Gallium Nitride and Silicon Carbide: Compound Materials for Radiation-Hardened Applications
For a long time, silicon-based devices have represented the baseline standard in the semiconductor landscape. Starting from 2007, due to Moore’s Law failure, composed material has been developed with a special focus on wide bandgap semiconductors as they leverage important features which make possible the realization of devices with superior performance when compared to traditional silicon counterparts like power electronics.
The most mature Wide Band Gap (WBG) semiconductor materials already developed are Gallium Nitride and Silicon Carbide. Devices based on these kinds of materials, such as GaN HEMTs and SiC MOSFETs are becoming the solutions of choice to manage high power levels in fast-speed switching equipment and exhibit better performance in many key applications if compared to Silicon power devices like IGBTs and Power MOSFETs.
Since the new WBG semiconductor devices are capable to manage higher power densities than traditional ones, it is possible to obtain significant size reductions with the same level of performance, simplifying thermal management, saving heatsink and related costs: improved breakdown voltage, high electron mobility, and saturation velocity make them the right choice in high-power and high-temperature applications.
Furthermore, higher values of critical electrical field make these compounds very attractive for the realization of power switches with outstanding values of specific Rds-on, i.e. the conduction-state equivalent resistance. Based on these new key factors, a significant reduction of power losses in the on-state can be achieved. At the same time, these kinds of new devices are also capable of reducing switching power losses thanks to the input capacitances lower than silicon power transistors.
Advantages of WBG Devices
The advantages of WBG devices in terms of efficiency and power density are undisputed.
The pursuit of improvements in these two key characteristics drives innovation in several global industrial areas today, such as data centers, renewable energy, consumer electronics, and EV and autonomous vehicles. GaN ad SiC components can provide great power density and more efficiency than their traditional competitors and these improvements result in a large range of benefits for both consumers and companies, whether it be a smaller form factor or faster charging rate in consumer adapters, or cooling costs saving and power waste in data centers.
Even in RF and microwave applications, requirements for higher and higher working frequencies cannot be easily managed with the available Silicon RF power devices.
Due to low breakdown voltage, it is impossible to design and fabricate silicon transistors capable of delivering radio-frequency output powers on the order of a few hundreds or thousands of watts and this issue has severely limited the use of solid-state devices in high-power RF and microwave applications. Recent improvements in the growth of wide bandgap semiconductor materials such as SiC and GaN, allow the production of devices with impressive RF performance, offering the opportunity of developing new designs based on microwave solid-state transistors, which exhibit performance previously achieved only by using microwave vacuum tubes.
Another very interesting advantage of Gallium Nitride and Silicon Carbide-based devices over standard silicon technology is the high level of radiation hardness shown by these two compound materials, which open doors to a large spectrum of applications in the military and space markets. Damages and malfunctions due to ionizing events in harsh radiation environments are tolerated much better by GaN and SiC devices as compared to their Silicon counterparts.
Power Devices: Effects of Radiation
In general, severe requirements for radiation-hardened electronic systems must be satisfied in several applications, so as to guarantee that electronic devices and circuits are able to tolerate damages caused by high levels of ionizing radiation and/or high-energy electromagnetic radiation typical of environments such as the space and high-altitude flights, particle accelerators and nuclear reactors, including nuclear accidents.
Unfortunately, some kinds of semiconductors are susceptible to radiation damage and radiation-hardened components can be realized by appropriate design and manufacturing variations, useful to reduce their susceptibility to radiation-induced damage.
For example, distributed MOSFET structures having enclosed gate layouts equipped with guard rings of annular form; exhibit enhanced resistance to the effect of ionizing radiation. Other examples of solutions are represented by ringed sources or double drain techniques.
The space environment, in particular, can present conditions capable to influence and, in many cases, degrading the characteristics of devices and materials affecting as a consequence the correct operation of important systems.
The main requirement for space-qualified devices is a high level of reliability for long-term operation. In fact, radiation effects produce interruptions, degradations and, in general, discontinuities in device performance.
Electronic devices used for space applications are subject for instance, to damages caused by protons and electrons which are trapped in the Earth’s magnetic field; space radiation flux consists mainly of 85% protons and 15% heavy nuclei; these effects are called Single Event Effects (SEEs). Another important effect due to space radiation is represented by the Total Ionizing Dose (TID).
The difference between the two concepts is quite simple: SEE is the result produced by a single high-energy particle hitting the electronic device, whereas TID is related to the effects produced by prolonged exposure to ionizing radiation.
In the case of electronic devices, TID is typically measured in “rad” (radiation absorbed dose) which is one of the units used to measure the total amount of radiation absorbed by a material, an object or a person; it reflects the total amount of energy deposited in the materials exposed to radioactive sources.
The absorbed radiation dose, expressed in rad, is the amount of energy (coming from whatever kind of ionizing radiation) deposited in any substance like water, air or tissue; a dose of 1 rad corresponds to a total energy of 100 ergs deposited in 100 grams of material. In the International System, the unit of measurement for the total irradiation dose is the Gray (Gy): 1Gy is equivalent to 100rad.
