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This paper reports the design and process flow of a fully integrated yet isolated low-voltage (LV) CMOS with high voltage (HV) lateral power MOSFET on a 6-inch 4H-SiC substrate for the development of HV SiC power ICs. The epi stack (N-epi/P-epi on N+ substrate) for the development of the power ICs was optimized to accommodate and isolate the HV devices and circuits from their LV counterparts. The devices reported in this work were fabricated at 150mm, production grade-Analog Devices Inc. (ADI) Hillview fabrication facility located in San Jose, CA. The HV lateral NMOSFET from this work demonstrated a breakdown voltage (BV) of 620V and a specific on-resistance (Ron,sp) of 9.73 mΩ⸱cm2 at gate-source voltage (Vgs) of 25V. A single gate oxide and ohmic process were used to fabricate the HV NMOS and LV CMOS devices and circuits. Junction isolation was implemented for isolating the HV and the LV blocks for the design of HV Power ICs. Finally, this work executed an HV capable three-metal layered back-end-of-the-line (BEOL) process, an imperative provision for developing reliable and robust power ICs. For future high-temperature applications, the static performances of the devices are characterized and are reported up to 200oC.

IEEE WiPDA 2021

Development of Isolated CMOS and HV MOSFET on an N-epi/P-epi/4H-SiC N+ Substrate for Power IC Applications

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

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.

Best Practices to Quantify Linearity Performance of GaN HEMTs for Power Amplifier Applications

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.

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

In this work, 15 kV 4H-SiC Split-Gate Vertical Planar MOSFETs (SG-MOSFET) are demonstrated for the first time. Both the traditional MOSFET and SG-MOSFET were fabricated on the same 6-inch SiC wafer and achieved maximum breakdown voltages (BV) of 15930 V and 15080 V, respectively. This is the first demonstration of the split gate architecture on SiC for voltage ratings above 10 kV, and this BV was achieved using a Hybrid-Junction Termination Extension (JTE) edge termination. The fabricated SG-MOSFETs showed a reduction in gate charge as well as no significant deviation in forward conduction and blocking behavior when compared to their MOSFET counterparts. These results display the SG-MOSFET’s potential for lower switching power losses in ultra-high voltage devices without compromising device static performance. In the following text, the layout, static performance, and gate charge of the MOSFET and SG-MOSFET are described to show the exciting potential of the high voltage SG-MOSFET.

Demonstration of High Voltage (15kV) Split-Gate 4H-SiC MOSFETs

Several designs of 1.2kV-rated 4H-SiC JBS diode integrated MOSFETs (JBSFETs) have been successfully fabricated in order to increase the 3rd quadrant device performance while having negligible effect on both the forward and blocking modes of operation. Incorporating the Schottky area in an efficient manner to improve overall device performance has been one of the critical paths in the development of 4H-SiC JBSFETs. Device design, fabrication, and electrical performances are discussed in this presentation.

Increased 3rd Quadrant Current Handling Capability of 1.2kV 4H-SiC JBS Diode-Integrated MOSFETs (JBSFETs) with Minimal Impact on the Forward Conduction and Blocking Performances

SuperCascode Power Switches (SCPS) use series-connected Low Voltage (LV) - High Current (HC) devices in a serial string to realize High Voltage (HV) – HC power switches. Cascading of SuperCascodes (CSC) is a modification of the earlier proposed SCPS concept aimed to scale the switching topology to very high (>100 kV) levels or applied to optimize previously reported HV SCPSs to achieve higher switching speed, reduced balancing network size and lower switching losses. However, certain practical challenges create engineering constraints in design which will be discussed and highlighted in this paper. Firstly, for a serial string of devices, scaling the devices adds stray inductances which lead to delays in triggering serial devices, de-synchronization of triggering and unexpected overvoltages that limit scalability. This paper provides an analytical model to enable optimization through simulation and provides experimental data verifying the model.

Analytical Method to Optimize the Cascaded SuperCascode Power Switch Balancing Network

Bidirectional, or four-quadrant switches (FQS) can be designed as back-to-back MOSFETs connected in common-drain (CD) or common-source (CS) topologies. CD-FQS and CS-FQS assembled from discrete 1.2 kV commercially available SiC Power MOSFETs were characterized to obtain capacitance and switching loss values. The CD-FQS exhibited a 1.6x larger turn-off loss compared to the CD-FQS. The CS-FQS exhibited a lower input capacitance, while the CD-FQS exhibited a lower output and reverse transfer capacitance.

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