With the rapid development of advanced manufacturing, aerospace systems, deep-earth energy exploration, and high-power electronics, temperature sensors are increasingly required to operate reliably under extremely harsh conditions. In environments involving high temperature, high pressure, corrosion, and radiation exposure, sensors must deliver not only high sensitivity, but also long-term stability and reliability.
Negative temperature coefficient (NTC) thermistors have long been widely used in industrial control systems, automotive electronics, battery management systems, and temperature compensation circuits due to their simple structure, fast response, and high sensitivity. However, conventional metal-oxide-based NTC thermistors are facing growing limitations in demanding applications.
Against this backdrop, diamond, as an ultra-wide-bandgap semiconductor material, is emerging as a highly promising platform for next-generation high-reliability temperature sensing technologies.
Challenges Facing Conventional NTC Thermistors
Most commercially available NTC thermistors are based on metal oxide ceramics or nanocomposite materials, typically involving Mn-, Co-, or Ni-based oxide systems. Although these devices benefit from mature manufacturing processes and relatively low cost, they still exhibit significant limitations in harsh operating environments.
| Challenge |
Limitation |
| Poor high-temperature stability |
Performance drift during long-term operation |
| Limited corrosion resistance |
Reliability degradation in acidic, humid, or chemically aggressive environments |
| Sensitivity degradation |
Reduced response after thermal aging |
| Narrow operating temperature range |
Difficult to meet ultra-high-temperature requirements |
| Limited service lifetime |
Higher failure rates under extreme conditions |
For example, aerospace engines, deep-well drilling systems, and nuclear industrial equipment often require sensors capable of stable operation at temperatures of several hundred degrees Celsius or higher. Conventional NTC materials struggle to maintain long-term reliability under such conditions.
As a result, researchers have increasingly focused on identifying new sensing materials suitable for extreme environments.
Why Diamond Is an Ideal Candidate
Diamond is not only the hardest known natural material, but also one of the most outstanding ultra-wide-bandgap semiconductors.
Its key material advantages include:
| Material Property |
Typical Value |
Technical Advantage |
| Bandgap |
~5.5 eV |
Enables high-temperature electronic operation |
| Thermal conductivity |
>2000 W/m·K |
Rapid thermal response and efficient heat dissipation |
| Breakdown field strength |
~10 MV/cm |
Suitable for high-power systems |
| Chemical stability |
Extremely high |
Excellent corrosion and oxidation resistance |
| Mechanical hardness |
Extremely high |
Suitable for harsh environments |
| Radiation resistance |
Excellent |
Ideal for aerospace and nuclear applications |
These properties make diamond highly attractive for applications such as:
- • High-temperature temperature sensors
- • High-power electronic devices
- • Aerospace electronic systems
- • Deep-well and geothermal exploration
- • MEMS devices for harsh environments
- • Radiation-resistant electronics for nuclear applications
In recent years, researchers have explored diamond NTC thermistors based on boron-doped diamond and grain-boundary-engineered polycrystalline diamond. However, several critical challenges remain:
- • Unclear conduction mechanisms
- • Poor device consistency
- • High fabrication complexity
- • Limited compatibility with large-scale manufacturing
Consequently, developing diamond NTC thermistors that combine high performance, reliability, and scalability has become an important research objective.
Breakthrough in Selective Epitaxial Diamond NTC Devices
Recently, a research team led by Ao Jinping from the School of Integrated Circuits at Jiangnan University, in collaboration with researchers from Xi’an Jiaotong University, published a study titled
“Investigation of diamond NTC thermistors based on selective epitaxial method” in Materials Letters.
The study presents a novel NTC thermistor based on selectively epitaxial
diamond thin films and successfully demonstrates stable negative temperature coefficient behavior without intentional doping.
This achievement provides a new technological pathway for the development of next-generation diamond temperature sensors.
An Unexpected Discovery Led to a New Concept
The research team had originally been investigating selectively epitaxial diamond for ultraviolet photodetectors and Schottky barrier diodes.
During these studies, the researchers unexpectedly observed that selectively grown diamond films exhibited measurable conductivity at room temperature, along with distinct NTC behavior.
This finding suggested that stable diamond thermistors could potentially be realized without conventional doping processes.
The discovery not only simplifies device fabrication, but also improves compatibility with scalable manufacturing technologies.
Figure 1. (a) Schematic illustration of the NTC device and (b) optical image of the fabricated device.
