Automotive and Aerospace Wide-Bandgap Device Testing

Wide-bandgap (WBG) devices such as silicon carbide (SiC) and gallium nitride (GaN) are enabling high efficiency in traction-inverter and on-board charger designs, allowing electric vehicles to travel farther and significantly reducing charging times.

WBG power devices are also a good candidate for aerospace electrical power distribution systems, particularly those targeting More Electric Aircraft (MEA) architectures. Compared with traditional, silicon-based power devices such as IGBTs, they offer higher power density, lower switching losses, and higher temperature tolerance.

WBG considerations

SiC and GaN devices operate with high voltage and current slew rates (dv/dt often >100 V/ns and di/dt at ~10 A/ns in fast-switching designs), high-frequency switching, and high junction temperatures. High dv/dt results in parasitic capacitive coupling, high common-mode noise, severe EMI, and high di/dt that results in ringing, voltage spikes, and stress on the device.

For qualification of a WBG device for an automotive traction inverter or MEA actuator drive, test methodologies are needed that consider the operation and failure modes of these devices. This approach is not normally inherited from the practices used with IGBTs and other silicon-based power devices.

This article reviews the specialized test frameworks and practices that are widely used to ensure the reliability and performance of WBG devices in these demanding applications.

Static characterization

The qualification of WBG power devices must evaluate static and dynamic characteristics due to their physical properties (Figure 1). For example, if we used only static test data, we would miss critical high-frequency behaviors that result in device failure or significant efficiency loss.

Static characterization, including DC/pulse testing and curve tracing, measures key device parameters when the device is on, off, or in a steady state of conduction. Dynamic characterization typically uses double-pulse testing (DPT) to evaluate the device’s transition between states under high-speed switching conditions. This includes characterization of transient losses, parasitics, gate drive instabilities, and dynamic degradation mechanisms.

The DC or static characterization deals with parameters such as on-resistance (R_DS(on)), threshold voltage (V_th), and leakage currents as a function of temperature. The R_DS(on) measurements are required to be performed over the entire operating temperature range for both SiC and GaN devices.

In SiC MOSFETs, this parameter increases with temperature, as phonon scattering degrades carrier mobility. So if a designer uses an inverter rated with the R_DS(on) at a 25°C value from the datasheet (for example, 30 mΩ) but the device is running at 150°C in the vehicle, the actual resistance increases to 50 mΩ.

It follows that:

• The conduction losses (I²R) will be more than 60% higher than expected.
• The system will be much hotter than estimated during early simulation.

In contrast, GaN HEMTs suffer from charge trapping. This phenomenon has been observed as a transient increase in R_DS(on)during high-voltage off-state stress prior to turn-on from a high-voltage off state (V_DS). This effect is sometimes called current collapse. Although GaN manufacturers are using different methods to reduce dynamic R_DS(on), it is still a parameter to be assessed for GaN device qualification.

Dynamic characterization

DPT is the common method used for the evaluation of switching energy, turn-on and turn-off times, and dv/dt and di/dt slew rates (Figure 2). For WBG devices, DPT has a more complicated setup than for silicon devices. The high slew rates of SiC and GaN devices make parasitic inductance in the power loop, which can be ignored at silicon switching speeds, a source of voltage overshoot and ringing.tic inductance in the power loop, which can be ignored at silicon switching speeds, a source of voltage overshoot and ringing.

Special clamping circuits are needed to measure dynamic R_DS(on). Fast-clamping circuits use high-voltage diode arrays or active switch networks to bypass high off-state voltages, clamping the scope input to a low reference (e.g., 5 V) while still measuring a high-bandwidth V_DS(on) within nanoseconds of gate turn-on.

Also, engineers should characterize switching energy over a matrix of bus voltage, load current, and temperature, as WBG switching loss does not scale as predictably with these variables as silicon does, especially with GaN devices, where dynamic on-resistance and trapping effects shift behavior pulse to pulse.

MADTHOR (Figure 3) is an advanced T&M system for power transistors developed by MinDCet, a semiconductor and microelectronics company from Leuven, Belgium. MADTHOR combines dynamic and static testing in one unit. It can characterize WBG semiconductors and traditional silicon MOSFETs and IGBTs at high-voltage ratings.

The characterization of dynamic parameters requires specialized sensing technology:

• Voltage probe: Conventional passive voltage probes are inappropriate due to excessive ground-lead inductance as well as introducing false ringing. Differential probes offer galvanic isolation but often do not provide the high common-mode rejection ratio that high-frequency power devices require.
  High-bandwidth, optically isolated probes (e.g., Tektronix’s IsoVu) are a good choice for measuring high-side V_GS and V_DS signals without corruption from common-mode voltage transients (see Figure 4).

• Current sensing: For accurate current measurement in fast-switching GaN and SiC devices, sensors must handle high switching rates and high dv/dt without adding parasitic inductance. The coaxial shunt resistors offer a high wideband response from DC to high frequencies but result in insertion losses.
  Rogowski coils, which feature a high bandwidth, provide excellent isolation and transient tracking; however, they cannot measure direct current. Alternatively, modern chips integrate galvanic isolation and compact protection directly into the power stages to minimize loop layout parasitics.
  The Tektronix TICP Series, a family of isolated current probes, is specially designed for WBG measurements. IsoVu technology powers them, and they offer galvanic isolation, high common-mode rejection ratio, and low insertion inductance, making them ideal for measuring fast-switching SiC and GaN devices on high-side floating shunts.
• Time-skew calibration: Large errors in E_on and E_off are due to a small propagation delay offset (skew) between voltage and current channels. Accurate deskewing with special calibration fixtures is required before high-speed dynamic measurements can be made.

