Clemens Ostermaier

Towards understanding the origin of threshold voltage instability of AlGaN/GaN MIS-HEMTs

GaN-power HEMTs with insulated gate structure suffer from threshold voltage drifts (ΔVth) under forward gate bias stress. We present a systematical approach to characterize the phenomenon and understand the dominant physical mechanisms causing this effect. We found out that ΔVth is caused by traps with a broad distribution of trapping and emission time constants. This distribution is analyzed using so called Capture Emission Time (CET) maps known from the study of bias temperature instability (BTI) in CMOS devices. Physical models, which could explain the broad distribution of time constants, are discussed.

Modeling small-signal response of GaN-based metal-insulator-semiconductor high electron mobility transistor gate stack in spill-over regime: Effect of barrier resistance and interface states

We provide theoretical and simulation analysis of the small signal response of SiO2/AlGaN/GaN metal insulator semiconductor (MIS) capacitors from depletion to spill over region, where the AlGaN/SiO2 interface is accumulated with free electrons. A lumped element model of the gate stack, including the response of traps at the III-N/dielectric interface, is proposed and represented in terms of equivalent parallel capacitance, Cp, and conductance, Gp. Cp -voltage and Gp -voltage dependences are modelled taking into account bias dependent AlGaN barrier dynamic resistance Rbr and the effective channel resistance. In particular, in the spill-over region, the drop of Cp with the frequency increase can be explained even without taking into account the response of interface traps, solely by considering the intrinsic response of the gate stack (i.e., no trap effects) and the decrease of Rbr with the applied forward bias. Furthermore, we show the limitations of the conductance method for the evaluation of the density...

On the fly characterization of charge trapping phenomena at GaN/dielectric and GaN/AlGaN/dielectric interfaces using impedance measurements

Charge trapping phenomena at interfaces of GaN-based semiconductors with a dielectric are one of the major concerns in modern MIS-HEMT technologies. Fundamental questions about the nature and the behavior of interface defects must still be answered. We address these questions by investigating devices with and without an AlGaN layer at the the interface with the dielectric, using MIS capacitor test structures. We consider different methodologies to perform and analyze impedance measurements and the results are compared and discussed. Special attention is paid to the uncertainties and limitations inherent to different techniques, as well as the challenges due to the composite structure of GaN/AlGaN devices. We introduce an on the fly technique which allows the extraction of the device drift during stress. This enables us to suggest a lower and upper boundary for the amount of device degradation. The experimental results indicate the presence of similar defects at GaN and AlGaN surfaces, which therefore appear to be intrinsic to the III-N material.

Characterization and modeling of single defects in GaN/AlGaN fin-MIS-HEMTs

Charge trapping in gallium-nitride (GaN) metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs) is a serious reliability challenge but is still poorly understood. A promising opportunity to investigate physical defect properties has become available through nanoscale GaN fin-MIS-HEMTs which are small enough to be sensitive to the impact of individual defects. In this work, we extract the properties of four pre-existing single defects, which are identified by their correlated random telegraph noise (RTN) signals and step heights. We use a two-state non-radiative multi-phonon (NMP) model to extract their vertical positions, their trap levels and their apparent activation energies. We show that they are in close proximity to each other within the barrier layer and also share a similar trap-level despite their different capture and emission times. Furthermore, TCAD simulations are used to estimate the influence of the vertical and horizontal positions of single-traps which are in reasonable agreement with the experimental data.

Characterization of Interface Defects With Distributed Activation Energies in GaN-Based MIS-HEMTs

Charge trapping is one of the main reliability issues for GaN-based MIS-high-electron-mobility-transistor technologies. In this paper, we focus on the defects located at or close to the interface with the dielectric, which are responsible for the threshold voltage instability at positive gate bias conditions. We present a methodology to analyze the experimental data based on the nonradiative multiphonon model for charge trapping. In particular, we show how to extract the density of interface traps as a function of their activation energy from stress and recovery experiments performed at various temperatures. Our approach is applied to two GaN/AlGaN/SiN samples with different trapping properties, at temperatures ranging from −190 °C to 200 °C. We evaluate their response to forward bias stress and finally, we extract the activation energy distribution for electron capture and emission over a continuous energy range.

