Diamond Field-Effect Transistors With V2O5-Induced Transfer Doping - Scaling to 50-nm Gate Length
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-05-06 |
| Journal | IEEE Transactions on Electron Devices |
| Authors | Kevin G. Crawford, James Weil, Pankaj B. Shah, Dmitry Ruzmetov, Mahesh R. Neupane |
| Institutions | DEVCOM Army Research Laboratory |
| Citations | 28 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research details the successful fabrication and characterization of highly scaled hydrogen-terminated (H-terminated) diamond Field-Effect Transistors (FETs) utilizing Vanadium Pentoxide (V2O5) for robust surface transfer doping.
- Record Performance: Devices achieved a peak drain current (Id,peak) of approximately 700 mA/mm and a peak transconductance (gm,peak) of 153 mS/mm, among the highest reported for diamond Metal Semiconductor FETs (MESFETs).
- Critical Scaling: The gate length (Lg) was successfully scaled down to 50 nm, demonstrating the potential for improved performance through dimensional reduction.
- Robust Doping Mechanism: V2O5, a high electron affinity transition metal oxide, was used as a stable surface acceptor, inducing a 2-D hole gas (2DHG) and improving device stability compared to volatile atmospheric adsorbents.
- Inverse Scaling Observed: Peak output current and transconductance scaled inversely with gate length, confirming expected performance gains from scaling.
- Channel Stability: Repeat measurements over 17 iterations showed high stability, with a standard deviation of only 3.1 mA/mm for the 200 nm device.
- Material Improvement: Sheet resistance of the H-terminated diamond surface was reduced from 14.2 kΩ/square to 6.8 kΩ/square after V2O5 deposition.
- Limitation Identified: Short channel effects, including Drain-Induced Barrier Lowering (DIBL) and gate instability (hot carrier injection), limit OFF-state performance at the smallest dimensions.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Peak Drain Current (Id,peak) | ~700 | mA/mm | 50 nm Lg, Vds = -10 V |
| Peak Transconductance (gm,peak) | 153 | mS/mm | 50 nm Lg, Vds = -4 V |
| Minimum Gate Length (Lg) | 50 | nm | Scaled device dimension |
| ON-Resistance (RON) | 6 | Ω.mm | 50 nm Lg, Vgs = -3 V |
| Sheet Resistance (Rsheet) Post-V2O5 | 6.8 | kΩ/square | VDP measurement |
| Sheet Resistance (Rsheet) Pre-V2O5 | 14.2 | kΩ/square | VDP measurement |
| Effective Mobility (”eff) (Estimated) | 23.5 | cm2 V-1 s-1 | Calculated using 400 nm Lg C-V data |
| Sheet Hole Density (p) (Estimated) | 3.9 x 1013 | cm-2 | Calculated using Rsheet |
| Maximum Gate Capacitance (Cg) | 1.15 | ”F/cm2 | 400 nm Lg, Vgs = -3 V |
| V2O5 Film Thickness | 40 | nm | Encapsulation layer |
| V2O5 Deposition Anneal Temperature | 350 | °C | In situ anneal prior to V2O5 deposition |
| Gate Leakage (Ig) at Id,peak | ~1 | ”A/mm | 50 nm Lg |
| Diamond Band-Gap | 5.47 | eV | Intrinsic material property |
| Diamond Thermal Conductivity | >20 | W cm-1 K-1 | Intrinsic material property |
| Diamond Breakdown Field | 10 | MV/cm | Intrinsic material property |
Key Methodologies
Section titled âKey Methodologiesâ- Substrate Preparation: Single crystal diamond (100) was cleaned using H2SO4:NHO3 solution to remove metallic and organic contaminants, followed by hydrogen termination in a Chemical Vapor Deposition (CVD) reactor.
- Sacrificial Layer/Ohmic Contact: A 100 nm thick Au layer was thermally evaporated onto the substrate to serve as both a sacrificial layer and the basis for ohmic contacts.
- Device Isolation: Electrical isolation was achieved by patterning and etching the Au sacrificial layer between device regions using a KI2 solution. The exposed H-terminated diamond surface was then treated with O2 plasma to remove hydrogen termination in these isolation areas.
- Probe Pad Deposition: Ti/Au probe pads were written and deposited to overlap the Au ohmic metal, ensuring robust contact.
- Source-Drain Definition: The source-drain region was patterned and etched using a carefully controlled KI2 solution.
- Gate Stack Fabrication: Gate dimensions (50 nm to 800 nm) were defined using e-beam lithography, followed by the deposition of a metal gate-stack consisting of Al/Pt/Au (50/25/45 nm).
- Surface Annealing: A 350 °C in situ anneal was performed prior to the V2O5 deposition to drive off atmospheric molecules from the diamond surface, crucial for V2O5 work function stability.
- V2O5 Encapsulation: The substrate was encapsulated with 40 nm of V2O5 deposited via thermal evaporation to induce stable surface transfer doping.
Commercial Applications
Section titled âCommercial ApplicationsâThe unique combination of diamondâs intrinsic properties (ultrawide band-gap, high thermal conductivity, high breakdown field) and the high-performance scaling demonstrated by this V2O5-doped FET technology targets several high-demand sectors:
- Radio Frequency (RF) Electronics: High transconductance (153 mS/mm) and current density (700 mA/mm) make these devices ideal for high-frequency power amplifiers and switches in 5G/6G infrastructure and military communications.
- High-Power Switching: Diamondâs 10 MV/cm breakdown field and superior thermal management (>20 W cm-1 K-1) enable next-generation power electronic devices (e.g., inverters, converters) operating at high voltages and temperatures.
- Harsh Environment Electronics: The stability provided by the V2O5 encapsulation suggests suitability for electronics operating under extreme conditions (high temperature, high radiation).
- Compact High-Density Circuits: The successful scaling to 50 nm gate lengths facilitates the integration of diamond FETs into dense, high-speed integrated circuits where small footprints are critical.
- Diamond Metal Semiconductor FET (MESFET) Development: This work validates V2O5 as a robust alternative to traditional volatile dopants, accelerating the maturity of diamond MESFET technology.
View Original Abstract
We report on the fabrication and measurement of hydrogen-terminated diamond field-effect transistors (FETs) incorporating V2O5 as a surface acceptor material to induce transfer doping. Comparing a range of gate lengths down to 50 nm, we observe inversely scaling peak output current and transconductance. Devices exhibited a peak drain current of ~700 mA/mm and a peak transconductance of ~150 mS/mm, some of the highest reported thus far for a diamond metal semiconductor FET (MESFET). Reduced sheet resistance of the diamond surface after V2O5 deposition was verified by four probe measurement. These results show great potential for improvement of diamond FET devices through scaling of critical dimensions and adoption of robust transition metal oxides such as V2O5.