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Analysis and comparison of GaN-based microwave semiconductor devices
Gallium nitride (GaN), a wide bandgap semiconductor, has emerged as a promising alternative to the first-generation silicon (Si) and second-generation compound semiconductors like gallium arsenide (GaAs) and gallium phosphide (GaP). As a third-generation semiconductor material, GaN has gained significant attention due to its superior physical and chemical properties. Compared to traditional materials, GaN offers unique advantages such as a wider bandgap, higher saturation drift velocity, higher breakdown electric field, and better thermal conductivity, making it one of the most attractive materials for next-generation electronic devices.
Recent advancements in GaN-based light-emitting devices have shown remarkable progress, with commercialization of green-to-violet LEDs abroad and initial industrialization of blue LEDs domestically. Additionally, numerous studies highlight GaN's potential in high-temperature and high-power microwave applications. This article explores the material properties that make GaN ideal for microwave device fabrication, introduces current research trends in GaN-based microwave components, and discusses the working principles and performance characteristics of GaN modulated doped field effect transistors (MODFETs), which show great promise in high-power applications.
**1. Material Properties**
The exceptional performance of GaN-based microwave devices stems from their outstanding material properties. As shown in Table 1, GaN exhibits the widest bandgap, the highest breakdown electric field, and better thermal conductivity compared to Si, GaAs, and SiC. These features make GaN an ideal candidate for high-power microwave devices.
Figure 1 illustrates the electron drift velocity of GaN, Si, SiC, and GaAs at 300 K as a function of the electric field. It is evident that GaN has a significantly higher saturation drift velocity than other materials, indicating its suitability for high-current and high-power applications. Moreover, GaN possesses a high electron mobility (up to 1,000 cm²/(V·s)), leading to lower parasitic resistance and improved device performance.
As a direct bandgap semiconductor, GaN can be alloyed with AlN to form a continuous bandgap ranging from 3.4 eV to 6.2 eV. This enables the formation of an AlGaN/GaN heterojunction, which generates a two-dimensional electron gas (2DEG) with enhanced mobility and drift velocity. Current research shows that AlGaN/GaN heterostructures are increasingly used in microwave device manufacturing.
Table 2 compares key parameters of AlGaN/GaN and AlGaAs/GaAs heterojunctions. The data clearly indicate that AlGaN/GaN structures offer superior material properties, making them more suitable for microwave applications. For instance, researchers at Xi'an University of Electronic Science and Technology have successfully grown high-quality AlGaN/GaN heterostructures on sapphire and SiC substrates, achieving a 2DEG mobility and surface charge density product of 2×10¹â¶/(V·s) on sapphire.
**2. GaN-Based Microwave Devices**
Thanks to its low heat generation and high breakdown field, GaN has become a key material for developing high-power microwave devices. With advances in growth techniques and breakthroughs in thin-film technology, various GaN-based microwave devices have been developed, including MESFETs, HFETs, MODFETs, and MISFETs.
**2.1 MESFETs**
GaN-based metal-semiconductor field-effect transistors (MESFETs) show great potential in microwave power amplification. They leverage GaN’s wide bandgap and simple fabrication process to deliver high performance. In 1993, Khan et al. fabricated the first GaN MESFET on sapphire using low-pressure MOCVD, employing an AlN buffer layer to improve film quality and Au/Ti as source-drain contacts. The device demonstrated a gate length of 1 μm and a transconductance of 23 mS/mm at -1 V gate bias.
Subsequent work by SCBinari et al. reported GaN MESFETs with improved microwave performance. Using organic metal vapor phase epitaxy, they grew undoped GaN on sapphire, achieving a cutoff frequency (fT) of 8 GHz and a maximum oscillation frequency (fmax) of 17 GHz. Further improvements led to a new design with a gate length of 1.5 μm, showing a maximum transconductance of 20 mS/mm and a predicted output power of over 1 W/mm under optimal conditions.
These developments suggest that with continued improvements in design and fabrication, GaN MESFETs could achieve even higher frequencies, potentially reaching 20–40 GHz.