Emerging GaN-based HEMTs for mechanical sensing within harsh environments

Gallium nitride based high-electron-mobility transistors (HEMTs) have been investigated extensively as an alternative to Si-based power transistors by academia and industry over the last decade. It is well known that GaN-based HEMTs outperform Si-based technologies in terms of power density, area specific on-state resistance and switching speed. Recently, wide band-gap material systems have stirred interest regarding their use in various sensing fields ranging from chemical, mechanical, biological to optical applications due to their superior material properties. For harsh environments, wide bandgap sensor systems are deemed to be superior when compared to conventional Si-based systems. A new monolithic sensor platform based on the GaN HEMT electronic structure will enable engineers to design highly efficient propulsion systems widely applicable to the automotive, aeronautics and astronautics industrial sectors. In this paper, the advancements of GaN-based HEMTs for mechanical sensing applications are discussed. Of particular interest are multilayered heterogeneous structures where spontaneous and piezoelectric polarization between the interface results in the formation of a 2-dimensional electron gas (2DEG). Experimental results presented focus on the signal transduction under strained operating conditions in harsh environments. It is shown that a conventional AlGaN/GaN HEMT has a strong dependence of drain current under strained conditions, thus representing a promising future sensor platform. Ultimately, this work explores the sensor performance of conventional GaN HEMTs and leverages existing technological advances available in power electronics device research. The results presented have the potential to boost GaN-based sensor development through the integration of HEMT device and sensor design research.

Physical-chemical stability of fluorinated III-N surfaces: Towards the understanding of the (0001) AlxGa1-xN surface donor modification by fluorination

Gallium nitride based high electron mobility transistors are widely known for their operational instabilities regarding interface defects to the dielectric. In this paper, we discuss a III-N surface treatment that results in an electrically more defined interface and hence a narrower distribution of electrically present interface states compared to the original, untreated interface. This surface modification is caused by a remote plasma fluorination of the III-N surface. We show that it is a very distinctive surface processing which cannot be reproduced by other plasma techniques or ion implantation. Applying physical and chemical analyses, the fluorination is found to have a remarkable stability towards temperatures up to 700 °C and is also stable in air for up to 180 h. However, an aqueous clean allows the surface to return to its original state. Even though the exact physical origin of the responsible surface donor cannot be inferred, we suggest that fluorine itself might not directly represent the new surface donor but that it rather activates the III-N surface prior to the dielectric deposition or even substitutes and hence reduces the concentration of surface hydroxides.Gallium nitride based high electron mobility transistors are widely known for their operational instabilities regarding interface defects to the dielectric. In this paper, we discuss a III-N surface treatment that results in an electrically more defined interface and hence a narrower distribution of electrically present interface states compared to the original, untreated interface. This surface modification is caused by a remote plasma fluorination of the III-N surface. We show that it is a very distinctive surface processing which cannot be reproduced by other plasma techniques or ion implantation. Applying physical and chemical analyses, the fluorination is found to have a remarkable stability towards temperatures up to 700 °C and is also stable in air for up to 180 h. However, an aqueous clean allows the surface to return to its original state. Even though the exact physical origin of the responsible surface donor cannot be inferred, we suggest that fluorine itself might not directly represent the new...

Thermal activation of PBTI-related stress and recovery processes in GaN MIS-HEMTs using on-wafer heaters

Threshold voltage instabilities are investigated in GaN-based metal-insulator-semiconductor (MIS) high-electron-mobility-transistors (HEMTs) with specially designed on-wafer heaters structures. The heaters are based on metal lines or 2-dimensional electron gas (2DEG) resistors and enable to choose the temperature during stress and recovery in the stress-recovery experiments independently. It allows to decouple thermal activation of capture (<; 0.9 eV) and emission (0.4-0.9 eV) processes at the dielectric/nitride interface, which is not possible in stress-recovery experiments performed at a common ambient temperature.

Review of bias-temperature instabilities at the III-N/dielectric interface

Abstract Two particular defects are commonly discussed at the III-N interface: the required donor states, known to exist from the formation of the two-dimensional electron gas (2DEG) below a hetero-barrier, and defect states at the interface or within the dielectric layer. It appears that the latter ones are responsible for the ongoing challenge to find a low-defect gate dielectric to reduce positive bias temperature instabilities (PBTI). This raises the question, why the natively given donor states behave almost like fixed charges. We review the known and verified characteristics for both defect types and the link between them. Moreover, we define a lifetime criterion for power switching applications to compare PBTI effects related to III-N interfaces.