With April marking another anniversary of Moore's Law, it prompts reflection on its lasting influence since its 1965 inception. While not a rigid scientific law, Moore's prediction of chip component doubling every two years has guided semiconductor industry progress for decades across computing and electronics sectors.
As we approach the projected limits of Moore's Law in the 2020s, the industry faces challenges sustaining rapid advancement amid physical and economic constraints. But even as the tempo of Moore's Law may be slowing, the evolution of semiconductor technology endures, manifesting in new forms and confronting emerging challenges. This ongoing evolution prompts a shift in focus from sheer transistor count to enhancing performance, power efficiency, and other critical metrics.
Against this backdrop of industry transformation, wide bandgap (WBG) semiconductors, notably silicon carbide (SiC) and gallium nitride (GaN), are fast emerging as pivotal players in power electronics. Offering distinct advantages over traditional silicon-based counterparts, WBG semiconductors promise superior performance and efficiency, particularly in demanding operating conditions. Their adoption represents a significant path forward in advancing energy-efficient power electronics, with implications for the continued evolution of Moore's Law, as they signify a new era of innovation and advancement in power electronics technology.
SiC versus GaN in Power Electronics Revolution
Wide band gap (WBG) semiconductors, exemplified by materials like silicon carbide (SiC) and gallium nitride (GaN), are hailed as the future energy-efficient power electronics, poised to surpass traditional silicon counterparts, particularly in the challenging operational environments. Their superiority stems from intrinsic characteristics such as higher breakdown fields and significantly greater electron saturation velocity compared to silicon, enabling the realization of robust power semiconductor devices capable of high-power and high-temperature operation.
SiC technologies have reached relative maturity, benefiting from advancements such as the ability to grow SiC via homoepitaxy with low defect density and enabling the utilization of vertical device structures capable of blocking voltages up to 20 kV.
Conversely, GaN requires heteroepitaxy and lateral device design, imposing limits on blocking voltages. Nonetheless, GaN devices offer a substantial advantage over SiC with approximately 70 times higher electron mobility, translating into exceptionally high switching speeds. For instance, while typical SiC MOSFETs operate within the range of 200-500 kHz, GaN power devices can achieve switching frequencies soaring up to 2 MHz.
These fundamental performance disparities, encompassing mobility, breakdown field, thermal properties, position SiC and GaN uniquely within the power device application spectrum, as illustrated in Figure 1. GaN, for example, finds its optimal utility in low to medium voltage commercial applications.
GaN power devices typically employ a lateral High Electron Mobility Transistors (HEMT) configuration (refer to Figure 2), leveraging the presence of a two-dimensional electron gas (2DEG) at the AlGaN/GaN interface. This design inherently results in normally-ON behavior, where current flows between the source and drain electrodes even at zero gate bias (Vg = 0).
The current is modulated by applying a negative bias at the Schottky gate electrode, thus depleting the 2DEG from electrons. This device is called Depletion-mode HEMT. Since normally-ON devices are not preferred in the power electronic applications due to high static power consumption, the mainstream solution has been to use a so-called cascode configuration with an additional silicon MOSFET transistor connected in series with the depletion-mode HEMT, as shown in Figure 2. This solution allows normally-OFF operation but adds a lot of complexity to the overall circuit design and backend-of-line (BEOL) processes.
Merits and Hurdles in Insulated Gate Structures
To date, several techniques have been developed to achieve naturally the normally-OFF operation of devices, termed Enhancement-mode HEMTs, featuring simplified gate driving schemes and fail-safe operation. These techniques include gate p-GaN cap design (refer to Figure 3), AlGaN barrier layer recess etch, and fluorinated gate designs. However, these devices still encounter challenges such as low gate breakdown (<10V), low threshold voltage, forward bias gate leakage, or poor thermal stability, or a combination of these factors. Particularly, high gate leakage diminishes the breakdown voltage, reducing power-added efficiency and increasing the noise figure, significantly impacting the amplification performance of power amplifiers.
The most promising approach thus far to enhance the Enhancement-mode operation of GaN devices is to introduce an insulator layer prior to gate metal deposition, creating a structure known as MIS-HEMT (Metal-Insulator-Semiconductor HEMT), as illustrated in Figure 3. The deposition of gate dielectric is typically achieved using techniques such as Atomic Layer Deposition (ALD), and the process may also involve slight gate recess etching to enable better modulation control of the channel.
