United States

We investigated the influence of aluminium concentration in the AlGaN cap layer on the stability of threshold voltage under gate and drain stress voltages. We found an empirical model that describes the de-trapping transients after a gate and drain stress in off-state condition. The devices examined are N-polar GaN MIS-HEMTs with a 2.6nm layer of Al x Ga 1-x N cap under the gate contact. The devices have three different Al concentration (x=22%, x=34%, x=46%) in the AlGaN cap layer. DC measurements show that the devices with a higher Al concentration have a lower gate leakage current. In this work we found out: 1) the amount of threshold voltage shift during stress in these devices, increases linearly with the value level of gate leakage current at the different bias conditions 2) a higher Al percentage in AlGaN cap layer can suppress gate leakage current and thus reduce the instability of threshold voltage under high gate and drain filling voltages.

IEEE WiPDA 2021

Detrapping Kinetics in N-Polar AlGaN/GaN MIS-HEMTs

The IEEE Workshop on Wide Bandgap Power Devices & Applications (WiPDA) provides a forum for device scientists, circuit designers, & application engineers from the Power Electronics & Electron Devices Societies to share technology updates, research findings, experience & potential applications.

**Topics of interest**

* Heteroepitaxial & Bulk Materials Growth
* Gate Dielectrics & Surface Passivation
* Device Structures & Fabrication Techniques
* Device Characterization & Modeling
* Very-High Efficiency or Compact Converters
* Safe Operating Areas of Wide Bandgap Devices, Including Short Circuit, Spike, & Transient Tolerance
* Harsh Environment (High Temperature) Operation & Reliability
* Packaging Power Modules & ICs
* Hard-Switched & Soft-Switched Application Analysis
* Gate Drive & Other Auxiliary Circuits
* High-Performance Passive Components
* Applications in Renewable Energy & Energy Storage, Transportation, Industrial Drives, & Grid Power Systems
* Wide Band Gap System Design Philosophies & Strategies
* Radio Frequency (RF) GaN (New)
* Technology Roadmap of Wide Bandgap Including Devices, Applications and Packaging (New)

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The IEEE Workshop on Wide Bandgap Power Devices & Applications (WiPDA) provides a forum for device scientists, circuit designers, & application engineers from the Power Electronics & Electron Devices Societies to share technology updates, research findings, experience & potential applications.

technical paper

We present a reduced physics-based thermal model to predict the thermal performance of Chip Scale Package (CSP) GaN FETs mounted to printed circuit boards (PCBs) and provide an easy-to-use thermal characterization tool for engineers and designers to quickly assess thermal management strategies. The heat conductance from the GaN FET to both the PCB side and to the “case” (i.e., die exposed surface) are modeled using resistive-network circuit modeling. The thermal resistance in each conductance path is presented for different parameters relating to PCB construction (stack-up, area, via density), FET size, and cooling approach (TIM material, heat spreader and heatsink). A comprehensive set of thermal finite-element simulations was run over the set of parameters. The results, presented in easy “look-up” style graphs, show that even with a small footprint, GaN FETs power-handling capability can be extended when effectively cooled using a simple PCB design and thermal management approach.

Quick Thermal Performance Estimation of Chip Scale Packaged GaN FETs using a Simple Circuit Model

This work reports an on-wafer study of avalanche behavior and failure analysis of in-house fabricated 1.3 kV GaN-on-GaN P-N diodes. DC breakdown is measured at different temperatures to confirm avalanche behavior. Diode’s avalanche
ruggedness is measured directly on-wafer using a modified unclamped inductive switching (UIS) test set-up with an integrated thermal chuck and high-speed CCD for real-time imaging during the test. The avalanche ruggedness of the GaN P-N diode is evaluated and compared with a commercial SiC Schottky diode of similar voltage and current rating. Failure analysis is done using SEM and optical microscopy to gain insight into the diode’s failure mechanism during avalanche operation.

On-Wafer Investigation of Avalanche Robustness in 1.3 kV GaN-on-GaN P-N Diode Under Unclamped Inductive Switching Stress

GaN transistors, and more recently integrated circuits, have been in mass production for over a decade [1] with several manufacturers reporting tens of millions of units shipped with billions of hours in actual end-use applications. The track record has been extraordinarily successful, and one of the key reasons is that GaN devices are far more rugged than their silicon MOSFET ancestors [2].
Recently, Efficient Power Conversion (EPC) did a series of tests to take eGaN FETs beyond their datasheet limits to quantify the effects of large amounts of overstress voltage and current. This paper will examine the results and those tests to show how operation under extreme conditions for long periods of time can be a practical new tool for power system design engineers.

Extreme GaN – What Happens When eGaN® FETs Are Exposed to Voltage Levels Well Above Data Sheet Limits

This paper studies switching behavior and dynamic on-state characteristics of lateral 10 mm depletion-mode β-Ga2O3 met-al-oxide-semiconductor field-effect transistors (MOSFETs). Improvements in device technology and fabrication enable hard-switching operation with an increase of the maximum blocking voltage to 400 V at 0.26 A load current in a double-pulse test configuration. Compared to static measurements, dispersion effects lead to an increased on-resistance in hard-switched operation, which becomes especially pronounced at blocking voltages above 200 V. The extend of dynamic Ron-increase further depends on several operation conditions such as switching speed, on-state duration and load current.

