World's first AlN-based high-frequency transistor for post-5G communication
- Expanding Application Areas of AlN from Power Conversion to Wireless Communications -

News Highlights:

Background

High-frequency transistors are the core devices of high-frequency power amplifiers used in wireless communications, satellite communications, radar, and other applications. Higher output power and higher frequency of wireless signals lead to improved communication services such as expanded coverage areas and higher communication speeds. Therefore, high-frequency transistors require semiconductor materials with both a large breakdown field³ and high saturation electron velocity⁴. In current 5G communications, high-frequency transistors using gallium nitride (GaN), a wide-bandgap semiconductor⁵, are widely used. Toward post-5G, ultra-wide-bandgap semiconductors⁵, such as aluminum nitride (AlN), diamond, and gallium oxide (Ga₂O₃), which have even larger breakdown electric fields, are attracting attention to achieve further increases in output power of high-frequency transistors.
AlN is predicted to have one of the largest breakdown electric fields and saturation electron velocities among semiconductor materials, and its Johnson's Figure of Merit (FoM)⁶—a performance index for high-power, high-frequency transistors—is five times higher than that of GaN, the highest among ultra-wide-bandgap semiconductors (Figure 1). In aluminum gallium nitride (AlGaN), a compound of AlN and GaN, the performance index improves as the Al composition increases. Therefore, high-frequency transistors using high-Al-content AlGaN (AlN-based semiconductors) as the channel layer⁷- possess high potential as next-generation power amplifiers. NTT has been the first in the world to successfully grow AlN semiconductors and has demonstrated the operation of AlN transistors and Schottky barrier diodes, showing their potential as power-device semiconductors. However, when using AlN-based semiconductors for high-frequency transistors, increasing the Al composition leads to fundamental issues such as insufficient current injection from electrodes into the semiconductor and increased channel resistance. For this reason, high-frequency operation had long been considered difficult for high-Al-content AlN-based transistors with Al composition exceeding 75%.

Figure 1. Predicted high-Power, high-frequency transistor-performance index of semiconductor materials based on material properties. Johnson's Figure of Merit (normalized to GaN).

Technology highlights

For this study, we developed the following two technologies to achieve high-frequency operation of AlN-based transistors (Figure 2).

Figure 2. Schematic of the AlN-based transistor and key technical features.

Research Results

Using these technologies that reduce Ohmic contact resistance and channel resistance, we fabricated AlN-based transistors in the high-Al-composition range (Al compositions of 78, 85, and 89%). Even in the Al-composition region above 75%, where drain current had previously been severely limited, we confirmed large drain current and excellent current linearity in the linear region of the transistor. As one example, the transistor with 85% Al composition exhibited a high drain current exceeding 500 mA/mm and high on/off ratio exceeding 10⁹ (Figure 3). With these improvements in transistor performance, we succeeded, for the first time in the world, in achieving RF-power amplification above 1 GHz in AlN-based transistors with Al composition exceeding 75%. The transistor with 85% Al composition also achieved a maximum frequency of oscillation (f_max)¹⁰ of 79 GHz in the millimeter-wave band (30-300 GHz)—the highest among AlN-based transistors reported to date (Figure 4). Since higher Al composition is advantageous for achieving higher output power in high-frequency transistors, the structure proposed in this study provides a design guideline for achieving the intrinsic potential of AlN, representing an important advancement toward the application of AlN-based high-power, high-frequency transistors.

Figure 3. (a) Top-view scanning-electron-microscope image of the AlN-based transistor (Al composition: 85%), and (b) drain current‐voltage characteristics as the gate voltage varied from +3 to −9 V.

Figure 4. (a) High-frequency characteristics of the AlN-based transistor (Al composition: 85%), and (b) trend of f_max as a function of the Al composition in AlN-based transistors.

Future Outlook

We succeeded, for the first time in the world, in achieving millimeter-wave power amplification in AlN-based transistors with Al composition exceeding 75%. This marks an important first step toward the evolution of wireless-communication infrastructure in the post-5G era, including expanded communication coverage and higher communication speeds. Going forward, we will design device structures capable of higher current and voltage operation to demonstrate high-power operation of these high-frequency transistors and continue research and development toward the practical implementation of AlN-semiconductor technology from power conversion to wireless communications.

Related Press Releases

Conference Information:

Glossary

¹Aluminum Nitride (AlN)
A compound semiconductor consisting of aluminum (Al) and nitrogen (N). AlN-based semiconductors collectively refer to AlN and high-Al-content AlGaN alloys with Al compositions of 50% or more.

²Power Devices
Power-conversion devices with functions such as DC-AC conversion, voltage step-up/step-down in DC systems, and frequency conversion in AC systems. They are widely used in home appliances, electric vehicles, railways, industrial equipment, and power infrastructure.

³Breakdown electric field
The electric-field strength at which a semiconductor material can no longer maintain electrical insulation and a sudden increase in current occurs. A higher breakdown field enables operation at higher voltages and higher output power.

⁴Saturation electron velocity
The maximum velocity that electrons can reach under a strong electric field. Higher electron velocity enables operation at higher frequencies.

⁵Wide-bandgap semiconductors and Ultra-wide-bandgap semiconductors
The bandgap is a fundamental material property that determines the electrical characteristics of a semiconductor. Materials with larger bandgaps exhibit higher breakdown electric fields. Silicon (Si) has a bandgap of 1.1 eV. Semiconductors with bandgaps around 3 eV, such as silicon carbide (SiC) and gallium nitride (GaN), are classified as wide-bandgap semiconductors. Materials with even larger bandgaps—including gallium oxide (Ga₂O₃), diamond, and aluminum nitride (AlN)—are referred to as ultra-wide-bandgap semiconductors.

⁶Johnson's Figure of Merit (FoM)
A performance index for high-power, high-frequency transistors. It is proportional to the product of breakdown electric field and saturation electron velocity.

⁷Channel layer
The semiconductor layer inside a transistor that serves as the path for current flow. By controlling the electron concentration in the channel layer via the gate voltage, the transistor regulates or amplifies current.

⁸Ohmic Contact
A metal-semiconductor contact with low electrical resistance that enables current to flow easily in both directions.

⁹Polarization Doping
A technique in which the composition of AlGaN is spatially graded to create polarization charges, thus generating a three-dimensional electron gas or three-dimensional hole gas.

¹⁰Maximum frequency of oscillation (f_max)
The upper frequency limit at which a transistor can function as a power amplifier (the frequency where the power gain becomes 1). It is an important parameter for high-frequency amplifiers and wireless-communication circuits.

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