Considering a specific device, its dose radiation threshold represents the minimum level of TID that causes a failure of the device: most commercial devices declared “radiation hardened” are able to withstand up to 5krad before a functional failure occurs.
Radiation-hardened products are generally tested for one or multiple concurrent effects for instance TID ELDRS (Enhanced Low Dose Rate Effects), displacement damage from neutrons and protons.
Regarding single event effects, they play a very important role in environments such as spacecraft and satellites because of the high flux of protons and ions present in the environment where these systems work.
A series of different types of SEEs in electronic circuits can be identified as follows.
SEU (Single Event Upset) represents a change of state, typically in a digital circuit, due to one single ionizing particle (ion, electron, photon, etc.) striking a sensitive point in an electronic device.
SET (Single Event Transient) occurs when an energetic subatomic particle strikes a combinational logic element. The charge deposited by the particle causes a transient voltage disturbance that can propagate to a storage element and is latched, resulting in a Single Event Upset.
SEFI (Single Event Functional Interrupt) is a temporary failure (or interruption of normal operation) on the affected devices caused by a single particle strike.
SEGR (Single Event Gate Rupture) is an event in which a single energetic particle causes a breakdown of the thin gate oxide of MOSFET, creating a conductive path through it. This event is revealed by an increase in gate leakage current and can result in either the degradation or the complete failure of the device.
SEL (Single Event Latch-up) indicates an abnormally high current state in a device and is caused by the passage of a single energetic particle through sensitive regions of the device structure resulting in a device functionality loss. SEL could cause permanent damage to the device and if the device is not permanently damaged, it is mandatory to execute a power cycling (off/on) to restore normal operation. SEL occurs in CMOS structures where an intrinsic parasitic p-n-p-n structure (in practice an SCR) is turned on by the absorption of a single particle and induces the creation of a short circuit between power and ground.
SEB (Single Event Burnout) represents an event in which a single energetic particle strike induces a localized high current state in devices that causes catastrophic failure.
All types of SEE events can cause system performance degradation, possibly to the point of total destruction. In order to ensure a high degree of reliability, it is necessary to select components in which the effects produced by radiation have been measured, recorded and declared.
As described above, electronic devices suffer radiation effects, especially due to electrons and protons and the main reasons for their production are Solar Energetic Particle Events where the Earth’s magnetosphere dips closest to the earth, causing more trapped radiation.
With reference to traditional silicon technology, long-term cumulative ionizing damage coming from protons and electrons is represented by two types of radiation damage for MOSFET devices working in hostile radiation environments: surface effects due to phenomena such as charge trapping at oxide interfaces and bulk damages as a result of ions displacement.
These effects produce a significant worsening of the device performance resulting in threshold voltage shifts (see Figure 1), transconductance degradation (see Figure 2), an increase of leakage current (and related power consumption), and alterations of the dynamic characteristics, reduction of efficiency in switching performance, etc.
Figure 1. Threshold voltage shift due to Irradiation Effects. Image used courtesy of Bodo’s Power Systems
Figure 2. Transconductance degradation due to Irradiation Effects. Image used courtesy of Bodo’s Power Systems
Incident radiation produces hole-electron pairs into the oxides, and as a consequence trapped holes into gate oxide induce threshold voltage shifts, whereas trapped holes into field oxide cause an increase in the leakage current. NMOS transistors result in more vulnerability than PMOS transistors as far as these phenomena are concerned.
As a possible solution to alleviate the above-mentioned issues, metal shielding can be adopted to cover the device.
This arrangement could be of help, but several relevant factors must be considered to assess its effectiveness, for instance, shield geometry to be studied with suitable ad hoc analysis techniques, shield material composition and device composition. Electrons can be effectively attenuated by an aluminum shielding even in case of high energies; aluminum shielding works well also for low-energy protons, but it is ineffective for high-energy protons, greater than 30 MeV.
WBG Advantages in Space and Military Systems
Power devices used for critical applications such as strategic military equipment, space missions and satellite applications, spacecraft, high-altitude flights and drones, data transmission, and robotics must be resistant to failures and malfunctions caused by ionizing radiation.
In these kinds of applications, WBG devices offer significantly higher performance than traditional silicon-based rad-hard devices.
This feature makes feasible the implementation of innovative architectures because size reduction combined with a decrease in weight, together with high efficiency and good reliability are the basic requirements for any device intended for the above-mentioned applications.
Gallium Nitride and Silicon Carbide are able of providing the highest level of efficiency with the smallest footprint available nowadays among all existing devices. They also have excellent performances as far as Electromagnetic Interference (EMI) is concerned, thanks to the reduction of parasitic capacitance values and consequently the decrease of the energy stored and released during the switching cycles. At the same time, the reduced dimensions improve the loop inductance, so that antenna effects are quite attenuated.
Rad-hard Silicon MOSFETs have already reached their limits due to no so recent technology because they have got large die sizes with the Performance Figure of Merit (FOM) being very high if compared to new WBG transistor, especially with enhanced-mode GaN devices.
