1 W/mm Output Power Density for H-Terminated Diamond MOSFETs With Al2O3/SiO2Bi-Layer Passivation at 2 GHz
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-12-23 |
| Journal | IEEE Journal of the Electron Devices Society |
| Authors | Xinxin Yu, Wenxiao Hu, Jianjun Zhou, Bin Liu, Tao Tao |
| Institutions | Nanjing University, Nanjing Institute of Technology |
| Citations | 19 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research demonstrates a novel bi-layer passivation technique for hydrogen-terminated (H-) diamond MOSFETs, achieving high stability and record output power density at RF frequencies.
- Novel Passivation Structure: A bi-layer dielectric consisting of a thin lower layer of ALD-Al2O3 (protecting C-H bonds) and a thick upper layer of PECVD-SiO2 (providing stability and encapsulation) was successfully implemented.
- Record Power Density: The device achieved an output power density of 1.04 W/mm at 2 GHz, the highest reported value for a diamond transistor operating at this frequency.
- Enhanced Stability: The surface current, which initially dropped after passivation, gradually restored and saturated at a high level, becoming significantly more stable than an unpassivated counterpart over 60 days.
- Excellent RF Performance: The 0.45 µm gate length MOSFET exhibited an extrinsic cutoff frequency (fT) of 15 GHz and a maximum frequency of oscillation (fmax) of 36 GHz.
- Low Contact Resistance: An extremely low Ohmic contact resistance of 0.87 Ω·mm was obtained, which is the lowest value achieved on H-diamond surfaces.
- High Current Density: The maximum current density reached -549 mA/mm after the surface current saturation period.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum Output Power Density | 1.04 | W/mm | At 2 GHz, VDS = -28 V |
| Extrinsic Cutoff Frequency (fT) | 15 | GHz | LG = 0.45 µm |
| Maximum Oscillation Frequency (fmax) | 36 | GHz | LG = 0.45 µm |
| Maximum Current Density (IDS,max) | -549 | mA/mm | After 60 days saturation |
| On-Resistance (Ron) | 28 | Ω·mm | After 60 days saturation |
| Ohmic Contact Resistance (Rc) | 0.87 | Ω·mm | Lowest reported on H-diamond |
| Sheet Resistance (Rsh) | 6.4 | kΩ/sq | 2DHG channel |
| Specific Contact Resistance (ρc) | 1.18 x 10-6 | Ω·cm2 | - |
| Power Added Efficiency (PAE) | 13.69 | % | At 2 GHz |
| Associated Gain | 3.22 | dB | At 2 GHz |
| Saturation Velocity (vsat) | 4.24 x 106 | cm/s | Calculated from fT |
| Al2O3 Thickness | 50 | nm | ALD layer |
| SiO2 Thickness | 200 | nm | PECVD layer |
| Gate Length (LG) | 0.45 | µm | RF MOSFET |
| Diamond Thermal Conductivity | 22 | W·cm-1·K-1 | Intrinsic property |
| Critical Breakdown Field | 10 | MV/cm | Intrinsic property |
Key Methodologies
Section titled “Key Methodologies”The H-diamond MOSFETs were fabricated on a 5x5x0.3 mm3 CVD (100)-oriented single crystal diamond substrate using the following key steps:
- Hydrogenation:
- Performed via Microwave Plasma Chemical Vapor Deposition (MPCVD).
- Parameters: 700 °C, 2.2 kW power, 10 min duration.
- Result: Formation of the 2DHG channel (Rsh = 6.4 kΩ/sq).
- Ohmic Contact Formation:
- 50 nm Au film deposited by Electron Beam (EB) evaporation.
- Unmasked Au removed using potassium iodide (KI) solution.
- Device Isolation:
- Achieved by exposing the surface to a low power oxygen plasma for 5 min.
- Surface Preparation (Annealing):
- Substrate annealed in the ALD chamber at 350 °C for 10 min to remove surface adsorbates.
- First Passivation / Gate Dielectric (Al2O3):
- 50 nm thick Al2O3 deposited by Atomic Layer Deposition (ALD) at 350 °C.
- Reactants used were trimethylaluminum and deionized water.
- Gate Fabrication:
- Gate windows defined by EB lithography.
- Gate metals (20/500 nm Ti/Au stack) deposited by EB evaporation.
- Second Passivation (SiO2):
- 200 nm thick SiO2 deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD) at 280 °C.
- This layer encapsulates the gates and provides long-term stability.
- Test Pad Deposition:
- Final 20/500 nm Ti/Au metal stack deposited after opening the Al2O3/SiO2 windows.
Commercial Applications
Section titled “Commercial Applications”The development of highly stable, high-power diamond MOSFETs using this bi-layer passivation technique is critical for several high-performance electronic sectors:
- High-Power RF Amplification: Diamond’s high breakdown field and thermal conductivity make it ideal for power amplifiers (PAs) in radar systems, satellite communications, and high-power microwave generators.
- 5G/6G Infrastructure: The high fmax (36 GHz) and high power density are essential for developing next-generation base station transmitters and high-frequency communication modules.
- Harsh Environment Electronics: The intrinsic stability of diamond combined with the robust, acid/alkali-resistant SiO2 passivation layer enables devices suitable for high-temperature or chemically aggressive operating environments.
- Power Electronics: While tested at RF, the high current density (-549 mA/mm) and low Ron (28 Ω·mm) indicate potential for high-efficiency switching devices operating at high voltages.
- Thermal Management: Utilizing diamond substrates inherently provides superior heat dissipation, crucial for densely integrated monolithic microwave integrated circuits (MMICs).
View Original Abstract
We have demonstrated a novel method of depositing ALD-Al<sub>2</sub>O<sub>3</sub>/PECVD-SiO<sub>2</sub> bi-layer dielectric to passive the surface channels of the hydrogen-terminated diamond (H-diamond). After Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> passivation, the surface current increased with time and then tended to be saturated. Afterwards, it became much more stable and showed a larger current than an unpassivated counterpart. The H-diamond MOSFETs were fabricated by using this bi-layer passivation structure and an extremely low Ohmic contact resistance of <inline-formula> <tex-math notation=“LaTeX”>$0.87~\Omega \cdot $ </tex-math></inline-formula>mm was obtained. The H-diamond RF MOSFET with gate length of <inline-formula> <tex-math notation=“LaTeX”>$0.45~{\mu }\text{m}$ </tex-math></inline-formula> achieved a high current density of −549 mA/mm and an extrinsic <inline-formula> <tex-math notation=“LaTeX”>${f} {\mathrm{ T}}/{f}{\max }$ </tex-math></inline-formula> of 15/36 GHz. By load-pull measurement, a high output power density of 1.04 W/mm was obtained at frequency of 2 GHz. The results reveal that it is a promising solution for high-stable and high-power diamond transistors by using the Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> bi-layer passivation.
Tech Support
Section titled “Tech Support”Original Source
Section titled “Original Source”References
Section titled “References”- 2014 - High-reliability passivation of hydrogen-terminated diamond surface by atomic layer deposition of Al2O3 [Crossref]
- 2015 - Isotope analysis of diamond-surface passivation effect of high-temperature H2O-grown atomic layer deposition-Al2O3 films [Crossref]