the frequency domain. A recursive thermal network with scalable model parameters has been developed and added at the temperature node of the HBT compact model HiCuM. This network has been verified through numerical simulations and by low-frequency s-parameter measurements and found to be in excellent agreement for various device geometries.

AlGaN/GaN HEMT Reliability Assessment by means of Low Frequency Noise Measurements

Although impressive results have been published for GaN-based transistors in a large frequency range reliability demonstration is becoming an important subject of concern. In this article the conditions of a long term DC life test is presented and a detailed description of pre- and post-test characterization by means of Low Frequency Noise measurements (LFN) is discussed. The transistor parameters (IDSS, Ron, Vp) and the drain noise spectra presented an evolution strictly related to the biasing point during the stress. This demonstrates that LFN measurement is a useful tool to investigate degradation in GaN HEMTs.

Thermal Impedance Modeling of Si–Ge HBTs From Low-Frequency Small-Signal Measurements

In this letter, the thermal impedance of SiGe heterojunction bipolar transistors has been characterized using low-frequency small-signal measurements. Theoretical works for thermal impedance modeling using different electrothermal networks, developed until date, have been verified with our experimental results. We report for the first time the experimental verification of the electrothermal model for thermal impedance developed by Mnif using a nodal and recursive network. A generalized expression of the frequency-domain thermal impedance has been selected for electrical compact transistor model (HiCuM) improvement, parameter extraction, and electrothermal network verification.

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

Link between low frequency noise and reliability of compound semiconductor HEMTs and HBTs

On the basis of papers published on the fundamental and excess LF noise sources in compound semiconductor transistors, two main issues are addressed in this paper: i) the characterization of LF noise sources linked to parasitic effects induced by innovative component architecture in emerging technologies such as GaN-based HEMTs and GaAsSb HBTs. ii) the identification of LF noise sources linked to degradation mechanisms in compound semiconductor devices issued from mature technologies. Examples of experimental correlation between LF noise and parasitic effects (i.e. potential electrical performance penalization) or failure mechanisms in III-V HEMTs and HBTs are described with data related to recent technological developments.

Electro-thermal characterization of Si-Ge HBTs with pulse measurement and transient simulation

This paper describes a new and simple approach to accurately characterize the transient self-heating effect in Si-Ge Heterojunction Bipolar Transistors (HBTs), based on pulse measurements and verified through transient electro-thermal simulations. The measurements have been carried out over pulses applied at Base and Collector terminals simultaneously and the time response of Collector current increase due to self-heating effect are obtained. Compared to previous approach, a complete calibration has been performed including all the passive elements such as coaxial cables, connectors and bias network. Furthermore, time domain junction temperature variations, current of heat flux and lattice temperature distribution have been obtained numerically by means of 3D electro-thermal device simulations. The thermal parameters extracted from measurements using HiCuM HBT compact model have been verified with the parameters extracted from electro-thermal transient simulation. It has been shown that, the standard R-C thermal network is not sufficient to accurately model the thermal spreading behavior and therefore a recursive network has been employed which is more physical and suitable for transient electro-thermal modeling.

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.