Related to the previous figure, an important parameter to assess devices performance is the FoM (Figure of Merit) defined as FoM = RDS(ON) x Ciss
Figure 3. Performance cooperation. Image used courtesy of Bodo’s Power Systems
It represents the deviation from the ideality of the device -- the lower its value, the better the system efficiency.
Enhanced-mode GaN HEMTs transistors are quite easy to drive because they require up to 40 times less gate charge if compared to the best rad-hard MOSFETs. This is due to physical dimensions that are favorable for GaN devices: they can be mounted directly on the ceramic substrate avoiding any additional external package.
In this way, it is possible to eliminate any wire bounding and, consequently, the related degeneration inductances disappear, allowing high switching rates, limited only by resistances and capacitances associated with the gate and drain nodes.
As a consequence, the operating frequencies can be increased a lot, thus achieving switching times of the order of nanoseconds. For these high-speed applications, particular attention should be paid to the layout design phase.
Several important players in the field of WBG advanced solutions have developed radiation-hardened, high-performance GaN FET devices suitable for DC-DC converters and, in general, for switching power supply applications in space-borne systems. These components have been characterized against destructive Single Event Effects (SEE) and tested for a high level of Total Ionization Dose (TID).
These 100 V and 200 V GaN FETs with maximum drain current of up to 60 Amps are capable of performing up to 10 orders of magnitude better than silicon MOSFETs, reducing the package size by approximately 50%.
They also reduce power supply size, weight and cost, saving a part of the heat-sink due to lower switching power loss. Furthermore, these devices are the best in class because they show the best FoM with 5mΩ RDS(on) and 14nC gate charges.
For instance, the SGRB series of DC-DC converters from VPT make use of advanced GaN technology and has been expressly designed and qualified for space applications and, in general, harsh radiation environments. The above mentioned series is radiation tolerant, has low noise and exhibits very high levels of efficiency, above 95%, which compares favorably to traditional radiation-hardened silicon products.
A wide family of Adaptor Modules, based on high-reliability GaN devices, is available for multi-function power applications. They exploit eGaN switching HEMTs with high-speed gate driver circuits.
Electronics solutions have become more and more widespread in today’s aerospace environments and applications and all the developers are working hard on an increasing number of systems, such as satellite and spacecraft equipment. High efficiency coupled with good reliability are essential for the success of a space mission project.
Additionally, the capability of wideband semiconductors to operate in high-temperature environments has important implications, because this paves the way for their applications in extreme heat environments, at the same time requiring less cooling for proper operations.
Usually, irradiation tests of electronic devices are performed in accordance with MIL-STD-883E and ESA-SCC 22900 standards.
These standards define and regulate methods and requirements applicable to the steady-state irradiation testing of integrated circuits and discrete semiconductor devices suitable for space applications and describe measurements and procedures for testing microelectronics devices suitable for military and aerospace electronic systems, including basic environmental tests to determine resistance to destructive effects of natural elements and conditions typically in military and space operations.
On the one hand, SiC power MOSFETs demonstrate good tolerance to gamma-rays and neutron irradiation, but at the same time, they have a low tolerance to Single Event Effect phenomena (SEE) induced by high-energy heavy ions. On the other hand, GaN transistors tested with gamma-rays exhibit a remarkable hardness with respect to the total dose and displacement damage.
In more detail, under irradiation with gamma-rays, neutrons and heavy ions in a large range of energies [20MeV÷550MeV], SiC power MOSFETs show good performances in terms of sensitivity to Total Ionizing Dose, whereas they present a quite poor SOA (Safe Operating Area) for what concerns Single Event Effects (SEE); instead, irradiating GaN transistors with gamma-rays, neutrons, and heavy ions and low energy protons, they exhibit a very good SOA toward SEE.
After an irradiation test, gate leakage currents of the device can increase by up to an order of magnitude, with a threshold voltage reduction of up to 1 Volt and a remarkable drop in transconductance value.
According to the previous discussion, wide bandgap devices have become strategically important for the development of next-generation space systems.
While important results have already been demonstrated, a lot of research and development work remains to be done to mature these new technologies and ensure their suitability for use in space applications.
European Space Agency (ESA) has been working for over 10 years to enhance the quality of crystalline materials for the realization of devices with improved reliability and performance.
Further research work activity has been planned to improve material growth processes while optimizing device performance, in order to qualify WBG devices for use in space applications. More additional tasks have been foreseen, for instance, to develop advanced packaging with appropriate solutions.
In particular, ESA based in Estec (The Netherlands) has launched an extensive portfolio of activities specifically related to WBG materials, such as SiC, GaN and Diamond. Their specific purpose is to better understand and optimize the manufacturing process of WBG components, so as to achieve high-reliability operation in space applications.
One of the world’s first demonstrations of a GaN-based X-band telemetry transmitter, installed on board the satellite called “PROBA-V” has been developed in this framework, in order to stimulate the creation of a European supply chain dedicated to WBG component technology.
Further activities are planned with the purpose of demonstrating the superior performance of these new technologies in terms of high operating frequencies, high operating voltages and increased working temperatures, for instance in advanced sensors for photonic applications.
This article is co-authored by Giuseppe Vacca and Cristoforo Marzocca and originally appeared in Bodo’s Power Systems magazine.