High-value resistors in the megaohm range are widely available in a range of package styles. Manufacturing resistors in the gigaohm range (1 GΩ and above) presents greater challenges. These ultra-high-value components require specialised materials and production techniques.
In this post, we consider the electrical properties of high-value resistors. We cover the key technical considerations and the challenges faced by resistor manufacturers.
High Value Resistors Applications
Very high-value resistors are often used to minimise measurement errors. Their primary application is in low electronic signal measurement equipment.
Applications include biomedical systems where measurements in the picoampere range are required, high gain amplifiers such as photodiode applications and instrumentation.
High-value resistors are also found in high-voltage applications. These include high voltage power supplies, variable frequency motor drives and electric vehicle battery management.
Comparing Resistor Technologies
It is impractical to construct high-value wirewound resistors. This is due to the maximum resistivity and minimum wire diameter of wire alloys. The wire length required for very high resistance makes manufacture within an acceptable form factor impractical.
Due to their construction bulk metal foil and thin film resistors maximum resistance tends to be in the 20MOhms range. However, some specialist devices are available up to approximately 100MOhm.
In contrast, thick film high-value resistors in the GigaOhm range are readily available. Devices with low TeraOhm resistance values are available from specialist manufacturers.
Electrical properties Of Thick Film High-Value Resistors
Optimising any electronic component for a high resistance value often involves compromises. When selecting a thick film resistor it is important to consider:
- Tolerance ranges.
- Temperature coefficient of resistance (TCR).
- Voltage coefficient of resistance (VCR).
- Power ratings.
- Noise.
Tolerance – To achieve high resistance values the resistance path must be very long and/or very narrow. Controlling material and manufacturing consistency across the resistance path is a significant challenge. Therefore, thick film high-value resistors tend to have tolerance values from 1% upwards.As a manufacturer increases the target ohmic values, the tolerance band tends to widen accordingly.
TCR – The TCR of high-value thick film resistors tends to be higher due to factors including material composition, film thickness and complex resistor track geometry. The combination of these factors makes it more challenging to control the TCR.
VCR – VCR is typically measured in parts per million per volt (ppm/V). Due to the inherent resistor material properties of high-value resistors, they often exhibit a positive VCR. This means resistance increases slightly with increased voltage.
Power Rating – High-value thick film resistors generally have lower power ratings than low-value resistors. In high-voltage applications, a high voltage across a low-value resistor will (according to Ohm’s Law) cause a high current to flow. Without adequate thermal management, this could damage the resistor.
To achieve very high resistance values, the resistive film must be extremely thin, or have a high proportion of insulating material. This combination reduces the thermal conductivity and makes it more difficult to transfer heat away from the resistive element. Hence high-value resistors have lower power ratings (a few Watts maximum) to avoid overheating the resistor.
Noise – Thick film resistors usually generate more thermal noise than other resistor types. This effect is more pronounced in high-value resistors due to their higher resistance and lower current-carrying capacity.
Manufacturing Challenges
High-value resistors require precise geometric patterns to fine-tune resistance and depositing a very thin resistive layer is crucial. Controlling the thickness of the film to achieve GigaOhm resistances without defects is challenging.
The resistance value of a thick film resistor depends on the composition of the resistor paste. Specifically the composition of the glass, the metal oxide conductive particle size and the number of conductive particles per unit area.
The higher the metal oxide to glass ratio, the lower the resistivity. Hence high-value resistors use high resistivity paste with a relatively low number of conductive particles. Manufacturers can use specialist GOhm inks, but they are very sensitive to external influences.
The firing process must be carefully controlled. Temperature variations can affect the microstructure of the resistive film, leading to inconsistencies in resistance. The resistive film can shrink and cause cracking or delamination.
Ensuring the long-term stability and reliability of GigaOhm resistors is difficult. Any degradation in the resistive film or changes in the interface between the film and substrate can lead to significant shifts in resistance.
Thick film resistors with resistance values in the GOhm range and above are ultra sensitive to environmental issues such as electrical noise will upset the resistance value.
High-value resistors are particularly sensitive to moisture ingress. Even a fingerprint on the resistive area can cause changes in resistance values.
Conclusion
Resistors, particularly in the gigaohm range, present unique material and manufacturing challenges.
Thick film technology enables resistances up to low teraohms but with trade-offs in tolerance, TCR, VCR, power rating, and noise characteristics. Manufacturing challenges include precise geometric patterning, controlled paste composition, film deposition, and firing.
Engineers must consider the challenges faced by thick film high-value resistor manufacturers when specifying components. Material and manufacturing challenges can be overcome, but at a cost.