Swati Shikha

Bio: Swati Shikha is an academic researcher from Indian Institute of Science. The author has contributed to research in topics: Leakage (electronics) & Stress (mechanics). The author has an hindex of 3, co-authored 4 publications receiving 17 citations.

On the Role of Interface States in AlGaN/GaN Schottky Recessed Diodes: Physical Insights, Performance Tradeoff, and Engineering Guidelines

Abstract: In this paper, the impact of donor and acceptor states at the Schottky interface of fully recessed AlGaN/GaN Schottky diode is physically modeled using device TCAD and detailed experiments. This allowed us to develop physical insights into recessed AlGaN/GaN diode’s reverse breakdown, reverse leakage, and ON-state performance as a function of interface states and provided design guidelines to engineer fully recessed AlGaN/GaN Schottky diode for the maximum reverse breakdown and least reverse leakage without compromising its ON-state performance. It has been observed that donor states are responsible for high reverse leakage and reduced breakdown performance in Schottky diodes. On the other hand, the presence of acceptor states at the interface improves the diode leakage and breakdown voltage. Experiments involve a number of dry and wet surface treatments to: 1) validate computational findings and 2) find ways to cure or passivate donor states affected Schottky interface/recessed region. The introduction of acceptor traps at the Schottky interface has been proposed and experimentally verified using the Fluorine implant to cure donor state-affected Schottky interface, which improves the breakdown and reverse leakage characteristics significantly.

Time Dependent Shift in SOA Boundary and Early Breakdown of Epi-Stack in AlGaN/ GaN HEMTs Under Fast Cyclic Transient Stress

Abstract: This experimental study reports first observations of (i) SOA boundary shift in AlGaN/GaN HEMTs and (ii) early time-to-fail of vertical AlGaN/GaN epi-stack under fast changing (sub-10ns rise time) cyclic pulse transient stress, which otherwise qualified for 600 V DC stress. It is shown that a epi stack qualified for 10 years lifetime under DC stress, fails faster under cyclic transient stress. The drain-to-substrate leakage exhibits different trends under DC and pulse stress. Integrated electrical and mechanical stress characterization routine involving Raman/ PL mapping, PFM and CL spectroscopy along with atomistic simulations reveals material limited unique failure physics under transient stress condition. Failure analysis using cross-sectional SEM and TEM investigations reveal signature of different degradation and failure mechanism under transient and DC stress conditions. A failure model is proposed for rapid breakdown of AlGaN/GaN epi-stack under cyclic transient stress and it is experimentally validated.

Time Dependent Early breakdown of AIGaN/GaN Epi Stacks and Shift in SOA Boundary of HEMTs Under Fast Cyclic Transient Stress

Abstract: This experimental study reports first observations of (i) SOA boundary shift in GaN HEMTs and (ii) early time to fail of vertical AIGaN/GaN Epi stack under fast changing (sub-10ns risetime) cyclic transient stress conditions for a 600V qualified commercial grade HEMT stack. It is shown that a stack qualified for 10 years lifetime under DC stress, fails faster under cyclic transient stress. Integrated electrical and mechanical stress characterization routine involving Raman/PL mapping and CL spectroscopy reveals material limited unique failure physics under transient stress condition. Failure analysis using cross-sectional TEM investigations reveal signature of different degradation and failure mechanism under transient and DC stress conditions. A failure model is proposed for failure under cyclic transient stress.

UV-Assisted Probing of Deep-Level Interface Traps in GaN MISHEMTs and Their Role in Threshold Voltage & Gate Leakage Instabilities

Abstract: This work demonstrates UV assisted probing of deep level traps in dielectric/GaN interface. The deep level donor traps lead to threshold voltage and gate leakage instabilities in GaN MISHEMTs. While UV exposure excites the deep traps, gate bias can sweep the trap energy state and trigger de-trapping. The recovery transient is evaluated to study the trap time constant. Besides, this work reveals a non-destructive technique to probe intrinsic traps as the UV exposure does not change the trap density across the device.

