The Challenges of Characterizing Wide Bandgap Semiconductors
Wide bandgap semiconductors (WBGs) are seen as the future of power electronics. But the challenges to test and simulate their characteristics are slowing widespread adoption.
When used in power electronic devices, wide bandgap semiconductors (WBGs) like gallium nitride (GaN) and silicon carbide (SiC) offer higher operating temperatures, voltages, and frequencies, according to Keysight Technologies. Compared to their silicon counterparts, they also provide lower power loss due to their low on-resistance.
Types of measurements under considerations for WBG power devices. Image used courtesy of Keysight Technologies
Even with these obvious benefits, WBGs are difficult to characterize, especially when designers are:
- Measuring currents greater than 100 A
- Measuring voltages greater than 3,000 V
- Measuring sub-milliohm on-resistance
It may also be challenging to characterize WBGs with a quantitative GaN current collapse measurement and a junction capacitive measurement at thousands of volts of DC bias.
The Pitfall of Improper WBG Characterization: EMI and Oscillation
We know that WBGs are generally higher performing than traditional silicon—so, why wait to use these devices broadly?
The ability to characterize these devices is crucial to the functionality and robustness of the design. A recent EE Power article on simulating WBG power circuits explains that these devices are capable of higher frequency operation. But even when not used at these higher frequencies, they can cause some catastrophic electromagnetic interference (EMI) behaviors that can kill an entire system.
Depiction of Keysight Technologies' method to model and simulate WBG devices. Image used courtesy of EE Power
The only way to mitigate this properly is to understand the device parameters and design EMI issues out of the circuit. The article states that WBGs are especially susceptible to harmful EMI due to their higher speed capability, which can cause “false turn-on field-effect transistors (FETs).”
Furthermore, EE Power contributor Ryo Takeda states that the parasitic capacitances and inductances in the device can cause oscillations if the device is not properly set up and used. This can be designed out if the values of these parasitics are known. Some parasitic models can ensure the device is connected to a circuit that will not put it in an oscillatory state.
Simulations and Models for WBG Devices
Engineers have begun to find ways to obtain accurate simulations and models for WBG devices. Power Magazine outlines a few tests to obtain characteristic values of the WBG devices.
On-State Characterization
One such test is the on-state characterization test, which has two source measure units (SMUs), one measuring the current going through a WBG device and another pulling that device to the on-state.
The configuration of an SMU for on-state characterization of power devices. Image used courtesy of Power Magazine
If the device were a MOSFET, this would be done by having the first SMU connected to the drain or source to measure the current while the second SMU is connected to the gate, applying a bias voltage that guarantees it is in the on-state. This test measures the on-resistance, which should be minimal.
Off-State Characterization
The same test can characterize the off-state of the device, except the second SMU is applying a bias voltage that will put the device in the off-state. Alternately, the second unit can be omitted completely because the input can be left floating or shorted to ground and turned off in many cases. This test confirms the leakage value, which is integral to the lossiness of the device.
Mathematical Modeling
Takeda explains that designers can also use mathematical equations based on experimental results to create a model of a DC-DC converter made from a WBG. This is a very effective tool while researchers continue to delve into the device physics of WBGs.
It gives design engineers a good idea of how to implement their designs based on parameters derived from experiments and tests instead of modeling the device based on its physics. The mathematical approach allows engineers to develop accurate IV curves and on- and off-state S-parameters for WBG devices.
Test setup for determining the characterization of on-state S-parameters. Image used courtesy of EE Power
The Race to Characterize WBGs
It's likely that WBGs are going to take the power electronics world by storm. The timeline for how quickly WBGs will be adopted depends on our ability to accurately characterize these devices before making them an integral part of future designs.
Universities and semiconductor manufacturers are heavily investing research funds into understanding WBG physics to accelerate their industry-wide adoption. In the meantime, engineers can continue to fill knowledge gaps regarding WBGs by using mathematical modeling and other characterization techniques.
