16 May 2020
NexGen Power Systems Inc. is fabricating vertical power devices (vertical GaN) using homoepitaxial GaN on GaN substrates. Vertical GaN devices are capable of switching at even higher frequencies and operating at higher voltages, which should lead to a new generation of more efficient power devices.
“Vertical GaN devices are 90% smaller than silicon. Capacitance is directly related to the area of the device,” said Dinesh Ramanathan, CEO & co-founder at NexGen Power Systems. “The smaller the device, the lower the capacitance. The lower the capacitance, the greater the switching frequency. Vertical GaN delivers 67% lower switching losses than Si MOSFET in most typical applications, especially power supplies.”
Gallium nitride (GaN) is a high bandgap material that allows devices to operate at higher temperatures and withstand higher voltages compared to silicon. Moreover, GaN’s higher dielectric breakdown allows building thinner and therefore, lower resistance devices. Lower characteristic RDS(on) leads to smaller devices with lower capacitance.
The advantage of growing low defect-density epitaxial layers on low defect-density bulk GaN substrates is that it leads to vertical power devices with higher reliability under-voltage and thermal stress, compared to lateral GaN devices manufactured on non-GaN substrates.
Vertical GaN is capable of operating at high breakdown voltage (Figure 1) which enables vertical GaN to power the most demanding applications, such as power supplies for data center servers, electric vehicles, solar inverters, motors, and high-speed trains.
Traditional power devices and lateral GaN-on-Si
Power electronics use solid state devices to process or convert electrical power. Power converters or adapters are ubiquitous and are available in all shapes and sizes. Most converters called switched mode power supply (SMPS) use capacitors, inductors, transformers, and semiconductor switches to transfer power from an input with a given voltage and current to output at a different voltage/current configuration (Figure 2).
Capacitors, inductors, and transformers are passive and physically large components. To reduce the size of the SMPS, they must operate at high frequencies. To operate at high frequencies, they need a better semiconductor switch which can overcome the limitations of incumbent silicon-based switches which generally top out at a couple of hundred kHz.
Over the past three decades, silicon devices such as MOSFETs and IGBTs have dominated the power device market. Recently, silicon MOSFETs have seen only incremental performance gains. “Silicon has reached its limits. There’s nothing that you can fundamentally gain from silicon power devices right now, based on its material properties,” said Dinesh Ramanathan.
Silicon carbide (SiC) is another alternative to silicon, but a GaN generally has more attractive fundamental material properties.
Current GaN devices are made on hybrid substrates: thin layers of GaN on silicon or silicon carbide creating GaN-on-Si or GaN-on-SiC HEMT (high electron mobility transistor) structures (Figure 3).
Lateral GaN-on-Si (or GaN-on-SiC) devices combine materials with a mismatched coefficient of thermal expansion (CTE) which compromises both reliability and performance. Moreover, in a typical GaN HEMT device, the channel is very close to the surface (in the order of a few hundred nanometers) which creates passivation and cooling problems. In a lateral GaN-on-Si device, the drain-source separation determines the breakdown voltage of the device. A larger drain-source separation increases the channel resistance and limits the current capability. To compensate for this and increase current carrying capability, the device must be made wider. The combination of higher voltage and higher current requirements results in devices with a large area and therefore higher capacitances. Hence, lateral devices are limited to a breakdown voltage of approximately 650 V.
Avalanche breakdown is a key property of Si and SiC devices to protect themselves under short term overvoltage conditions. The absence of p-n junctions in lateral GaN-on-Si HEMTs prevents these devices from avalanche breakdown. Further, GaN-on-Si HEMTs are difficult to cool from the top due to the sensitivity of current conduction close to the device surface. The buffer layers separating the Si substrate from the GaN layer limits the efficiency of bottom side cooling. This means that often custom packages have to be created to cool GaN-on-Si HEMTs, further increasing their costs.
Vertical GaN power devices
Where the lattice mismatch between GaN and Si or SiC degrades GaN’s electrical properties and affects reliability, when GaN devices are grown on GaN substrates, both the lattice and the CTE are of course perfectly matched – it is the same material. As a result, very thick layers of GaN can be epitaxially grown on bulk GaN substrate, which allows the creation of very high-voltage devices.
Vertical GaN technology unlocks the full potential of GaN’s superior material properties as it is based on GaN grown homo-epitaxially on GaN substrates (Figure 4). Further, vertical GaN devices use all three spatial dimensions: higher breakdown voltage by increasing the thickness of the drift layer and low RDS(on)/current capability by increasing device area, effectively creating a 3-D device which decouples breakdown voltage and current capability (RDS(on)).
