GaN Transistors Ready for Launch Into Space

Article By : Maurizio Di Paolo Emilio

GaN power transistors are an ideal choice for power and RF applications to support extreme space missions with exceptionally high electron mobility...

GaN power transistors are an ideal choice for power and RF applications to support extreme space missions. Through its new eGaN solutions, EPC Space guarantees radiation hardness performance and SEE (single-event effects) immunity, with devices that are specifically designed for critical applications in commercial satellite space. These devices have exceptionally high electron mobility and a low-temperature coefficient with very low RDS(on) values.

“EPC Space is a joint venture between VPT and EPC. VPT is a leader in power conversion for avionics, military, space, and industrial applications, while EPC is a leader in GaN-based power conversion technology. EPC Space is the successor to Freebird Semiconductor, established in 2015,”said Bel Lazar CEO at EPC Space.

Modern telecommunications satellites have a structure that is designed to optimize the process of placing them in a proper orbit and to better enable their function. The central part is where most of the electronic equipment, the propulsion system, and the relevant tanks are located.

The electronics onboard the various satellites in orbit around the Earth and in the exploration satellites in the farthest areas, are continuously exposed to gamma rays, neutrons, and heavy ions. The flow of space radiation consists primarily of 85% protons and 15% heavy nuclei. The effects of radiation can lead to degradation, interruptions, and discontinuities in the performance of the device.

This bombardment can damage the semiconductor in multiple ways, including the destruction of the crystal. In particular, it may cause traps in the non-conduction zone, or generate a cloud of electron-holes pairs that unbalance the operation of the device by creating short circuits. In eGaN devices, energetic particles from space cannot generate a momentary short circuit because electron hole pairs cannot be generated.

Space radiation
The charged particles and gamma rays create ionization that can alter the parameters of the device. These changes are estimated in terms of total ionizing dose parameter (TID). The absorbed ionizing dose is commonly measured in Rads that is the absorbed energy of 100 ergs per gram of material. The duration of satellite missions can last for years so that a large TID value can be accumulated. Some deep space missions require 10 mega rads and silicon cannot support them. The rad-hard requirements determine the design of an electronic component from scratch to withstand the effects of radiation.

Figure 1: Cross-section of a typical silicon MOSFET [Source: EPC Space]
Figure 1: Cross-section of a typical silicon MOSFET. (Source: EPC Space)

Figure 1 is a cross-section of a typical silicon MOSFET. It is a vertical device with the source and gate on the top surface and the drain on the bottom surface. The gate is separated from the channel by a layer of silicon dioxide. In a silicon-based MOSFET, the radiation disrupts the electrons on this oxide base by triggering a positive charge in the gate that reduces the voltage threshold until the transistor goes from normally off (or enhancement mode)  to normally on (or depletion mode) states. To achieve an equivalent operation, you will need a negative voltage to turn the MOSFET off.

The single-event effects (SEE) occurring in the space environment due to high-energy radiation are unpredictable and can occur at any time during a spacecraft’s mission. The SEE are comprised of several phenomena;  transient effects (or soft errors) such as single event transient (SET), single event upset (SEU), catastrophic effects such as single event burnout (SEB), single event gate rupture (SEGR) and single event latch-up (SEL). The mechanism underlying every SEE consists of the accumulation of charge in a sensitive area of a device following the passage of the particle.

A single event gate rupture is caused by the energetic atom causing such a high transient electric field across the gate oxide that the gate oxide breaks, as illustrated in figure 2. A single event burnout, or SEB, is caused when the energetic particle transverses the drift region of the device where there are relatively high electric fields.

 

Figure 2: Single event gate rupture (SEGR) in a MOSFET caused by the energetic atom creating a high transient electric field across the gate oxide that, rupturing the gate oxide [Source: EPC Space]
Figure 2: Single event gate rupture (SEGR) in a MOSFET caused by the energetic atom creating a high transient electric field across the gate oxide that, rupturing the gate oxide. (Source: EPC Space)

The energetic particle loses its energy by generating a large number of electron pairs and holes. The latter causes a momentary short-circuit in the device that destroys it. In some cases it can even cause damage to other components, in this case, reference is made to a single event upset, or SEU.

“The thing that happens is it, let’s say, misses the gate and it comes barreling through the other part of the device, the energy of this particle not only causes damage to the crystal, but it also causes a huge cloud of electrons and holes, and they conduct and, in doing so, the device experiences a momentary short circuit. And that’s called single event upset,” said Alex Lidow CEO at EPC.

eGaN Transistor
GaN (eGaN) devices in Enhancement mode are constructed in a different way compared to a silicon MOSFET. All three terminals are located on the top surface. As in a silicon MOSFET, the conduction between source and gate is modulated by polarizing the gate electrode from zero volts to a positive value (5V). The gate is separated from the channel below by a layer of aluminum nitride and gallium. This layer does not accumulate charge when subjected to gamma radiation (Figure 3).

