pH Measurement at Elevated Temperature with Vessel Gate and Oxygen-Terminated Diamond Solution Gate Field Effect Transistors

At a Glance

Metadata	Details
Publication Date	2022-02-25
Journal	Sensors
Authors	Shuto Kawaguchi, Reona Nomoto, Hirotaka Sato, Teruaki Takarada, Yu Hao Chang
Institutions	Waseda University
Citations	7
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Temperature Diamond SGFETs

Executive Summary

This research successfully demonstrates the viability of Boron-Doped Diamond (BDD) Solution Gate Field Effect Transistors (SGFETs) for pH sensing in harsh, high-temperature environments, specifically targeting applications in the food industry.

High-Temperature Operation: C-O BDD SGFETs were proven to operate stably in electrolyte solutions up to 95 °C, significantly exceeding the limitations of conventional glass electrodes.
System Achievement: A highly sensitive pH sensing system was realized by combining a pH-insensitive C-O BDD SGFET with a highly pH-sensitive Stainless Steel (SUS304) “Vessel Gate.”
Performance Metric: The combined system achieved a high sensitivity of -54.6 mV/pH at 80 °C, corresponding to 77.9% of the theoretical Nernst response (70.1 mV/pH).
Material Mechanism: Increased temperature led to enhanced activation of boron acceptors (Activation Energy: 0.15 eV), resulting in a tripling of channel conductance and transconductance, confirming the material’s suitability for high-temperature electronics.
Surface Chemistry: The use of chemically stable, oxygen-terminated (C-O) diamond ensures robustness and biocompatibility, essential for industrial and biosensing applications.
6CCVD Value Proposition: This work validates the need for high-quality, precisely doped MPCVD diamond substrates, a core specialization of 6CCVD, for developing next-generation solid-state sensors.

Technical Specifications

The following hard data points were extracted from the research paper detailing the device performance and material properties:

Parameter	Value	Unit	Context
Maximum FET Operating Temperature	95	°C	C-O BDD SGFET operation limit
pH Sensing Temperature (System)	80	°C	Highest temperature tested for combined system
System pH Sensitivity (80 °C)	-54.6	mV/pH	Vessel Gate + C-O BDD SGFET combination
Nernst Response (80 °C)	70.1	mV/pH	Theoretical maximum response at 80 °C
System Efficiency (80 °C)	77.9	%	Sensitivity relative to Nernst response
BDD Substrate Type	Polycrystalline	N/A	Material used for device fabrication
Substrate Dimensions	5 x 5	mm²	Device footprint
Channel Length (L)	200	µm	SGFET geometry
Channel Width (W)	5	mm	SGFET geometry
Source/Drain Metalization	Gold (Au)	N/A	Electrode material
Metalization Thickness	100	nm	Deposited via E-beam evaporation
Boron Activation Energy (E_i)	0.15	eV	Corresponds to 6-8 x 10¹⁹ atoms cm^-3 concentration
Transconductance Increase (RT to 95 °C)	3	Times	Observed increase using Ag/AgCl electrode

Key Methodologies

The fabrication and testing of the C-O BDD SGFETs relied on precise MPCVD growth and controlled surface termination processes:

Substrate Selection: Black polycrystalline diamond substrates (5 x 5 mm²) were utilized.
BDD Layer Deposition: The boron-doped layer was deposited onto the polycrystalline diamond using Microwave Plasma Chemical Vapor Deposition (MPCVD).
Acid Cleaning: The surface was cleaned in a mixture of nitric acid (HNO₃) and sulfuric acid (H₂SO₄) at 200 °C for 30 minutes.
Hydrogen Termination (C-H): The diamond surface was fully hydrogen-terminated using microwave-enhanced plasma CVD.
Electrode Fabrication: Gold (Au) source and drain electrodes (100 nm thickness) were deposited via electron-beam evaporation, defining the 200 µm channel length.
Insulation: Conductive parts were covered with insulating epoxy resin.
Oxygen Termination (C-O): The channel region was converted to oxygen termination using a plasma reactor in an oxygen atmosphere.
pH Measurement: Sensitivity was calculated from the V_GS shift required to maintain a constant drain current (I_DS) using Carmody buffer solutions.
Gate Configuration: Measurements were performed using both a standard Ag/AgCl glass electrode and a Stainless Steel (SUS304) Vessel Gate.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced MPCVD diamond materials and customization services required to replicate, optimize, and scale this high-temperature pH sensing technology.

Applicable Materials

Research Requirement	6CCVD Solution	Technical Advantage
Boron-Doped Polycrystalline Diamond (BDD)	High-Purity PCD (BDD)	Cost-effective, scalable material for large-area sensor arrays (up to 125 mm).
High-Performance FETs	Optical Grade SCD (BDD)	Eliminates grain boundaries, offering superior carrier mobility, uniformity, and reduced gate leakage current for enhanced device stability at high temperatures.
Precise Doping Control	Custom BDD Doping Profiles	Guaranteed control over boron concentration (N_A) and depth ($h$) to achieve the critical carrier density (< 10¹³ cm^-2) necessary for optimal SGFET modulation.

Customization Potential

The success of this research hinges on precise material engineering, which aligns perfectly with 6CCVD’s custom capabilities:

Custom Dimensions & Scalability: The paper used small 5 x 5 mm² substrates. 6CCVD offers PCD plates/wafers up to 125 mm and SCD substrates up to 10 mm thick. This capability is essential for scaling up the Vessel Gate system for industrial deployment or creating multi-sensor arrays.
Thickness Control: The paper highlights the crucial nature of controlling the boron-doped depth. 6CCVD guarantees SCD and PCD layer thicknesses from 0.1 µm to 500 µm, providing the necessary precision for depletion layer engineering.
Metalization Services: The study used 100 nm Au electrodes. 6CCVD provides in-house custom metalization using robust materials such as Au, Pt, Pd, and Ti, ensuring reliable, high-adhesion contacts that withstand the harsh chemical and thermal environment (95 °C+).
Surface Quality: While the paper used polycrystalline diamond, 6CCVD can provide SCD with ultra-low roughness (Ra < 1 nm). This superior polishing minimizes surface defects, ensuring highly uniform and repeatable plasma termination (C-O or C-H) crucial for stable pH sensitivity.

Engineering Support

6CCVD’s in-house PhD team specializes in diamond material science and device physics. We can assist researchers and engineers with material selection, optimizing doping profiles, and designing metal contact schemes for similar High-Temperature pH Sensing projects, ensuring maximum Nernst efficiency and long-term operational stability.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

Diamond has many appealing properties, including biocompatibility, ease of surface modification, and chemical-physical stability. In this study, the temperature dependence of the pH-sensitivity of a oxygen-terminated boron-doped diamond solution gate FET (C-O BDD SGFET) is reported. The C-O BDD SGFET operated in an electrolyte solution at 95 °C. At 80 °C, the pH sensitivity of C-O BDD SGFET dropped to 4.27 mV/pH. As a result, we succeeded in developing a highly sensitive pH sensing system at −54.6 mV/pH at 80 °C by combining it with a highly pH sensitive stainless-steel vessel.

Tech Support

Original Source

DOI: https://doi.org/10.3390/s22051807

References

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