# High temperature characterizations of a monolithic 900 V GaN power device for current source inverter applications

Van-Sang Nguyen, René Escoffier, Anthony Bier, Stéphane Catellani

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Van-Sang Nguyen, René Escoffier, Anthony Bier, Stéphane Catellani. High temperature characteri-
zations of a monolithic 900 V GaN power device for current source inverter applications. The 26th
European Conference on Power Electronics and Applications, GDR SEEDS France & EPE Associa-
tion, Mar 2025, Paris, France. 10.34746/epe2025-0183 . hal-05105997

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# High temperature characterizations of a monolithic 900 V GaN power device for current source inverter applications

Van-Sang NGUYEN¹, René ESCOFFIER², Anthony BIER¹, Stéphane CATELLANI¹

¹Univ. Grenoble Alpes, CEA, LITEN, Campus INES, Le Bourget du Lac, France

²Univ. Grenoble Alpes, CEA, LETI, Grenoble, France

E-Mail: van-sang.nguyen@cea.fr & rene.escoffier@cea.fr

Tel : +33 (0) 479792750 / URL: www.cea.fr

## Acknowledgements

This work has been realized with the participation from members of INES.2S.

## Keywords

«HEMT», «DC-AC», « Gallium Nitride (GaN)», « Current Source Inverter (CSI) », « Monolithic power integration ».

## Abstract

Based on the recent works [1-4] on the current source inverter for photovoltaic (PV) application with Silicon Carbide (SiC) devices, this work presents the design and the characterizations of a monolithic GaN (Gallium Nitride) device where the GaN transistor and the diode in series connection are built up on a single chip for a specific current source inverter topology. This work presents the details of this monolithic 900 V GaN device in the first section, and then the static characterizations by using of an automatic industrial equipment (B1506 from Keysight) with a high temperature thermostream will be shown in the second section. A specific double pulse test-bench is detailed for high temperature

dynamic characterization with the high speed current and voltage probes, this monolithic GaN device can be heated-up locally up to 175°C by using an infrared beam station.

## Introduction

Today wide band gap (WBG) semiconductors such as GaN and SiC made out already the high efficient / compact energy conversion systems design [5-6]. Several conversion topologies have been studied in order to take advantage of these high-speed WBG devices. A current source inverter topology (CSI) is shown Fig. 1, this topology has a natural voltage step-up characteristic which could be interesting for a high efficiency - compact PV inverter or a high speed motor driver where a conventional step-up stage could be swiped out.

As presented in SiC based previous works [1-4], a CSI topology operates at a fixed input current while the voltage is sinus modulated. Thereby, the switching devices in this topology are voltage-bidirectional and current-unidirectional [5-6]. A single MOSFET and a single Schottky diode as the discrete WBG devices can be connected in series to achieve a voltage-bidirectional switch, see Fig. 1.

<figure><img src="image_2.png" /><figcaption>Fig. 1: CSI topology inherited voltage step-up characteristic and its devices [3]</figcaption></figure>
Until now, the semi-conductors manufacturers provide only separated high voltage wide band gap (WBG) transistor (SiC and GaN) and high voltage WBG diode device (SiC only at this moment) with or without the packaging. In this work, a monolithic GaN device combines a GaN transistor and a GaN diode in series connection on a single chip. This GaN monolithic device could increase the efficiency; also decrease the systems volume and the overall cost of the conversion systems.

# Monolithic device

## GaN technology

To achieve a high threshold voltage, a higher maximum Gate to Source voltage and to limit the leakage current, the recessed-gate metal insulator semiconductor (MIS) structure is chosen in our process for the E-mode GaN devices as the cross-section of this HEMT (high electron mobility transistor) in Fig. 2. In this figure, the gate recessed in the 2DEG (two-dimensional electron gas) channel, a distance between the Drain and the Gate has been optimized for withstanding the high voltage of 900 V in this work, and for a low On-state resistance.

<figure><img src="image_3.png" /><figcaption>Fig. 2: Lateral GaN HEMT</figcaption></figure>

On-going works aim to reduce several well-known disadvantage of this manufacturing option such as the high channel On-state resistance, the threshold voltage hysteresis and the gate dielectric quality.

