Multimode GaN Transistors with Ultrawide Threshold Voltage Control Enabled by Passivation-Induced...

Area of Science:

• Materials Science
• Semiconductor Physics
• Device Engineering

Background:

• Gallium nitride (GaN) transistors are crucial for high-frequency and high-power applications.
• Achieving both depletion-mode (D-mode) and enhancement-mode (E-mode) operation in a single GaN transistor, known as multimodality, presents a significant challenge.
• Existing methods for threshold voltage (VTH) tuning often involve complex fabrication steps or compromise device performance.

Purpose of the Study:

• To demonstrate a novel method for achieving ultrawide and tunable threshold voltage (VTH) in GaN transistors.
• To enable multimode operation (D-mode and E-mode) within a single GaN device.
• To explore the underlying physics of stress-strain-polarization coupling (SSPC) for device performance enhancement.

Main Methods:

• Epitaxial growth of ultrathin aluminum nitride (AlN) barriers with high interface quality.
• Introduction of tensile stress via plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiN) passivation.
• Utilizing stress-strain-polarization coupling (SSPC) to modify the two-dimensional electron gas (2DEG) density.
• Device simulations to analyze conduction mode evolution with varying passivation thickness (PT).

Main Results:

• Achieved a broadly tunable and ultrawide threshold voltage (VTH) window covering both D-mode and E-mode operation.
• Generated significant strain (up to -4.2%) along [0001] through SSPC, increasing 2DEG density by over an order of magnitude.
• Simulations clarified the transition between background-carrier, MOS-like, and D/E-mode high-electron mobility transistor (HEMT) conduction modes.
• Demonstrated SSPC as an etch-free, low-damage method for tuning polarization, carrier density, and VTH.

Conclusions:

• Multimode GaN transistors with continuously tunable conduction modes are realized through SSPC.
• This approach offers a compact and effective way to achieve VTH tunability and enhance 2DEG density.
• The developed GaN transistors possess multiple functionalities for diverse applications, including RF amplification, neuromorphic computing, and power conversion.