Switching Behavior and Dynamic On-Resistance of Lateral β-Ga2O3 MOSFETs Up to 400 V

A gallium oxide (β-Ga2O3) Schottky barrier diode (SBD) is proposed and characterized. The fabricated diode has an area of 300 μm × 300 μm with a 12 μm unintentionally doped (UID) (1.7 × 1016 cm−3) Ga2O3 epitaxial layer on Sn doped (5.4 × 1018 cm−3) (010) Ga2O3 substrate. The device has 1.02 kV breakdown
voltage and 500 mA forward current at 5.5 V. The reverse recovery characteristic and the rectification capability of the diode are experimentally studied. The Ga2O3 SBD shows a high potential for power device applications such as pulse power applications.

Reverse Recovery and Rectification Characteristic of β-Ga2O3 Schottky Barrier Diode

Higher cost, lack of availability, and higher on-resistance (RDSon) are just a few drawbacks of semiconductor devices at higher voltages. This applies to the wide-bandgap semiconductors like Silicon Carbide (SiC) MOSFETs as well, where individual devices are not commercially available at voltages above 3.3-kV at the time of preparing this paper. Series connection of individual semiconductor devices is an effective way to achieve higher-voltage switches. However, the inherent unequal static and dynamic voltage sharing problems between the devices in the stack need to be addressed. The majority of the existing voltage balancing schemes are developed for slow-switching Si-based semiconductors, and bring about a significant amount of additional losses in the control circuit or on the switches. Some other techniques are not practical on SiC MOSFETs, given the high switching speeds featured by these devices. This paper tries to survey and compare the voltage balancing techniques proposed by researchers for being used on the faster-switching wide-bandgap devices. The voltage balancing techniques proposed in literature are classified into 5 different categories of “passive solutions”, “self-powered and pulse-powered gate drivers”, “active gate control techniques”, “gate timing control methods” and “voltage clamping solutions”. Then, a comprehensive and unbiased comparison is conducted between different voltage balancing techniques from “cost”, “modularity”, “complexity”, “startup balancing”, “losses” and “voltage balancing” points of view. Finally, conclusions from the conducted comparison are discussed.

Study of Voltage Balancing Techniques for Series-Connected Wide-Bandgap Semiconductors Devices

GaN-based transistors offers the highest power density for radio-frequency (RF) applications. However, self-heating significantly limits device performance and reliability that must be managed for the transistors to perform. Exploiting the high thermal conductivity of polycrystalline diamond (300-2000 W/mK near room temperature), we have demonstrated polycrystalline diamond integration on top of the GaN channels for effective device cooling, while preserving its electrical performance. Various thicknesses of polycrystalline diamond were grown on top of the GaN channel to investigate its impact on the device temperature. Using Raman thermometry, a remarkably lower temperature (by 30-60 °C) was measured in the channel for devices with diamond compared to control devices without diamond. The effect of diamond thickness (250-650 nm) and device layout on the heat dissipation was carefully studied and discussed. We have shown that thicker diamond (~650 nm) along with the metal-diamond overlap improves channel cooling capabilities of the diamond heat spreader by a factor of two.

Diamond Integration on GaN for Channel Temperature Reduction

The target of manufacturing the GaN-on-Si HEMT with advanced CMOS process lines has been continuously pursued. This work reports on a low contact-resistance. Au-free ohmic contacts of an undoped AlGaN/GaN HEMTs on
silicon. These results are believed to promote the development of the GaN-on-Si HEMT compatible with the CMOS process line.

Low Contact Resistance CMOS-Compatible RF GaN-on-Silicon HEMTs

This paper reports two failure modes and degradation mechanism of normally-on AlGaN/GaN HEMTs of 0.15 µm gate length with Pt/Au metallization scheme providing stable Schottky contact. Results describe the device performance and final degradation under reverse bias off-state stressing at high VDS (up to 145 V) and stressing at elevated temperatures (up to 125 °C). Electrical stressing combined with microstructural diagnostic characterization confirms two microstructural failure modes. The first failure mode is observed as a local breakdown between gate head and source ohmic contact due to electrical overstress (EOS) causing electro-thermally induced material degradation. The second failure mode, seen also under different stress conditions, consists in pit formation at both gate edges, which is caused by stress induced local oxidation of the semiconductor surface. The first failure mode is related to a local breakdown of the nitride / oxide insulator in correlation with sudden increase of the device currents. The second failure mode causes local distortion of the 2-dimentional electron gas (2DEG) and partial degradation of the Schottky contact

Microstructural Degradation Investigations of OFF-State Stressed 0.15 µm RF AlGaN/GaN HEMTs: Failure Mode related Breakdown

In this paper, we discuss the limitations of using conventional linearity metrics (i.e., OIP3 and OP1dB) to evaluate the linearity performance of GaN HEMTs for power amplifier (PA) applications. Unlike Silicon and other traditional III-V devices, GaN HEMTs suffer from soft compression, making the use of conventional metrics “unfair” when compared to other transistor technologies. Through simulation-based evaluation of two different commercial GaN devices, we provide insight on best practices and limitations on quantifying the linearity of GaN HEMTs using various linearity performance metrics. We propose using communication standard-based metrics such as adjacent channel power ratio (ACPR) and error vector magnitude (EVM) to account for realistic device performance at the circuit and system level. Using Keysight ADS Verification Test Bench (VTB), we show that a device with 1.3 dB higher OIP3 results in 1.17% and 1.1 dB worse EVM and ACPR, respectively, under backed-off conditions, when compared to a device with lower OIP3.

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Detrapping Kinetics in N-Polar AlGaN/GaN MIS-HEMTs

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Quick Thermal Performance Estimation of Chip Scale Packaged GaN FETs using a Simple Circuit Model

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