Radiation Robustness of Gallium Nitride Power Devices

Gallium nitride (GaN) offers better characteristics than silicon (Si) and silicon carbide (SiC), such as a bigger bandgap and a higher critical electric field. Even though GaN devices are being used in many different applications, there are still unanswered concerns about their robustness, stability, and reliability.

Is gallium nitride completely robust against radiation?

For power devices, robustness against radiation and extreme temperatures has become a more important requirement. Apart from their traditional uses, GaN power devices are considered excellent options for space, aerospace, and military applications. In most cases, these applications call for equipment that can withstand harsh temperatures and radiation.

GaN is naturally radiation-hardened because of its high ionization threshold energy and displacement threshold energy. However, the radiation hardness of GaN devices is dependent on the device design and material quality.

GaN HEMTs are lateral transistors that have high electron mobility. Radiation damage has different effects on these transistors than it does on Si and SiC FETs.

Three categories into which radiation effects can be divided are

● Total ionizing dose (TID)

● Displacement damage

● Single-event effects (SEEs)

Fig. 1(a) summarizes these effects and their relationship to local physical processes in GaN HEMTs.

Fig. 1: (a) Radiation types and strike location; (b) Compilation of VSEB values found for E-mode HEMTs. Source: IEEE Transactions on Power Electronics

● The deposition of energy by ionizing radiation due to either photons or ions is what causes TID effects.

● Displacement damage effects result from the displacement of atoms in the irradiated device due to energy transfer.

● SEEs are the result of the device's brief ionization response to a radiation particle striking it suddenly.

TID and displacement damage both cause changes in the device that depend on the overall dose of radiation absorbed.

While SEEs are transient events that may cause instantaneous device degradation or failure,. They have the potential to cause irreversible hard-switching faults in converter circuits, and SEEs in particular are a major source of concern for power devices.

Total Ionizing Dose (TID)

P-gate GaN HEMTs are anticipated to be generally resistant to TID effects because they do not contain gate oxide. The TID effects in GaN HEMTs are bias-dependent, according to a recent study, which is also noted in SP-HEMTs and HD-GITs.

When devices rated at 80 V and 200 V are compared, the higher-voltage device exhibits a greater threshold voltage shift. Radiation research on composite GaN devices has not been extensively published, but because these devices employ a Si mosfet with gate oxide, they should be more vulnerable to TID effects.

Further research is required to understand the effects of TID exposure on GaN devices in switching conditions where self-heating occurs. Annealing has been found to reverse some of these effects in GaN HEMTs.

Studies on how TID affects the dynamic properties of GaN HEMTs are rarely conducted.

● When exposed to 300 kRad of Co-60 irradiation (1 MeV X-rays), HD-GITs do not show much of a change in their dynamic ON resistance.

● Moreover, SP-HEMTs exhibit negligible alterations following 300 kRad of 10 keV X-ray TID exposure.

Displacement Damage

In GaN HEMTs, displacement damage effects are mostly dependent on the irradiating particle's energy, mass, and charge. The amount of nonionizing energy lost when the radiation particle hits the material of the device can be used to measure these effects.

A range of HD-GITs and SP-HEMTs are tested under bias conditions, subjected to proton and neutron irradiation up to a maximum total fluence of 6×10¹⁵cm^-2

● When devices are unbiased, they can function up to maximum fluence

● When devices are biased by significant off-state biases, failures are noted.

Similar outcomes are observed where SP-HEMTs are subjected to 1 × 10¹² to 1 × 10¹⁵ cm^-2of total fast neutron fluence.

● As fluence increases, gate-leakage current is seen to decrease

● While threshold voltage and dynamic ON resistance do not alter in response to exposure,

The other way around, a research-grade SP-HEMT was studied after being exposed to a 5 MeV proton dosage of 2 ×10¹⁵ cm^-2. The results showed

● Fivefold rise in dynamic ON resistance

● A negative threshold voltage shift.

The formation of defects in the p-GaN gate is the reason for these outcomes.

Remarkably, when high-energy protons are irradiated into HD-GITs, up to a fluence of 2 ×10¹⁴ cm^-2

● A slight positive threshold voltage shift is observed.

To fully understand how the HD-GIT and SP-HEMT designs react to radiation displacement damage, more research needs to be done.

Single-Event Effects (SEE)

SEEs are brief occurrences brought on by a high-energy radiation particle colliding with a biased device construction. When devices are biased, a significant track of electron-hole pairs can form, which can cause instantaneous failure or device deterioration.

Although SEE can be significant in high-energy neutron and proton attacks, heavy ion strikes from high-energy cosmic rays are the main source of SEE in devices that are in operation.

The linear energy transfer (LET) of the ion and the range of ion within the structure are the main parameters used to estimate the possible damage from a heavy ion attack.

● LET is linked to the magnitude of charge creation and transfer.

● The range of the ion will dictate the portions of the device that are vulnerable to SEE events.

Both bias conditions and device structure have an impact on a device's sensitivity to SEE.

SEE Types

In power devices, three SEE categories are significant:

● Single-event burnout (SEB)

● Single-event transients (SETs)

● Single-event gate rupture (SEGR)

Most of the work tests how well the devices work against SEE effects using MIL-STD-750 Method 1080. This involves setting the devices to an off-state bias and watching the drain current while they are irradiated until a failure transient is seen.

A number of 40 V, 100 V, and 200 V-rated SP-HEMTs are tested at 276 MeV at the rated voltage while exposed to I¹²⁷ irradiation. SEB is seen in devices with higher voltage ratings that have cumulative damage from strikes.

SETs can cause more off-state drain and gate leakage, which are two more ways that devices can break down when HEMTs are exposed to heavy ions.

SEGR is not anticipated to be significant because most GaN HEMT devices lack gate oxide.

Summarizing the Key Points

● Gallium nitride power devices offer superior characteristics, such as a higher critical electric field and bigger bandgap, making them a promising option for power electronics.

● Gallium nitride is naturally radiation-hardened due to its high ionization threshold energy and displacement threshold energy.

● The radiation robustness of gallium nitride devices is dependent on their design and material quality.

● Total ionizing dose, displacement damage, and single-event effects are the three categories into which radiation effects can be divided.

● Gallium nitride HEMTs are anticipated to be generally resistant to total ionizing dose effects because they do not contain gate oxide.

● Single-event effects have the potential to cause irreversible hard-switching faults in converter circuits and are a major source of concern for power devices.

● The article highlights the need for continued research in this critical area to ensure the robustness, stability, and reliability of gallium nitride devices in extreme conditions such as radiation and high temperatures.

Reference

Kozak, Joseph Peter, Ruizhe Zhang, Matthew Porter, Qihao Song, Jingcun Liu, Bixuan Wang, Rudy Wang, Wataru Saito, and Yuhao Zhang. “Stability, Reliability, and Robustness of GaN Power Devices: A Review.” IEEE Transactions on Power Electronics 38, no. 7 (July 2023): 8442–71. https://doi.org/10.1109/tpel.2023.3266365.