In the same way, the lateral 900 V GaN diode by using the 2DEG channel has been made as its cross-section on Fig. 3. A low forward voltage is critical in this design with an optimized distance between the anode and cathode for a low dynamic resistance. In additional, the total capacitive charge of this lateral diode is less critical than a vertical device.

<figure><img src="image_4.png" /><figcaption>Fig. 3: Lateral GaN Diode</figcaption></figure>

With a similar process for the transistor and the diode, the combination is implemented in order to have a monolithic 900 V GaN device for CSI topology where a transistor is in series connection with a diode in a single die as Fig. 4.

<figure><img src="image_5.png" /><figcaption>Fig. 4: Lateral 900 V GaN device (transistor and diode in series): fingers, schematic and layout</figcaption></figure>

For the applications, this device has been designed with several terminals. The drain of the transistor and the cathode of the diode are the power terminals of the device. A sensing source pad and gate pad are for the control of HEMT. In order to verify the functionality of this monolithic GaN device, the next section presents the high temperature static characterization with a thermostream and high temperature dynamic test-bench with an infrared beam.
# Static characterization

## Device packaging

This 900 V - 25 A GaN device is assembled in a DFN8 packaging as shown in Fig. 5. A high number of the wire-bonding are for the connections of Drain and Cathode, several wires are used to connect the power source.

With these three power terminals and the control signal pads (gate and sensing source), the independent characterizations of the transistor and diode can be implemented.

<figure><img src="image_6.png" /><figcaption>Fig. 5: Wire-bonding of DFN packaging</figcaption></figure>

## B1506 with ThermoStream

The static characterisations in this work are performed by an automatic station B1506A from Keysight®.

<figure><img src="image_7.png" /><figcaption>Fig. 6: High temperature static characterization with B1506A and a thermostream.</figcaption></figure>

This station characterizes the transistors and the diodes up to 3 kV. A thermostream is associated with B1506 for a range of the characterization between -55°C and +200°C.

In this work, the GaN transistor and the GaN diode in the monolithic device are characterized between 25°C and 175°C, up to 900 V.

## Characterization of GaN Transistor

By using this test-bench, in this provisional paper, the threshold voltage, the I/V curves of this GaN device are extracted under different temperatures. At the ambient temperature, Fig. 7 shows the threshold voltage of transistor, the transistor conducts 2 mA when the gate-source voltage

crosses 1.05 V and this value increases to 1.57 V when the current reaches 100mA. At 175°C, the threshold voltage of transistor is 1.26 V for a 2mA of $I_{DS}$. The threshold voltages of this 900 V GaN transistor increase slightly for a higher temperature.

As well known, this value is extremely critical for GaN based high power applications where the transient voltage $dv/dt$ and transient current $di/dt$ are usually very high due to the high voltage applied on the devices. These transient values might be the main reason at the undesired switching event of the transistor. Design of the grid-tie inverter need to take into account carefully this issue due to the high value of DC bus voltage and the potential perturbation from the grid. These static results suggest the correct maximum driving voltage of GaN transistor in order to obtain the best performance of the monolithic device.

<figure><img src="image_8.png" /><figcaption>Fig. 7: Experimental results: threshold voltage of 900 V HEMT inside this monolithic device</figcaption></figure>

Fig. 8 shows the current/voltage curves and the static resistances of this GaN transistor with different driving voltages at 25°C and 175°C. These curves can give the static ON-state resistance of the transistor. At a high temperature (175°C in this case), the On-State resistance is not yet optimized, the ongoing works investigate the impacts of the temperature on the packaging and the wire-bonding to understand these changing values.
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>Fig. 8: Experimental results: current/voltage characterization of 900 V HEMT</figcaption></figure>

The parasitic capacitances of the GaN transistor inside this monolithic device are characterized up to a drain-source voltage 600 V at 25°C and 175°C in Fig. 9.

These results show the slight differences on the input capacitance of the GaN transistor. At 25°C, transistor has 374 pF of Ciss meanwhile this value of Ciss is 392 pF at 175°C.