The introduction of the insulated gate structure, however, brings about another challenge: III-V-insulator interfaces are notorious for their poor quality, characterized by a high density of defect states within the band gap. These defect levels serve as sources of high dispersion in device capacitance and provide pathways for additional gate leakage. Furthermore, the trapping of charge carriers at surface states can deplete the two-dimensional electron gas (2DEG) of electrons, increasing dynamic ON-resistance and leading to phenomena such as current collapse, ultimately resulting in poor device performance such as reduced output power.
More specifically, trap states situated at the dielectric/AlGaN interface initiate dynamic charge/discharge processes, which are particularly problematic in wide bandgap GaN-based materials where traps may be deeply embedded within the bandgap, contributing to severe operational instability due to their slow de-trapping behavior. The instability of threshold voltage in AlGaN/GaN MIS-HEMTs represents a significant challenge. Notably, a substantial shift in threshold voltage induced by the “spill-over” of electrons from the 2DEG channel towards the dielectric interface under forward gate bias conditions may occur. Additionally, the degradation of current linearity in the transfer characteristics of AlGaN/GaN MIS-HEMTs results in gain loss and the deterioration of large signal linearity in power amplifiers.
Novel Kontrox Passivation Enhancing AlGaN/GaN Power Device Attributes
With Kontrox passivation technology, the surface and interface quality of Nitride-based compound semiconductors can be significantly enhanced. By creating a thin, high-quality crystalline oxide passivation layer on the nitride material (e.g., AlGaN) prior to gate dielectric deposition, interface states can be effectively eliminated.
Leveraging its expertise in semiconductor chip surface enhancement, Comptek Solutions, the pioneer of Kontrox technology, has reported a reduction of interface state densities by over 1-2 orders of magnitude. As a result, the gate-stack performance can be elevated to the technologically feasible levels in MIS-HEMT enhancement-mode transistor designs.
In Figure 4, the standard AlGaN/GaN MIS-structure with HfO2 high-k dielectric exhibits significant frequency-dependent capacitance dispersion, indicating a substantial impact of interface states and the resulting trapping/de-trapping mechanisms for charge carriers at the interface. Such behavior can introduce additional capacitance components in normal transistor operation, affecting achieved channel current, voltage swing, and ultimately, the power gain cut-off frequency (=highest frequency at which power gain can be obtained from the device) and power added efficiency. In contrast, Kontrox-passivated gate stacks demonstrate record-low CV dispersion in AlGaN/GaN MIS-stack, indicating efficient channel modulation with minimized charge carrier trapping processes.
In summary, the adoption of Kontrox passivation technology in GaN power electronics offers several key benefits:
Reduction of defect states by 1-2 orders of magnitude at dielectric-nitride interfaces.
Key enabler for Enhancement-mode operating MIS-HEMT.
Effective channel modulation at the gate stack with low gate leakage, high breakdown voltage and low ON-state resistance.
Stable threshold voltage and high transconductance, enhancing dynamic range of input voltage for improved power gain cut-off frequency and power-added efficiency.
Enablement of transistor scaling to smaller gate-lengths by eliminating parasitic capacitance components of defect states and minimizing gate leakage.
Passivating Ahead in WBG Semiconductor Tech
The relentless pursuit of advancements in WBG semiconductor technology fuels ongoing exploration in materials science, device physics, and manufacturing processes. By pushing the boundaries of semiconductor performance and investigating novel materials and device architectures, researchers and engineers worldwide are poised to chart new frontiers in power electronics technology, continually striving to enhance performance, power efficiency, and other critical metrics in alignment with the evolving demands of the digital era.
Addressing the challenges posed by interface states and trapping mechanisms, Kontrox passivation offers a pathway to significantly improve the surface and interface quality of Nitride-based WSG compound semiconductors. By mitigating interface states and achieving efficient channel modulation, this novel technology paves the way for unprecedented performance and efficiency gains in GaN power electronics.
To explore more about the benefits Kontrox passivation has in store for enhancing your power electronics applications, don't hesitate to contact the Comptek team.
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