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The power electronics industry is adopting Gallium Nitride (GaN) devices for use in switching power converters, and engineers need to understand how GaN HEMTs perform in-circuit. Given that dynamic resistance varies with operating conditions, in-circuit characterization provides an invaluable measurement tool. Aerospace has developed a method that works independently of the power stage topology, using inexpensive and widely available measurement hardware. Instead of the traditional clamp, the circuit actively controls the open-loop power-stage and directly senses the on-state voltage. This approach enables measurement in the application’s power stage, which includes the true current-voltage loci and parasitic elements, providing more insight than pulsed IV or double pulse measurements.

2021 IEEE WiPDA 

A Generalized Circuit for Measuring GaN Dynamic Resistance

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

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.

Comparison of the Capacitances and Switching Losses of 1.2 kV Common-Source and Common-Drain Bidirectional Switch Topologies

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.

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

In this paper, a direct bonded GaN intelligent power module (IPM) with liquid cooling is demonstrated. A half bridge module with GaN high electron mobility transistors (HEMTs) is bonded on active metal brazed (AMB) substrates which are pre-brazed on a copper liquid-cooled heat exchanger. The design of this power module, such as the selections of the GaN dies and the placement of the GaN dies with respect to the AMB dimensions, is determined based on thermal simulations, estimated electrical parasitics and experiments. The prototype GaN IPMs are demonstrated in a buck converter configuration. The modules are tested up to 110 W with top surface temperature of the PCB module kept below 125 °C, measured by an infrared camera (FLIR). The proposed IPM with liquid cooling offers an effective packaging solution for electric vehicle applications.

Compact GaN Power Modules with Direct Bonded Liquid-Cooled Heat Exchanger Suitable for EV Applications

This paper presents an approach to measure the online junction temperature of wide bandgap (SiC and GaN) power transistors using quasi turn-on delay as a temperature-sensitive electrical parameter with the possibility of adjustable temperature sensitivity. A double pulse test setup is implemented for both of the power devices and a time-to-digital converter is used for the quasi turn-on delay measurement which requires two timestamps to measure the intermediate time. The switching signal from the function generator is used as the first trigger to start the timer and for the second trigger to stop the timer, the voltage drop across the parasitic inductor between the Kelvin and power source is used. The novelty of the proposed method is to measure quasi turn-on delay independent of other electrical parameters such as DC-link voltage.

Online Junction Temperature Monitoring of Wide Bandgap Power Transistors using Quasi Turn-on Delay as TSEP

Exposure to an extreme ambient temperature leads
to increased losses in a Gallium Nitride Field Effect Transistor
(GaN-FET) when operating in a switch-mode power stage. The
output capacitance of Gallium Nitride (GaN) devices is decreased
at higher voltages but increased at higher temperatures. This
paper highlights the effect of output capacitance in a halfbridge switching stage and offers analysis towards the switching
losses. A power stage was build and tested with two timing
optimizations for room temperature and high temperatures,
respectively. Trading off dead time and the loss mechanisms
involved, a loss reduction of 16.1% was achieved. A loss reduction
was further improved to 26.1% after, thorough investigation of
the switch node transient responses.

A Generalized Circuit for Measuring GaN Dynamic Resistance

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

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