“AC systems require high-performance Power Factor Correction circuits with a significant reduction of harmonic distortion. Vertical GaN’s high switching frequency enables new control algorithms and delivers all of this with smaller implementations and higher efficiency”, said Ramanathan.
Figure 4: Vertical GaN vs. GaN-on-Si Device Construction [Image: NextGen Power Systems]
Figure 5 shows the diagram of the enhancement mode vertical GaN junction field-effect transistor (eJFET) and GaN-on-Si high electron mobility transistor (HEMT). NexGen Power Systems said that it was able to demonstrate a drift thickness of >40µm producing diodes with a breakdown voltage >4000V and transistors with specific resistance of 2.8mΩ.cm2. For the same current capability, the vertical GaN device size is approximately six times smaller than a 650V GaN-on-Si HEMT but delivers a much larger breakdown voltage of 1200V. The vertical GaN eJFET has avalanche capability that protects the device when the specified breakdown voltage is exceeded.
Vertical GaN devices are built to conduct current through the drift layer, which is inside the bulk of the transistor. Therefore, there is no mechanism for dynamic RDS(on) variation which is created by charges trapped due to surface interface impurities. The extension of the depletion region of the gate-source diode into the channel controls the current flow between drain and source. In situations when the breakdown voltage is exceeded avalanche initially occurs through the reverse polarized gate-source diode, subsequently causing the avalanche current to increase the gate-source voltage and the channel to open and conduct.
Due to the small output capacitance, switching losses in applications are very small. In contrast to lateral GaN devices, heat is optimally transferred through a homogeneous material — without additional layers — directly to the package lead frame (Figure 6) from the top and bottom of the device.
“The advantage of this device is the fact that it has just p-n junctions made from GaN. We don’t have 2D electron gases and complicated layers of materials. We have an enhancement mode JFET which is a well-understood device, and because it has p-n junctions, it avalanches, so you don’t have a destructive breakdown. Because it all happens in the bulk of the device, it can absorb a fair bit of energy during an avalanche, and after the event, the device recovers and operate like normal. So, it has a built-in safety mechanism. And therefore, it’s a much more reliable and a much more robust device”, said Ramanathan.
Using vertical GaN in power circuits
The NexGen vertical GaN FET is a junction field-effect transistor (JFET) with similarities to FinFETs used in silicon logic devices.
The voltage difference between gate and source (VGS) controls the current between the drain and the source. When VGS is below the threshold voltage (Vt), the JFET channel is closed. When VGS is larger than Vt, the channel opens, and the current can flow between the source and the drain. This current flows within the bulk of the device. Electron mobility is high and together with the smaller capacitance of the p-n junctions, creates a device with very small output (Coss) capacitance. This allows the devices to operate efficiently at high frequencies and enables applications with switching frequencies above 1MHz.
The symmetric construction of the JFET allows the source and drain to exchange function if the drain terminal voltage drops below the source terminal voltage, channel current can then flow in a reverse direction. This resembles the function of a body diode in Si MOSFETS but without the losses and potential reliability issues caused by minority carrier/reverse recovery charge removal.
“NexGen’s Vertical GaN eJFETs can be driven by well-established and cost-efficient standard low-cost Si MOSFET Drivers (Figure 7) with only minor modifications to existing designs. This enables quick adoption of the devices with superior properties”, said Ramanathan.
NexGen’s Vertical GaN technology combines the properties of devices, which were previously considered incompatible and, therefore, impossible to achieve. Power conversion in Automotive, consumer, solar, motors, and data centers are the main applications where one can experience the potential of this new technology. It offers lower losses at higher switching frequencies and better avalanche robustness than other switching devices and competes effectively on cost with silicon devices.
“From mobile phones to laptop computers, electronic devices are getting smaller and more portable. With Vertical GaN, the power supply system can be small, lightweight, lower cost and portable too”, said Ramanathan.
He continued with a particularly striking example: “In a data center rack, there are a certain number of racks units reserved for the power supply, which is converting AC power to DC power — we reduce the size of that power supply by 50%. Let’s look at a 30-kilowatt rack, 11 rack units are needed for power supply and 31 provide compute services. With our higher switching frequency, we can reduce the power supply size from 11 to 5 rack units, which means we free up 6 rack units to be added to the compute rack. 6 additional compute racks mean an increase of computing density by 20%”.
Vertical GaN allows you to address the full range of power conversion applications that can currently only be served by a multitude of technologies.