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 9: Experimental results: parasitic capacitances of 900 V HEMT</figcaption></figure>

## Characterization of GaN Diode

The static characterizations of the GaN Diode inside this monolithic device at the high temperature is presented in Fig 10. The forward voltage of the diode is about 0.6 V with a leakage current of 26 mA at 25°C.

With a similar forward voltage, the leakage current increases to 105 mA at 175°C.

<figure><img src="image_13.png" /></figure>

<figure><img src="image_14.png" /><figcaption>Fig. 10: Experimental results: forward voltage / current of 900 V Diode inside this monolithic device</figcaption></figure>

By using the dynamic characterizations, the experimental results will be shown for the On-state resistance of this diode in different temperatures.

In the next part, the dynamic characterizations of the transistor and the diode of this monolithic GaN device are detailed in the different temperatures between 25°C and 175°C.

## Dynamic characterization

### High temperature set-up

In this section, a high temperature dynamic set-up is depicted in Fig. 11 where a specific profile of the infrared beam is applied on the low-side device - Device 2 (which is the DUT), heated-up from 25°C to 175°C while keeping the auxiliary components close to the ambient temperature as recent works [7-8].
<figure><img src="image_15.png" /><figcaption>Fig. 11: Dynamic test-bench and its schematic</figcaption></figure>

In Fig. 12 (a), a design's 3D view of the dynamic characterization bench is shown where GaN Device 1 (on the high side) and GaN Device 2 (on the low side) are similarly in a DFN8 packaging.

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 12: Dynamic characterization bench: (a) view 3D with circuit's elements, (b) actual set-up</figcaption></figure>

A coaxial Shunt current sensor allows to record the drain-source current. Two isolated optical probes with a high bandwidth are connected nearby the DUT in order to measure the drain-source voltage and the Anode-Cathode voltage. All the measurements are synchronized including the gate-source voltage, which is measured by a

low voltage isolated probe with a bandwidth of 1 GHz as Fig. 12(b). Finally, the turn-on and turn-off energies of this monolithic device could be characterized. Two identical gate driver circuits are implemented vertically for GaN Device 1 and GaN Device 2 as in Fig. 12.

The DUT has been characterized from 25°C to 175°C in this work. Fig. 13 shows the experimental results of the dynamic characterization where the low-side DUT performed under 600 V / 20 A at 175°C. In this figure, the control signals of the characterization bench set-up are shown. The GaN devices are controlled by two signals $V_{GS_HS}$ for the high side device, which is always turn-on and $V_{GS_LS}$ for the DUT as a DPT signal in order to increase the current in the inductive load ($I_{INDUCTANCE}$). Then the Anode-Cathode voltage of the diode, Drain-Source voltage and current of the transistor are measured.

<figure><img src="image_18.png" /><figcaption>Fig. 13: Experimental results at 175°C: DPT main waveforms</figcaption></figure>

The Anode-Cathode voltage and Drain-Source voltage and current are superimposed from 25°C to 175°C in Fig. 14.

<figure><img src="image_19.png" /><figcaption>Fig. 14: Experimental results from 25°C to 175°C</figcaption></figure>

The Anode-Cathode voltages ($V_{A-C}$) are measured by an isolated low-voltage high-speed probe with its maximum range of 2 V. During the turn-on state of GaN transistor, these voltages are
measurable from the forward voltage as the equation:

$$V_{A-C} = V_{F} + R_{AC} \times I_{DS} \quad (1)$$

Where V<sub>A-C</sub> and V<sub>F</sub> are the Anode-Cathode voltage and the forward voltage of the diode. R<sub>AC</sub> is the dynamic resistance of the diode and I<sub>DS</sub> the current through the DUT. After turn-off, the DC voltage is held entirely by the GaN transistor as Fig. 14.

At the low temperatures, the Anode-Cathode voltages are in the measure range of the only available isolated probe. Otherwise, the high temperature Anode-Cathode voltage are clipped but can be estimated by a fitting curve calculation using equation (1).

