# GaN and SiC Device Characterization by a Dedicated Embedded Measurement System

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## Abstract

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## 1. Introduction

_{DS,on}of the devices. A frequency dependent characterization for R

_{DS,on}in GaN devices is proposed in [6]; it is performed by finite element simulation covering different GaN devices. Results provide evidence that there is a relevant increase in the R

_{DS,on}with frequencies above 1 MHz and the layout influence. The dependence of R

_{DS,on}on drive gate conditions is discussed in [10], where the double-pulse and multi-pulse testing methods are applied under hard switch conditions. The double-pulse method is adopted by [11] under soft switching conditions and by [12] that simplifies the current sensor using the inductor current, minimizing the power loss associated to a resistive shunt sensor. All the above cited papers highlight the importance of the device characterization in order to design a converter based on a suitable measurement system.

_{DS,on}, the threshold voltage an the input capacitance. The measurement error proved to be low, thus assessing the goodness of the method. Experimental results are proposed comparing three products: a 900 V 15 A GaN-Cascode, a 650 V 15 A GaN E-mode and a 900 V 11.5 A SiC MOSFET.

_{on}of WBG devices, and significant obstacles for switching up to a few MHz are highlighted, even though no test on commercial devices is shown. In our test, we were able to verify these assertions by witnessing a decline in cascode performance as switching frequency increased. Instead, the survey [9] suggests settings for some defining factors for commercially available devices, such as the R

_{on}. Our findings are consistent with this, and the tested devices also display a greater breakdown voltage. The issues associated with dynamic R

_{on}measurement are also discussed in [9], which asserts that a temporal dependence is expected and demonstrates R

_{on}’s deterioration. We show corresponding curves versus switching frequency given at environmental temperature. Finally, [14] considers only the dynamic R

_{on}; however, like in our approach, it is measured in operating conditions and varying the temperature as well. In addition, it provides various devices’ rise and fall times, whereas we show the time-domain curve in operating conditions.

## 2. Wide Bandgap Power Switching Devices: Figures of Merits and Losses

#### 2.1. GaN HEMTs Devices

#### 2.1.1. Cascode HEMTs

#### 2.1.2. Enhancement-Mode Devices

#### 2.2. SiC MOSFETs

#### 2.3. Device Losses

_{DS,on}, the input capacitance C

_{iss}and the device threshold voltage variation ΔV

_{th}.

_{DS,on}represents the on-resistance of the switching device, I

_{d}is the on-state current, t

_{on}is the on-state conduction time and T

_{s}the switching period. Therefore, in order to minimize conduction losses, a power switching device with a low R

_{DS,on}must be chosen. Due to the most recent available power device technologies, such as Silicon Carbide and Gallium Nitride, it is possible to lower the on-resistance and achieve higher performance with respect to conventional Silicon device theoretical limit.

_{DS,on}shows a variation with the frequency and its dynamic value is different from the one measured in static condition. In addition, it can be influenced by the layout, requiring a measurement “in situ” to correctly identify the device model.

_{d}and the drain-source voltage ${V}_{ds}$ are different from zero at the same time. Therefore, the mathematical expression for these losses is

_{s}needs a precise calculation that can be retrieved only by an accurate model simulation or directly by measurements.

## 3. Embedded Measurement System

^{®}Cortex

^{®}-M7 32-bit RISC core has been selected due to its embedded high resolution timer (HRTIM) capable of reaching 480 MHz of internal clock frequency. The Cortex

^{®}-M7 core features a floating point unit (FPU) which supports Arm

^{®}double-precision (IEEE 754 compliant) and single-precision data-processing instructions and data types. STM32H743 devices incorporate high-speed embedded memories with a dual-bank Flash memory of up to 2 Mbytes, up to 1 Mbyte of RAM (including 192 Kbytes of TCM RAM, up to 864 Kbytes of user SRAM and 4 Kbytes of backup SRAM), as well as an extensive range of enhanced I/Os and peripherals connected to APB buses, AHB buses, 2 × 32-bit multi-AHB bus matrix and a multi-layer AXI interconnect supporting internal and external memory access.

_{DS,on}, is measured as the ratio between the drain voltage V

_{D}and the drain current I

_{D}. A gate signal with 90% duty cycle switches the device and, during conduction state, an average value of V

_{D}and I

_{D}is calculated to carry out the on-resistance calculation.

_{iss}of the considered switching power device is derived from the selected gate resistor and the equivalent gate time constant related to the gate voltage rising time. This relation can be expressed as

_{G}the external gate resistance. The time constant is calculated as the time interval between the start time of the rising transient of the gate voltage and the time instant when it reaches the 63% of the total voltage span.

#### 3.1. Hardware Implementation

#### 3.2. Firmware Implementation

## 4. Results

_{dd}is 60 V, and the operating current I

_{d}is 0.4 A, while for SiC MOSFET, they are 50 V and 2 A, respectively. This is due to the higher bias current level needed by SiC devices to work in saturation region. For each figure of merit, uncertainty is provided.

