Gallium Nitride: Power semiconductor switches for future EVs.
Electric vehicles are fast replacing conventional vehicles. The range and endurance have also increased dramatically for EVs, thanks to better technologies which lead to higher energy density for batteries as well as converters and motor controls. Innovative technologies are still being developed for more robust and efficient on-board converters.
Switching Devices play a crucial role when it comes to increasing efficiency of DC/DC converters, inverters as well as Motor control drives. The majority of losses takes place within the power semiconductor switch in a converter. Wide bandgap semiconductors-based power switches have already started replacing IGBTs due to their high frequency of operation, robustness and higher operating temperatures. Out of these wide bandgap semiconductors, Gallium Nitride (GaN) stands out due to its unique and robust characteristics.
Gallium Nitride: Comparison from a material perspective and how it translates to system level.
Table 1 Device properties for Si, SiC and GaN
As shown in Table 1, device properties 𝐸𝐺 (Band gap energy in eV), 𝐸𝐶 (Critical electric field strength in MV/cm), 𝑉𝑠𝑎𝑡 (Saturation velocity in cm/s), 𝜀𝑠 (dielectric constant) and 𝜇𝑛 (Mobility in cm2/s) are mentioned for three materials.
As observed from Table 1, The higher bandgap and higher electrified strength directly impact the Drain-source on resistance 𝑅𝐷𝑆_𝑂𝑁 of the switch.
From the equation it is evident that increase in 𝐸𝐶 leads to decrease in 𝑅𝐷𝑆_𝑂𝑁 of the switch. This translates to lower losses when the device is conducting.
Here, 𝑊𝐷 is the width of drift region. Due to higher 𝐸𝐶, smaller device can be modelled but heat dissipation becomes a challenge in this case because the width of the drift region decreases.
Also, a smaller device area and lower ε𝑠 leads to a decrease in value of parasitic capacitance. To conclude:
At a system level the above parameters translate to a greater power density, lower switching loss, faster switching (In range of GHz), higher breakdown voltage and higher operating temperature. Due to the higher band gap of GaN, the performance at higher temperature has negligible effect which reduces the cooling requirement by significantly reducing the overall weight in an EV.
Challenges associated with GaN based devices
Since GaN is a new material, there are many challenges which need to be addressed at Material as well as System level.
When it comes to RF applications, GaN based devices has already made in-roads to commercial markets but very few options are available for high power switching devices with high breakdown voltage that can fulfil the need of an EV application. GaN based high breakdown voltage devices are needed not only for EV cars but for heavy duty vehicles such as EV buses and trucks. High breakdown voltage can solve challenges faced with charging time in an EV and can help in increasing the range of EVs. Devices in range of 1200V to 1600V can address this issue.
SweGaN, a spin-off from Linkoping University, Sweden is one such company which works on development of GaN based wafers and is working in this direction with its industry partners to make such high break down voltage devices possible. They are working on technologies with industry partners to make devices available for OEMs in the range of 1200V to 1600V.
One of the Academic Partners of SweGaN, Chalmers University of Technology (Sweden) has been able to demonstrate GaN HEMT (High Electron Mobility Transistor) based on SweGaN’s technology by achieving a breakdown voltage of 1622V. Not only were they able to achieve a high breakdown voltage but also showcased very low specific on-resistance of 3.61mΩꞏcm2.
This demonstration by Chalmers University perfectly demonstrated the potential for Power devices based on SweGaN’s material.
Right now, the issue with the GaN based devices is that they are Normally-ON i.e., they operate in Depletion mode. This poses a design challenge at the system level as many modifications need to be done in the converter circuitry. One way to overcome this is by using a Cascade configuration like the one being used for Silicon Carbide power switches. This ensures that few modifications are needed in the converter circuitry. As shown in figure 1 below, a normally ON High voltage GaN JFET is being controlled by a low voltage Silicon MOSFET. The combination package works as a normally OFF device. SweGaN is also working to demonstrate this capability with devices made from its wafers in collaboration with industrial and academic partners.
Gallium Nitride based devices are still in the development phase, compared to other technologies they have huge potential when it comes to EV and hybrid vehicles.
Converters can be miniaturized by using GaN based switches.
SweGaN, along with its industry and academia partners is working in the direction to address the above-mentioned challenges. Once these challenges are addressed, GaN based devices based on SweGaN’s wafers have a huge potential in the EV market and can shift the technology of on-board converters and Motor drive technology in a completely new direction.
Vatsal Sonikbhai Shah, Device Testing Engineer SweGaN