<figure><img src="image_20.png" /><figcaption>Fig. 15: Double pulses and switching energies</figcaption></figure>

In addition, the measured Drain-Source voltage of the transistor after the first turn off, the GaN transistor shows the mitigation of the current collapse phenomena at the high temperature. From 100°C, the transistor could turn-on properly before the next turn-off. However, only at 175°C, the transistor could remove mostly the current collapse phenomena at the beginning of the turn-

on. For this reason, the turn-on energy of 104.9 µJ and turn-off energy of 39.9 µJ are shown only at the 175°C as Fig. 15. The current collapse is mitigated at the high temperature, however, this phenomena still affects negatively on the turn-on energies of the GaN HEMT as the turn-on energy is much higher than turn-off energy in Fig. 15. Fig. 16 shows the Anode-Cathode voltage of the dynamic characterization at 25°C and 175°C. The original voltage at 175°C is clipped by the range of the isolated probe then corrected. The forward voltage of the diode at 25°C is 0.59 V, which is very closed to the static measurement. At the higher the temperature, the forward voltage of the diode decreases, Fig. 16 shows the forward voltage of 0.54 V at 175°C. Following these results, the dynamic resistance should be more optimized to reduce the conduction losses of the systems.

<figure><img src="image_21.png" /><figcaption>Fig. 16: Dynamic anode-cathode voltage of GaN diode at 25°C and 175°C</figcaption></figure>

From the experimental results, Fig. 17 shows the extracted on-state anode-cathode resistance by using the equation (1) of the diode from 25°C to 175°C.

<figure><img src="image_22.png" /><figcaption>Fig. 17: On-state anode-cathode resistance of GaN diode from 25°C to 175°C</figcaption></figure>

This implementation of the monolithic device is our first step of a full-custom device for CSI applications before others implementations of the GaN based monolithic devices. For the high performance conversion systems with CSI
topology by using these GaN devices, the integrated GaN diode should be improved by decrease the forward voltage and the on-state anode-cathode resistance.

The forward voltage of this Schottky diode could be decrease to 0.2 V by changing the process. For example, the incorporation of fluorite (CaF2) [9] or localized N-type doping can modify the GaN output work and reduce. Moreover, the dynamic resistance of the diode could be optimized to several of tens mΩ by increasing the finger number of the diode and by improving the ohmic contact of the cathode.

Otherwise, a GaN based monolithic bidirectional-transistor with two independent gates and a common source could be a good option for a high performance CSI system if the current collapse could be controlled by a better way.

Conclusion & Perspectives

This work presents a monolithic GaN device where a 900 V transistor and 900 V diode are made on a single die of a recessed MIS gate process. This voltage-bidirectional and current-unidirectional device implements in the CSI topology which inherent the voltage step-up characteristic. With this monolithic device, there is no interconnections between transistor and diode which is a key element for a higher performance. This work then focus on the high temperature static and dynamic characterizations of the device by using two specific experimental benches.

This work also suggested the possibilities of a higher performance device by improving, in particular, the integrated diode.

References

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[2] J. Martin et al. «A high efficiency 5.3kW Current Source Inverter (CSI) prototype using 1.2kV Silicon Carbide (SiC) bi-directional voltage switches in hard switching», PCIM Europe 2016

[3] L-G. Alves Rodrigues et al. "Characterization of 1.7 kV SiC MOSFET Modules for Medium/High Power Current Source Inverter in Photovoltaic Applications" PCIM 2017

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[5] B. Sahan et al. "Comparative evaluation of three-phase current source inverters for grid interfacing of distributed and renewable energy systems" IEEE Transactions on Power Electronics, 2304-2318, 2011

[6] Abu-Khaizaran, M, Birzeit Univ, Bir Zeit, Palmer, P. "Commutation in a high power IGBT based current source inverter" Power Electronics Specialists Conference, 2007. PESC 2007. IEEE

[7] V-S. Nguyen et al. "A High Precision Dynamic Characterization Bench with a Current Collapse Measurement Circuit for GaN HEMT Operating at 175°C" PCIM 2021

[8] V-S. Nguyen, R. Escoffier, S. Catellani, J. Martin "Design, implementation and characterization of an integrated current sensing in GaN HEMT power device by using the current-mirroring technique" EPE ECCE 2022, Sept 2022.

[9] V.Maurya, J.Buckley et al. "Influence of fluorine implantation on the physical and electrical characteristics of GaN-on-GaN vertical Schottky diode" Microelec. Eng. Vol.274, 1 April 2023, p.111975