Enhancing p-GaN HEMTs with Composite Layers: Achieving Low On-Resistance, High Breakdown Voltage, and Improved Reliability

The rise of gallium nitride (GaN)-based high-electron-mobility transistors, or HEMTs, has completely changed the game in power electronics. GaN is a material that outshines traditional silicon in just about every way that matters for high-power devices. It offers a wider bandgap, faster electron movement, and excellent breakdown voltage, which makes it ideal for applications that demand efficiency and high performance—think electric vehicles, renewable energy systems, and advanced power supplies. Out of all the different GaN HEMT designs, p-GaN gate HEMTs stand out because they enable something called “normally-off” operation, meaning the device stays off by default unless activated. This is a major safety feature and a must-have for integrating power devices into real-world systems. But even with all these benefits, p-GaN HEMTs are far from perfect. One major issue engineers constantly face is the trade-off between keeping resistance low and ensuring the device can handle high voltages. Here is the problem: in a conventional p-GaN HEMT, the electric field tends to pile up near the gate edge when the device is in the off-state. This puts stress on the breakdown voltage, meaning it will fail at lower voltages than it should. To avoid this, designers often have to limit the thickness and aluminum content of the barrier layer, which reduces the stress but comes with its own drawback—it lowers the density of the two-dimensional electron gas (2DEG). The 2DEG is what makes these transistors so good at conducting electricity, and when it decreases, the resistance goes up, hurting efficiency. Finding a way to keep resistance low while maintaining high breakdown voltage has been a persistent challenge. Then there is the problem of current collapse, which happens when hot electrons get trapped at the surface of the device during operation. When this happens, these trapped charges act like invisible “virtual gates” that block the flow of current, raising resistance and slowing down performance. Over time, this can make the device unreliable, especially when used in high-power or high-voltage conditions like electric grids or industrial applications.

To address these issues, new study published in Semiconductor Science and Technology and led by Professor Junji Cheng, Queyang Wang, Yikai Liu, Guo Ding, Minming Zhang, Bo Yi, Haimeng Huang and Hongqiang Yang from the University of Electronic Science and Technology of China, decided to take a fresh approach. They introduced two important layers to the p-GaN HEMT design: a composite passivation (CP) layer and a composite barrier (CB) layer. The CP layer combines silicon nitride (Si₃N₄) with a high-permittivity (HK) material. Together, these help smooth out the electric field distribution and reduce the chances of hot electron trapping, which in turn minimizes current collapse. The CB layer, on the other hand, includes AlN and AlₓGa₁₋ₓN, which increase the 2DEG density without sacrificing the device’s breakdown voltage. This smart combination allows the device to tackle the limitations of conventional designs head-on, offering lower resistance, better voltage performance, and improved long-term reliability.

To see how well their new p-GaN HEMT design would perform, the researchers ran detailed simulations using Sentaurus TCAD software. They wanted to understand how the addition of CP layer and CB layer would impact three key things: on-resistance, breakdown voltage, and current collapse. To make the results as realistic as possible, they built their simulations using advanced physical models that took into account real-world factors, like carrier movement, recombination, surface traps, and polarization effects. One of the first major improvements they noticed was in the electric field distribution. In traditional p-GaN HEMTs, the electric field tends to spike around the gate edge during off-state operation, which often leads to breakdown. By adding a high-permittivity (HK) material to the CP layer, the researchers discovered that these spikes could be smoothed out. The HK film created polarization-bound charges on its lower surface, which absorbed much of the electric field lines coming from the channel. This small adjustment dramatically reduced stress at the gate edge, spreading the electric field more evenly across the surface. Thanks to this, the breakdown voltage improved significantly, jumping from 460 V in the conventional design to 505 V—an increase of nearly 10%.

The composite barrier layer also played a critical role, especially when it came to reducing resistance. The team incorporated a thin AlN interlayer and carefully increased the aluminum content in the AlGaN layer. This boosted the piezoelectric polarization at the GaN/AlGaN interface, which in turn increased the density of the  2DEG. With a denser 2DEG, the researchers saw a substantial drop in on-resistance. Their simulations showed that resistance fell by almost 30%, going from 0.61 to 0.43 mΩ·cm². The benefits were most noticeable around the gate area, where electron flow is critical for the device’s performance. Switching efficiency was another area where the new design excelled. In typical power devices, higher on-resistance means more energy gets wasted as heat during operation. However, with the CP-CB design, the on-state voltage drop at a rated current of 10 A was cut down from 2.2 V to just 0.93 V. This improvement led to a sharp reduction in energy losses. At a switching frequency of 200 kHz, the total energy loss per cycle was slashed in half, dropping from 57.46 μJ to just 28.46 μJ. Finally, the team addressed reliability issues caused by current collapse. Normally, trapped electrons near the gate-drain interface form “virtual gates,” which block current and increase resistance. The HK film in the CP layer offered a clever solution: when electrons were trapped, the HK material produced positive bound charges that balanced out the negative ones, reducing their effect. Simulations showed that the degree of current collapse fell dramatically—from 44.8% in conventional devices to just 16.2% in the new design. This represented a 28.6% improvement, making the device far more stable under stress. Moreover, according to the authors, structure can automatically reduce the current density in the saturation region, which helps to enhance the device’s ability to resist short circuits, and expand the safe operating area, as shown in Figure 2(e) and Figure 7 in the SST paper. In a statement to Advances in Engineering, the lead author, Professor Junji Cheng said: As far as we known, there have been no reports of a technology that can simultaneously increase the HEMT’s on-state current density and reduce its saturation current density, so it’s also an innovation in this work.

In conclusion, Professor Junji Cheng and his team have made a significant leap forward in improving p-GaN HEMTs, solving some long-standing challenges that have limited their use in power electronics. Their research introduces two key innovations: CP layer and CB layer. Together, these layers achieve something that has been difficult to balance—low on-resistance and high breakdown voltage. This balance is critical for power devices because it reduces energy loss while ensuring the device can handle the high stresses of demanding applications. One of the most impressive findings from their work is the dramatic reduction in energy loss during switching cycles—by more than 50% compared to conventional designs. This is a game-changer for technologies like electric vehicles, renewable energy systems, and industrial power supplies, which often operate at high frequencies. By improving efficiency and minimizing wasted energy, this new design can lower power consumption and extend the lifespan of devices, making it not only practical but also environmentally friendly. These improvements are especially timely as industries around the world push for more sustainable and efficient technologies. Another highlight of the study is how it addresses current collapse, a major issue for p-GaN HEMTs. The CP layer in this design introduces a clever mechanism that offsets the impact of these trapped electrons, helping the device maintain stable performance even under harsh conditions. This makes the proposed HEMTs particularly appealing for tough environments like those in automotive or aerospace applications, where reliability is a top priority. What really stands out about this work is its practicality. The team showed that these improvements don’t require overhauling existing manufacturing processes. Instead, the use of advanced materials like high-permittivity films and carefully designed barriers can be easily integrated into current production methods. This makes it much easier for the industry to adopt these advancements, potentially accelerating their use in real-world applications.