An enhanced two-dimensional hole gas (2DHG) C–H diamond with positive surface charge model for advanced normally-off MOSFET devices
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2022-03-10 |
| Journal | Scientific Reports |
| Authors | Reem Alhasani, Taichi Yabe, Yutaro Iyama, Nobutaka Oi, Shoichiro Imanishi |
| Institutions | Waseda University |
| Citations | 19 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled “Executive Summary”This research successfully models and validates the normally-off (enhancement mode) operation of a p-channel C-H diamond MOSFET, a critical requirement for safe power electronic devices, using a fixed positive interface charge sheet model.
- Core Achievement: Demonstrated normally-off operation (Vth = -3.5 V) in a C-H diamond MOSFET, overcoming the typical normally-on behavior associated with the two-dimensional hole gas (2DHG) layer.
- Mechanism: The normally-off state is achieved by introducing a fixed positive interface charge (Qf = 1 x 1011 cm-2) at the Al2O3/C-H diamond interface, which prevents hole accumulation and channel formation at zero gate voltage.
- Doping Strategy: Deep donor nitrogen (N) doping (1.7 eV from the conduction band minimum) in the diamond bulk pins the Fermi level, enabling the necessary band bending for inversion channel control.
- Performance Metrics: The simulated maximum drain current density (IDS Max) reached -290 mA/mm, closely matching the experimental result of -305.0 mA/mm obtained using an Al2O3/SiO2 gate stack.
- Device Feasibility: The device exhibited a high breakdown voltage of 1275 V (for a 20 µm gate-drain distance), confirming its suitability for high-power applications.
- Modeling Tool: The characteristics were analyzed using the two-dimensional Silvaco Atlas TCAD simulator employing the drift-diffusion model and incomplete ionization model for impurities.
Technical Specifications
Section titled “Technical Specifications”| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum Drain Current Density (Simulated) | -290 | mA/mm | Positive charge model, VGS = -40 V, VDS = -30 V |
| Maximum Drain Current Density (Experimental) | -305.0 | mA/mm | Al2O3/SiO2/C-H Diamond, VGS = -40 V |
| Threshold Voltage (Vth) | -3.5 | V | Simulated normally-off operation (Enhancement mode) |
| Transconductance (gm) | 0.4 | mS/mm | Simulated, VDS = -0.5 V |
| Breakdown Voltage (Experimental) | 1275 | V | Device with LGD = 20 µm |
| Fixed Positive Interface Charge (Qf) | 1 x 1011 | cm-2 | Required density for normally-off simulation |
| Nitrogen (Donor) Concentration (ND) | 2 x 1016 | cm-3 | Substrate doping concentration |
| Boron (Acceptor) Concentration (NA) | 2 x 1015 | cm-3 | Substrate doping concentration |
| Nitrogen Donor Level (ED) | 1.7 | eV | Fixed Fermi level position from Conduction Band Minimum (CBM) |
| Diamond Bandgap (Eg) | 5.5 | eV | Material property |
| Electron Affinity (EA) | -1.3 | eV | C-H diamond surface property |
| Hole Mobility (Surface) | 100 | cm2/Vs | Parameter used in modeling |
| Gate Oxide Thickness (tox) | 200 | nm | ALD-Al2O3 layer |
| Channel Length (LCH) | 4 | µm | Simulated device dimension |
Key Methodologies
Section titled “Key Methodologies”The device operation was analyzed using a combination of experimental fabrication and two-dimensional (2D) device simulation (Silvaco Atlas TCAD).
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Device Structure Modeling:
- Modeled a C-H diamond MOSFET structure with a 4 µm thick diamond substrate.
- Gate stack consisted of 200 nm ALD-Al2O3 (simulated) or 2 nm SiO2 under Al2O3 (experimental).
- Source/Drain contacts were modeled as ideal Schottky contacts (SBH of 0.1 eV) using Au/Ti.
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Doping and Fermi Level Pinning:
- Substrate was doped with low Boron (acceptor, 2 x 1015 cm-3) and higher Nitrogen (donor, 2 x 1016 cm-3).
- The deep donor level of Nitrogen (1.7 eV from CBM) was used to fix the Fermi level position in the bulk, which is necessary for controlling the inversion channel.
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Interface Charge Modeling:
- Three fixed interface charge sheet models were investigated: Negative (normally-on), Neutral (Vth = 0 V), and Positive (normally-off).
- The positive charge model (Qf = 1 x 1011 cm-2) was selected to reproduce the normally-off operation observed experimentally.
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Simulation Physics:
- The 2D drift-diffusion model was used for carrier transport analysis.
- The incomplete ionization model for impurities was applied, recognizing that nitrogen in diamond is hard to ionize (deep donor).
- Poisson’s equation was used to define the space charge based on the electrostatic potential and charge density (including mobile and fixed charges).
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Experimental Validation:
- 2DHG diamond MOSFETs were fabricated using a thin SiO2 layer (2 nm) under the gate to introduce positive charge effects.
- Experimental IDS-VDS and Vth distributions confirmed that the SiO2 layer successfully induced normally-off operation (Vth = -3.5 V), validating the positive interface charge model used in the simulation.
Commercial Applications
Section titled “Commercial Applications”The development of high-performance, normally-off p-channel diamond MOSFETs addresses critical needs in high-power electronics where safety and efficiency are paramount.
- Power Electronics and Inverter Systems: Diamond’s wide bandgap (5.5 eV) and high thermal conductivity (22 W/cmK) make it ideal for high-power FETs, enabling low switching losses and high efficiency in inverter systems.
- High Breakdown Voltage Devices: The demonstrated 1275 V breakdown voltage supports applications requiring robust, high-voltage switching capabilities.
- Complementary Power MOSFETs (CMOS): The successful realization of a normally-off p-channel device is essential for developing diamond-based complementary power MOSFETs, potentially using vertical FETs or trench gates.
- High-Frequency Devices: Diamond’s high carrier mobility (4500 cm2/Vs for electrons, 3800 cm2/Vs for holes) supports high-frequency operation in RF power amplifiers and switching circuits.
- Smart Inverter Systems: Enabling low on-resistance and high toughness for next-generation smart grid and electric vehicle (EV) power management systems.