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| Fig. 1: The comparison of the material figure of merit for unipolar devices, generated from Eq. (1) below using the parameters of Table 1. This illustrates the relationship between breakdown voltage and specific on-resistance (RON, SP). GaN exhibits a lower specific on-resistance at the same breakdown voltage than either Silicon (Si) or 4H-Silicon Carbide (4H-SiC). This reduction in RON, SP for GaN devices directly impacts the conduction losses in power devices, and makes GaN particularly advantageous for high-efficiency applications. Lower on-resistance reduces power dissipation, allowing GaN-based systems to operate at higher voltages with improved energy efficiency and thermal performance. (Image Source: H. Chen) |
Gallium Nitride (GaN) has emerged as a key material in the development of energy-efficient power semiconductors. Its superior electronic properties enable higher energy efficiency, smaller device sizes, and faster switching speeds compared to traditional materials like Silicon (Si) and even Silicon Carbide (SiC). These advantages stem from GaN wide bandgap and exceptional electron mobility, which allow it to operate at higher voltages and temperatures while maintaining lower power losses. This article examines the physical characteristics of GaN, its performance in power conversion, and the cost-benefit dynamics that determine its viability in the semiconductor market. Additionally, we compare GaN efficiency with Si-Insulated Gate Bipolar Transistor (Si-IGBT) and SiC-MOSFET, focusing on key performance metrics including conduction loss, switching loss, and overall energy efficiency in power conversion applications.
GaN key advantage lies in its wide bandgap of 3.4 eV, significantly higher than Si 1.1 eV and SiC 3.2 eV. This allows GaN to operate at higher voltages and temperatures with lower power losses due to leakage currents. Another critical factor is GaN small effective electron mass, which translates into higher electron mobilitynearly 2000 cm2/Vs, much higher than Si (1400 cm2/Vs) and SiC (900 cm2/Vs)[1]. This property makes GaN highly suitable for high-frequency power switching applications, where fast electron transport is essential for reducing switching losses.
In the power conversion scheme, the converter nonidealities are mostly considered as:
Inductor resistance (rL)
Transistor and diode voltage drops (VQ and VD)
Switching losses (rsw).
These introduced inefficiencies impact the overall energy performance of power devices. These parasitic elements, often overlooked in idealized models, can significantly increase power losses, especially in high-frequency applications where switching speeds are elevated. For instance, switching losses escalate with the operating frequency, primarily due to rapid transitions that generate transient currents and heat. GaN devices, with their high electron mobility and reduced specific on-resistance, inherently mitigate some of these losses by facilitating faster switching with lower resistive paths, though parasitics still impose practical limitations requiring careful design considerations.
The intrinsic material properties of GaN, such as its high electron mobility and low channel resistance, further support efficient operation by reducing both conduction and switching losses. High electron mobility allows faster current conduction, directly contributing to lower switching losses in GaN-based converters. This is crucial because channel resistance and mobility are inversely related; materials with lower channel resistance, like GaN, reduce the amount of energy dissipated as heat during conduction. When compared to Si and SiC, GaN reduced on-resistance enhances its efficiency in power conversion applications, as less energy is lost during each switching cycle. However, nonidealities in GaN devices, such as parasitic capacitances and series resistances within the circuit layout, remain important factors that influence final efficiency, underscoring the need to optimize both material choice and circuit design to fully leverage GaN's advantages.
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| Table 1: Material properties of Silicon, GaN, and SiC [2] |
The Figure of Merit (FoM) in power electronics is crucial because it provides a comprehensive and fast evaluation of a device's ability, allowing benchmarking of different materials. It is expressed as
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(1) |
with parrameters summarized in Table 1. The symbols and values that characterize the on-state and breakdown properties of semiconductor materials are given more specifically by
RON, SP: Specific on-resistance, measured in mΩcm. This is the resistance that characterizes the on-state losses in the device.
VBV: Breakdown voltage.
μ: Electron mobility, which affects how quickly electrons can move through the material. Higher mobility leads to lower resistance.
εr: Dielectric constant of the material, affecting the electric field distribution.
ε0: Permittivity of free space): A constant, approximately 8.854 × 10-4 Farad/cm.
There is a trade-off between RON, SP and breakdown voltage. As we increase the distance of the drift region, it can increase the breakdown voltage but also increase the resistance.
As shown in Fig. 1, GaN exhibits the lowest specific on-resistance (RON, SP) at the same breakdown voltage compared to Si and 4H-SiC. This significantly lower on-resistance contributes to reduced conduction losses, making GaN ideal for high-efficiency power applications. By minimizing RON, SP, GaN devices can operate at high voltages without sacrificing efficiency, further supporting compact and energy-efficient designs.
In GaN high-electron-mobility transistors (HEMTs), the two-dimensional electron gas (2DEG) structure offers a significant advantage. This 2DEG channel, formed at the heterojunction interface between GaN and AlGaN layers, creates a high-density electron region that is less affected by impurity scattering due to the absence of doping. Compared to traditional materials like Si and SiC, the 2DEG in GaN HEMTs enables much higher electron mobility and lower on-resistance, further reducing conduction losses. Additionally, the 2DEG structure allows for extremely fast switching speeds, which significantly minimizes switching losses in high-frequency applications. In contrast, Si and SiC face higher conduction resistance and switching losses at high frequencies, making it challenging to achieve both high efficiency and high-frequency performance simultaneously. Thus, the 2DEG structure provides GaN with a competitive edge in efficiency and frequency, positioning it as a strong alternative to Si and SiC in power conversion and RF amplification applications.
Despite its numerous advantages, GaN technology faces several critical challenges that impact its widespread adoption. One major issue is the presence of traps, which can lead to a phenomenon known as dynamic on-resistance [3]. Reliability is another concern, particularly with high-temperature reverse bias (HTRB) stress, which can cause device degradation over time [4]. Additionally, the cost of GaN devices remains relatively high compared to Si and SiC solutions. For GaN to compete effectively with these well-established materials, ongoing research and development are necessary to address these challenges and improve device robustness, manufacturing scalability, and cost- effectiveness.
GaN technology has established itself as a powerful contender in the field of energy-efficient power electronics, primarily due to its wide bandgap, high electron mobility, and low specific on-resistance, which enable high-speed switching and reduced power losses. These characteristics make GaN particularly advantageous in high-frequency and high-voltage applications, offering efficiency and compactness unmatched by traditional Si and even SiC devices. The unique 2DEG structure in GaN HEMTs further enhances its performance, supporting higher electron mobility and lower conduction losses. However, GaN adoption faces hurdles such as dynamic on-resistance issues due to traps, reliability concerns under high-temperature stress, and high manufacturing costs associated with complex fabrication processes. Overcoming these challenges through advancements in material quality, device reliability, and cost-effective manufacturing will be essential to unlocking GaN's full potential and ensuring its role as a mainstream solution for energy-efficient power conversion.
© Hugo Chen. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] Y. Zhong et al., "A Review on the GaN-on-Si Power Electronic Devices," Fundam. Res. 2, 462 (2022).
[2] A. Lidow et al., GaN Transistors For Efficientt Power Conversion, 2nd Ed (Wiley, 2014), p. 3
[3] S. Karboyan et al., "On the Origin of Dynamic Ron in Commercial GaN-on-Si HEMTs", Microelectron. Reliab. 81, 306 (2018).
[4] O. Chihani et al., "Effect of HTRB Lifetest on AlGaN/GaN HEMTs Under Different Voltages and Temperatures Stresses," Microelectron. Reliab. 88-90, 402 (2018).