Short-circuit and fault-tolerance test

One of the main differentiators between silicon and WBG devices is the short-circuit withstand time. It is also one of the most safety-critical parameters in automotive and aerospace applications in which a fault must be detected and cleared before destroying the device.

Silicon IGBTs can normally withstand short-circuit conditions for about 10 μs. SiC MOSFETs are normally rated for only 1–3 μs before thermal runaway occurs. Some GaN devices offer an even smaller margin. The short-circuit withstand time should be characterized over the full temperature and voltage range, and the test procedures should confirm that the intended protection scheme trips the device within the safe window of the device with margin. It should also be confirmed that repetitive short-circuit exposure (as required by some automotive specifications) does not degrade the device cumulatively.

Specialized high-energy, low-inductance test rigs capable of performing two key fault modes are required for the evaluation of WBG short-circuit ruggedness:

• Hard-switching fault: The power device is turned on directly into an existing short-circuit. I_D rises rapidly while the device is under full DC-link voltage.
• Fault under load: A short-circuit happens when the power device is already carrying nominal load current, causing severe dynamic thermal stress in the semiconductor lattice.

Testing includes unclamped-inductive-switching testing, avalanche energy capability, and surge-current rating checks for fault tolerance. Automotive load-dump events and aerospace bus transients can drive devices into conditions far beyond normal operation.

Power cycling and thermal characterization

WBG devices commonly operate at higher power densities and ambient temperatures than silicon-based devices, so thermal impedance characterization through transient thermal testing is not optional; it is imperative.

The structure function analysis from controlled power step cooling or heating curves allows engineers to deconvolve die, solder, substrate, and baseplate contributions and detect die attach or substrate delamination not otherwise detected until failure.

The power-cycling test measures the fatigue life of die attach, bond wires, and package interconnects by cycling the device through specified junction temperature swings (ΔT_j).

Automotive qualification (e.g., AQG 324) requires both active power cycling (self-heating by the device) and passive thermal cycling (external chamber cycling) with defined ΔT_j profiles and cycle counts, which are related to the expectations of the mission profile.

Aerospace programs will often add extreme cold-soak conditions, as equipment bays can reach well below –40°C at altitude before power-up. This can be a regime where wire-bond and solder-joint behavior differs significantly from the automotive qualification envelope.

Stress test of gate oxide and long-term reliability

Historically, the gate oxide interface of SiC MOSFETs has exhibited higher trap densities than the mature silicon processes. High-temperature gate-bias and high-temperature reverse-bias testing, run per JEDEC JESD22 methods and typically for 1,000 hours or more, screen for gate oxide degradation and V_th instability under sustained electrical and thermal stress.

The V_th drift and gate oxide lifetime are quantified under accelerated stress and extrapolated using bias temperature instability (BTI) and time-dependent dielectric breakdown (TDDB) testing. Both are relevant for devices with a true gate oxide, i.e., SiC MOSFETs and GaN MIS-HEMTs.

Most commercial GaN power devices are Schottky-gate and p-GaN-gate HEMTs, which do not have a similar oxide interface. Here, gate reliability is determined by Schottky-junction leakage and degradation of the p-GaN/metal interface under gate stress, characterized by high-temperature gate stress and gate-leakage drift testing rather than classical BTI/TDDB.

Application-specific tests

Automotive programs add the AEC-Q101 discrete semiconductor qualification, or the AQG 324 guideline at the module level, to the device-physics testing. These frameworks are specified with specific stress test groupings, minimum sample sizes, and pass/fail criteria for temperature cycling, humidity, mechanical shock and vibration, and electrostatic discharge based on automotive-specific human body model and charged device model limits.

ISO 26262 adds another layer of functional safety considerations. The test data should support the power stage failure mode, effects, and diagnostic analysis. That means describing switching and fault behavior with sufficient statistical rigor to yield credible failure-rate and diagnostic-coverage numbers, not just pass/fail qualification results.

Stressors that affect aerospace power systems are not usually emphasized in automotive testing. Partial discharge and corona testing are particularly important at altitude (e.g., flight levels above 40,000 feet), where the reduced atmospheric pressure leads to a lower dielectric breakdown voltage of the air gaps, as described by Paschen’s Law, and can cause corona inception at voltages that would be safe at sea level.

This is particularly important for high-voltage MEA buses. Testing requires altitude-vacuum chambers for the environment, integrated with dynamic power-cycling circuits, to measure dielectric breakdown and thermal performance over a range of pressures.

For avionics or space-adjacent applications, radiation and single-event-effect testing is necessary for devices, including single-event burnout, single-event gate rupture, and total ionizing dose. WBG devices have high electric fields; therefore, the failure modes induced by radiation are different than silicon.

Vibration and shock testing follows DO-160 environmental categories rather than automotive standards. Because the required temperature range is so extreme, often –55°C to 150°C or wider, package and interconnect qualification must go well beyond what most automotive profiles call for.°C to +150°C or wider, package and interconnect qualification has to go well beyond what most automotive profiles call for.

Cover image: Adobe Stock

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