Demonstration of Avalanche and Surge Current Robustness in GaN Junction Barrier Schottky Diode With 600-V/10-A Switching Capability

Abstract: In this letter, we achieved significantly enhanced avalanche ruggedness and surge current capability in GaN junction barrier Schottky (JBS) diode for highly reliable power operation. Based on the selective Mg-ion implantation technology, the GaN JBS diode obtains superior electrostatic performances, including 830-V breakdown voltage, 150-mΩ specific on -state resistance, and 0.5-V turning on voltage. Meanwhile, zero reverse recovery behaviors are observed even under extreme switching conditions of 600 V/10 A. During the reliability evaluation in the inductive load circuits, crucial avalanche capability with avalanche breakdown voltage over 965 V, avalanche energy up to 57.8 mJ, and more than 10 000 repetitive avalanche breakdown events are demonstrated. Together with the surge current tolerance up to 65 A and surge energy of 6.0 J, a large safe-operation-area under both forward and reverse inductive spikes is realized for the GaN-based rectifier.

Safe Operating Area of Polarization Super-junction GaN HEMTs and Diodes

Abstract: This article reports safe operating area (SOA) assessment in polarization super-junction (PSJ)-based GaN high-electron mobility transistor (HEMT) and Schottky diode. The degradation physics, which limits SOA in these devices under high-voltage and high-current-injection conditions is presented. Trap-induced SOA degradation and the role of PSJ in SOA improvement are unveiled. In PSJ-field-effect transistor (FET), the impact of PSJ length and its position on SOA robustness are studied. The role of self-heating and substrate effect on degradation are discussed. PSJ diodes with different configurations of Schottky contact are investigated. The correlation between PSJ length and failure threshold is discovered, besides power and field dependence of SOA boundary. Compared with their conventional counterparts, unique failure modes are discovered in PSJ-based GaN HEMT and diode.

Electro-Thermo-Mechanical Reliability of Recessed Barrier AlGaN/GaN Schottky Diodes Under Pulse Switching Conditions

Abstract: Pulse reliability of AlGaN/GaN recessed Schottky diode is studied under transient overstress conditions, typically encountered in power converters. Degradation of the Schottky diode during both free-wheeling operation (high forward current injection) and reverse blocking state (high-voltage stress) is studied. Defect generation and the associated degradation were found to be uncorrelated with the nature of interface formed due to various surface treatments (at metal/GaN Schottky interface). During forward conduction, trap-assisted Schottky interface degradation is studied using on-the-fly ${I}$ – ${V}$ and ${C}$ – ${V}$ characterization under high-current stress. Under high-voltage stress, in the reverse blocking mode, mechanical strain evolution and defects generation were found to be dominant degradation modes, which are studied in detail using on-the-fly micro-Raman spectroscopy. Post-failure analysis was performed using SEM, TEM, and EDX, which reveals distinct failure signatures at the safe operating area (SOA) boundary. TCAD simulations are used to gain deeper physical insights into the observed degradation mechanism. Finally, a qualitative failure model, explaining the distinct failure physics, is presented based on observations and findings, from various electrical, optical, Raman spectroscopy, and electron microscopy investigations.

Forward and reverse current transport mechanisms in tungsten carbide Schottky contacts on AlGaN/GaN heterostructures

Abstract: In this paper, the forward and reverse current transport mechanisms in as-deposited and 400 °C annealed tungsten carbide (WC) Schottky contacts on AlGaN/GaN heterostructures have been studied. In particular, under forward bias, the WC/AlGaN Schottky contacts exhibited a deviation from the ideal thermionic emission model due to the occurrence of a tunneling component of the current. From the temperature dependence of the ideality factor, a characteristic tunneling energy E00 in the range of 33–36 meV has been estimated. On the other hand, two different transport mechanisms have been identified under reverse bias. At low reverse bias (VR 2 V), the leakage current is dominated by a thermally activated process with an activation energy (0.27 eV) that is independent of the Schottky contact fabrication process. In this case, the temperature dependence of the leakage could be well described by a two-dimensional variable range hopping conduction associated with the presence of surface defects in the material.

Influence of surface trap states on RF/microwave performance of lateral AlGaN/GaN Schottky barrier diode

Abstract: In this paper, we have presented the influence of surface traps/states on RF and microwave performance of fully recessed Schottky anode AlGaN/GaN lateral Schottky barrier diode (L-SBD) using numeri...