Figure 3: Cross section of a typical enhancement-mode GaN (eGaN®) device [Source: EPC Space]
Figure 3: Cross-section of a typical enhancement-mode GaN (eGaN®) device. (Source: EPC Space)

“GaN inherently is radiation hard from a total dose, this is the accumulation of radiation throughout the lifespan of the device. However, to be able to withstand a single event you have to design them differently than a commercial device,” said Bel Lazar.

“In GaN devices, we have no oxide. So we have no single event, gate rupture. And there are no holes that can conduct well in GaN, so you have no single event upset,” said Alex Lidow.

To demonstrate the performance of the eGaN devices, EPC Space’s 100 V family of eGaN transistors were subjected to 500 kRad of gamma radiation. For the duration of the test, leakage currents from drain to source and gate to source, as well as the threshold voltage and on-resistance of the devices at various checkpoints were measured, confirming that there are no significant changes in device performance.

“For Single Event Effects, we developed a very interesting laser test, where we can actually simulate an energetic particle using a laser that is tightly focused. And we can remove the back surface of the device and shoot the laser through the gallium nitride and see what areas are vulnerable. Knowing the weakest parts of the devices has allowed us to improve our designs,” said Alex Lidow.

Figure 4 shows the primary failure mechanism for eGaN devices under heavy ion bombardment. The conditions are approximately the maximum possible with a beam of 85 LETs of gold atoms on the polarized device.

 

Figure 4: SEE primary failure mechanism for eGaN devices under heavy ion bombardment [Source: EPC Space]
Figure 4: SEE primary failure mechanism for eGaN devices under heavy ion bombardment. (Source: EPC Space)

The vertical axis is the leakage current of the device, while the horizontal axis is the number of heavy ions absorbed by the device per square centimeter. The dashed line shows the gate-to-source current leakage current, and the solid line shows the leakage current drain-source for three eGaN FBG10N30 100V. The current leakage, Ig, does not change during the bombardment, unlike the drain-source leakage, which increases as the bombardment increases.  This increase in drain-source leakage current is the primary failure mode for eGaN devices under heavy ion bombardment, and that is the mechanism that we have greatly improved thanks to the laser testing.

In addition, GaN is superior to neutron radiation because it has a much higher displacement threshold energy if compared to silicon (Figure 5).

Figure 5: Displacement energy compared with the inverse of the lattice constant for various crystals [Source: EPC Space]
Figure 5: Displacement energy compared with the inverse of the lattice constant for various crystals. (Source: EPC Space)

GaN can be used to create semiconductor devices such as diodes and transistors. A power supply designer could choose a GaN transistor instead of silicon for its small form factor and high efficiency. GaN transistors also dissipate less power and offer higher thermal conductivity compared to silicon devices with higher thermal management requirements. The new power devices are also intrinsically radiation-hardened (rad-hard) and provide a theoretical junction temperature operation of up to 600C.

“In space missions, the voltages involved are actually lower than most of the AC line voltages, so 200 volts and sometimes 300-volt devices are the best ones to use. And in that range, GaN is just much higher performance than silicon carbide, so it’s a better choice. Also, going forward, gallium nitride being a lateral device is much easier to integrate. So, we’re already flying integrated circuits in space and that will become better and better over time, with more density improvement from integrated circuits. The other thing is that silicon carbide, if it’s a transistor, It tends to be a MOS transistor. And that oxide is not a native oxide. So it has even bigger problems with total incident dose than a silicon MOSFET,” said Alex Lidow.

The electrical loads in a satellite can vary greatly, depending on the subsystems and functions to be performed. Protection for a satellite power system is essential to prevent failure of the supplied units that could degrade it and even put it out of service.

The key areas where GaN can be used is in both RF and power conversion.

eGaN FETs provide radiation tolerance, fast switching speed, improved efficiency, leading to smaller and lighter power supplies by increasing the frequency to allow smaller inductors and offer good efficiency. eGaN FETs are also smaller than the equivalent MOSFETs.

GaN power transistors are the ideal choice for power conversion applications in space. eGaN devices are more robust than hard rad MOSFETs when exposed to various forms of radiation. The electrical and thermal performances of the GaN have also demonstrated superior operation in a space environment.

Figure 6: photo of SGRB10028S converter from VPT using EPC Space GaN devices And the typical measured efficiency [Source: EPC Space]
Figure 6: Photo of SGRB10028S converter from VPT using EPC Space GaN devices And the typical measured efficiency. (Source: EPC Space)

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