Fabrication and DC Characterization of BST-Gated AlGaN/GaN MOS-HEMT Structures in Two- and Three-Terminal Configurations

Technical Summary: Fabrication and DC Characterization of BST-Gated AlGaN/GaN MOS-HEMT Structures

Problem Statement
While Gallium Nitride (GaN) based AlGaN/GaN High Electron Mobility Transistors (HEMTs) offer superior performance in power electronics and high-frequency applications due to their wide bandgap and high electron mobility, traditional Schottky-gate structures suffer from significant limitations. These include substantial gate leakage current, threshold voltage instability, surface state trapping, and current collapse, particularly under high electric fields. To address these issues, Metal-Oxide-Semiconductor (MOS) structures utilizing dielectric layers are employed to enhance electrostatic control and reduce leakage. Although high-k dielectrics like SiNx, Al₂O₃, and HfO₂ are commonly explored, there is a scarcity of published research regarding Barium Strontium Titanate (BST) as a gate dielectric for AlGaN/GaN MOS-HEMTs. Existing literature largely focuses on RF performance or breakdown voltage, with limited experimental data on the DC electrical properties and interface characteristics of BST-gated devices. Furthermore, no prior work has systematically compared the electrical properties of two-terminal and three-terminal BST-gated MOS-HEMTs to evaluate the impact of BST formation on transport phenomena and device operation.

Methodology
The study utilized commercially available AlGaN/GaN HEMT wafers grown via Metal-Organic Chemical Vapor Deposition (MOCVD). The fabrication process involved two distinct device configurations:

1. Two-Terminal Devices: Designed to analyze the electrical properties of the AlGaN/GaN structure before and after BST deposition. Ohmic contacts (Al/Cr/Au) were patterned using photolithography and thermal evaporation, followed by Rapid Thermal Annealing (RTA) at 870°C. BST films were subsequently deposited in the gate region using RF magnetron sputtering.
2. Three-Terminal MOS-HEMTs: Built upon the two-terminal structures, these devices incorporated a second photolithography step to define the gate region. A Ni/Au (10 nm/50 nm) gate stack was deposited over the BST layer to form the MOS gate, while Al/Cr/Au stacks served as source and drain contacts. The channel dimensions were approximately 100 μm in length and 25 μm in width.

Electrical characterization was performed using a Keithley 2634B Dual-Channel SourceMeter Unit at room temperature. Two-terminal devices were tested via symmetric I-V sweeps from ±1 V to ±12 V. Three-terminal devices were characterized using output curves (Ids–Vds) and transfer curves (Ids–Vgs) to extract critical parameters such as threshold voltage, transconductance, and on/off current ratios.

Key Results

• Two-Terminal Characterization: The I-V characteristics of the two-terminal devices demonstrated a transition from near-linear ohmic behavior to a significantly higher resistance state after BST deposition. At any given voltage sweep (e.g., ±12 V), the current decreased by approximately one order of magnitude (e.g., from 9.85 mA to 0.517 mA), confirming the effective blocking and insulating properties of the BST layer.
• Three-Terminal Characterization: The fabricated BST-gated MOS-HEMT exhibited effective gate control over the drain current.
  • Threshold Voltage ($V_{th}$): Extracted as approximately 0.028 V, indicating a device operating close to the normally-on mode.
  • Current Levels: The on-state current ($I_{ON}$) was measured at 5.76 × 10⁻⁸ A at $V_{gs} = 0.7$ V, while the off-state current ($I_{OFF}$) was 3.475 × 10⁻⁹ A at $V_{gs} = -4$ V and $V_{ds} = 1$ V. This yielded an on/off current ratio of approximately 16.
  • Transconductance: The maximum transconductance ($g_m$) was recorded as 4.03 × 10⁻⁶ S at $V_{ds} = 20$ V.
  • Output Characteristics: Drain current ($I_{ds}$) increased gradually from $4.715 \times 10^{-4}$ A to $9.347 \times 10^{-4}$ A as $V_{gs}$ varied from -4 V to +4 V at a fixed $V_{ds}$ of 20 V.

Significance and Claims
The paper claims that the successful fabrication and DC characterization of BST-gated AlGaN/GaN MOS-HEMTs provide a thorough insight into the viability of BST as a gate insulator. The study demonstrates that the incorporation of the high-k BST layer effectively reduces leakage current and enables gate modulation of the two-dimensional electron gas (2DEG) channel. By comparing two-terminal and three-terminal configurations, the work fills a gap in the literature regarding the DC performance of BST-based devices, specifically addressing the lack of data on interface transport phenomena and the comparative electrical properties of these specific device topologies. The results validate the functional formation of the 2DEG under the BST/AlGaN heterostructure and suggest that BST is a promising candidate for enhancing the DC electrical properties of GaN-based devices.