_{i}are the measured quantities, u

_{xi}is the absolute uncertainty related to each single quantity, while u is the calculated absolute uncertainty. As an example, the absolute uncertainty of the on-resistance measurement is described below. Since the expression of the resistance is $R=V/I$, the uncertainty is calculated as follows:

_{iss}(measured with Equation (3)) of the three considered devices almost doubles the low frequency value in the considered range of frequencies. This is due to electron trapping phenomenon occurring within the device switching period.

_{iss}, R

_{DS,on}, ΔV

_{th}). The experimentally obtained results show a measurement variance of 2 × 10

^{−15}for the gate equivalent time constant, 1.18 × 10

^{−6}for the R

_{DS,on}, and 0.0031 for the ΔV

_{th}. By comparing these variances with their correspondent KPIs’ mean values, an overall measurement error below 2% has been observed in the worst case. To experimentally evaluate whether the equivalent time sampling technique was a good measurement method, an initial test was performed aiming to answer the following question: how much error will be accumulated if a magnitude related to KPIs is sampled at a fixed frequency (thus removing the ${T}_{ET}$ term from the expression (5))?

^{−4}, while the mean value of the gate voltage samples was 3.53 V, hence experiencing an overall error of the proposed measurement method well below 1‰.

_{DS,on}behavior as a function of the switching frequency is shown. The measured values are referred to the on-resistance at a switching frequency of 10 kHz. This choice is motivated by the difference in the R

_{DS,on}values among the devices under test, allowing for a better visualization of the frequency behavior. In SiC devices, the variation is minimal, while GaN devices show a more consistent increase in the high-frequency region, approximately 25–30% at 1 MHz. R

_{DS,on}reference values at 10 kHz are the following: 408.4 mΩ for SiC, 165.8 mΩ for GaN E-mode and 163 mΩ for GaN Cascode.

_{th}are calculated as the difference between the positive threshold voltage variations during the on-state and the negative ones during the off-state. GaN cascode shows an almost null variation of its threshold voltage as a function of switching frequency. GaN E-mode’s threshold voltage variation increases at high switching frequencies of hundreds of mV. Finally, the highest variation occurs for the SiC device, whose threshold voltage increases over 4 V at increasing switching frequencies [19].

_{ds,on}behavior. The test with 50 pF capacitance shows almost negligible influence on the obtained values of R

_{ds,on}.

## 5. Conclusions

_{th}. During on-state, electrons can tunnel back into the oxide, causing a positive shift of V

_{th}confirming the results of [20]. Since the device is driven with 0–15 V gate voltage, the positive variation is much higher than the negative one, leading to a higher positive ΔV

_{th}.

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

## References

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**Figure 2.**Cascode GaN HEMT made by an enhancement-mode n-channel silicon MOSFET in series with a depletion-mode GaN HEMT.

**Figure 4.**Embedded measurement system board: Embedded MCU, Gate Driver, DUT with gate resistance, Load.

**Figure 5.**Drain sensing circuits: (

**a**) drain current sensing circuit; (

**b**) drain voltage sensing circuit.

**Figure 8.**Measured gate (blue) and gate driver (red) voltage transitions at 100 kHz switching frequency with a 50% duty cycle. Peak-to-peak voltage for gate driver voltage is 10 V.

**Figure 10.**Switching gate voltage at 100 kHz. (

**a**) Oscilloscope measurement. (

**b**) Waveform obtained by the embedded measurement system.

**Figure 11.**Drain voltage FFT at 100 kHz switching frequency: (

**a**) with gate resistance; (

**b**) w/o gate resistance.

Figure of Merit | Uncertainty | Type of Uncertainty |
---|---|---|

C_{iss} | 35 pF (worst case) | Absolute |

R_{DS,on} | 0.022 mΩ | Absolute |

ΔV_{th} | 0.007% | Relative |

Peak Frequency | Peak Amplitude w/ Gate Resistor | Peak Amplitude w/o Gate Resistor |
---|---|---|

100 kHz | 25 V | 26 V |

200 kHz | 8 V | 1 V |

300 kHz | 7 V | 10 V |

400 kHz | 6 V | 1 V |

600 kHz | 4 V | 1 V |

800 kHz | 2 V | 1 V |

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**MDPI and ACS Style**

Vella, A.; Galioto, G.; Vitale, G.; Lullo, G.; Giaconia, G.C.
GaN and SiC Device Characterization by a Dedicated Embedded Measurement System. *Electronics* **2023**, *12*, 1555.
https://doi.org/10.3390/electronics12071555

**AMA Style**

Vella A, Galioto G, Vitale G, Lullo G, Giaconia GC.
GaN and SiC Device Characterization by a Dedicated Embedded Measurement System. *Electronics*. 2023; 12(7):1555.
https://doi.org/10.3390/electronics12071555

**Chicago/Turabian Style**

Vella, Alberto, Giuseppe Galioto, Gianpaolo Vitale, Giuseppe Lullo, and Giuseppe Costantino Giaconia.
2023. "GaN and SiC Device Characterization by a Dedicated Embedded Measurement System" *Electronics* 12, no. 7: 1555.
https://doi.org/10.3390/electronics12071555