Figure 2. Scanning electron microscopy (SEM) images:
(a) enlarged view of the device morphology;
(b) voids on the tungsten (W) electrode stripes;
(c) etched surface of the tungsten mask and selectively epitaxial layer.
(d) Raman spectrum of the selectively epitaxial layer between the tungsten electrode stripes.
Innovative Device Fabrication Process
The researchers developed a fabrication route with strong process compatibility and manufacturing potential.
1. Buffer Layer Formation
A thin buffer layer was first grown on a type-Ib single-crystal diamond substrate to improve the quality of subsequent epitaxial growth.
2. Interdigitated Electrode Fabrication
Interdigitated tungsten (W) electrodes were fabricated using photolithography and magnetron sputtering techniques.
The interdigitated structure enhances the device response to temperature variations.
3. Selective Epitaxial Growth
Selective epitaxial diamond growth was carried out using a microwave plasma chemical vapor deposition (MPCVD) system, forming an approximately 200 nm thick active layer.
4. Oxygen-Terminated Surface Treatment
Ultraviolet ozone treatment was applied to achieve oxygen surface termination, further improving the electrical performance and operational stability of the device.
Key Process Innovation: In-Situ Formation of Ohmic Contacts
One of the most important innovations of this work is that stable ohmic contacts are formed directly during the selective epitaxial growth process.
During growth, the tungsten mask reacts in situ with the diamond surface, eliminating the need for additional metal deposition steps.
Compared with conventional fabrication approaches, this method offers several advantages:
| Conventional Process |
Proposed Process |
| Requires additional metal deposition |
Ohmic contacts formed in situ |
| More complex fabrication flow |
Simplified process |
| Limited contact consistency |
Improved contact stability |
| Moderate scalability |
Better compatibility with mass production |
This process innovation significantly enhances the commercial potential of the technology.
Device Performance
The fabricated device demonstrated stable operation across an ultra-wide temperature range.
| Parameter |
Performance |
| Operating temperature range |
150–723 K |
| High-sensitivity operating region |
150–477 K |
| Active layer thickness |
~200 nm |
| Electrode material |
Tungsten (W) |
| Device type |
Undoped NTC thermistor |
Within the key operating range of 150–477 K, the device exhibited:
- • High temperature sensitivity
- • Fast transient response
- • Excellent repeatability
- •Stable long-term operation
In addition, the device achieved significantly better low-temperature B-value performance compared with conventional boron-doped diamond thermistors.
Conduction Mechanism Systematically Clarified
The origin of conductivity in selectively epitaxial diamond films has long remained unclear.
Through systematic analysis, the researchers demonstrated that the conductivity primarily originates from tungsten-related impurities introduced during the selective epitaxial growth process.
This conclusion is highly significant because it:
- •Clarifies the physical origin of conduction
- •Provides theoretical guidance for process optimization
- •Improves device reproducibility
- •Supports the development of undoped diamond electronic devices
The findings may also provide valuable insights for future diamond-based sensors and electronic devices.
Future Prospects for Diamond NTC Thermistors
As selective epitaxial growth technology continues to mature, diamond NTC thermistors are expected to play an increasingly important role in several advanced application areas.
Aerospace Systems
Temperature monitoring for engines, hypersonic vehicles, and satellite electronics.
Deep-Well and Energy Exploration
Real-time sensing in high-temperature, high-pressure underground environments.
High-Power Electronics
Thermal management and reliability monitoring of power modules.
Harsh Industrial Environments
Long-term operation under corrosive and radiation-rich conditions.
Overall, this work not only expands the application scope of diamond electronic devices, but also advances the development of next-generation extreme-environment sensing technologies.
Diamond Material Solutions from JXT Technology Co.,Ltd.
JXT Technology Co.,Ltd. specializes in the development and supply of high-quality diamond materials, offering:
Product Portfolio
Typical Applications
| Product Type |
Main Applications |
| Single-crystal diamond substrates |
Power devices, quantum devices, UV photodetectors |
| Polycrystalline diamond substrates |
Thermal management materials, cutting tools, wear-resistant components |
| Diamond thin films |
MEMS devices, temperature sensors, optical windows |
Customization Services
The company provides customized solutions including:
- •Thickness customization
- •Size customization
- •Surface termination treatment
- •Epitaxy-grade specifications
These products and services can meet the diverse requirements of research institutes, university laboratories, and industrial customers.
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