Introduction

Electrical resistance measures how strongly a component opposes current. Many ordinary metals have higher resistance when warmed, because charge carriers are scattered more strongly. This is not a universal rule: semiconductor carrier populations and thermistor materials can produce a different temperature dependence.

Factors That Increase Resistance

Several factors cause electrical resistance to go up as temperature rises:

Thermal Vibrations of Atoms

All materials are made up of tiny atoms that are constantly vibrating and moving around. As temperature increases, the atoms vibrate more intensely. These stronger vibrations disrupt the flow of electricity, making it more difficult for electrons to travel through the material.

Thermal Expansion

Thermal expansion changes a conductor's length and cross-sectional area. Because resistance depends on both geometry and resistivity, expansion can change resistance, but extra space between atoms is not a simple additional obstacle. For ordinary metals, temperature-dependent scattering is the main explanation.

Electron-Lattice Interactions

The orderly arrangement of atoms in a material is called a crystal lattice. Higher heat causes greater interaction between electrons and the crystal lattice. Electrons bounce off the lattice more frequently, impeding the current flow.

Changes in Band Structure

Band structure refers to allowed energy levels for electrons in a material. As temperature rises, the relative position between lower energy valence bands and higher energy conduction bands changes. This alters the number of free electrons able to carry current, affecting resistance.

Specific Examples

Looking at how resistance changes with temperature in real materials helps illustrate the underlying scientific principles:

Metals

Metals like copper and aluminum are good conductors. But their electrical resistance steadily increases with temperature. Why? Greater thermal vibrations of atoms and expansion of the crystal lattice hamper electron mobility.

Semiconductors

In semiconductors, temperature changes both the number of mobile charge carriers and their mobility. In an intrinsic semiconductor, heating can create enough additional carriers to lower resistance despite increased scattering. Doping and the temperature range affect the outcome; heating does not universally raise semiconductor resistance.

Carbon

Carbon-based materials differ in structure and composition. Some carbon resistors have a negative temperature coefficient over their operating range. There is no universal temperature at which all carbon switches from decreasing to increasing resistance.

The Bigger Picture

The balance between carrier population, scattering and geometry determines a material's resistance-temperature curve. The positive coefficient familiar from copper and aluminum should not be generalized to every conductor or semiconductor.OpenStax distinguishes these temperature dependencies.

Understanding why resistance increases with temperature has useful applications. Materials with known resistance-temperature relationships, like platinum RTDs, can measure temperature. And materials engineered for low, stable resistance-temperature coefficients aid electronics design. So shedding light on the underlying science empowers innovation and progress.

Anomalies in Resistance-Temperature Dependency

While the general trend is for resistance to increase with temperature, there are exceptions to this rule. These can be found in certain types of conductive materials, and understanding these anomalies provides deeper insights into the behavior of electrical resistance.

Superconductors

Superconductors are materials that, when cooled to extremely low temperatures, display zero electrical resistance. The mechanism behind this phenomenon, known as superconductivity, involves the formation of electron pairs that move through the material without scattering off the lattice atoms. However, when the temperature rises above a certain critical temperature specific to the material, the resistance abruptly returns, marking the end of the superconducting state.

Thermistors

Thermistors are a type of resistor whose resistance varies significantly with temperature. Negative Temperature Coefficient (NTC) thermistors decrease in resistance as temperature increases, an inversion of the typical behavior. This property makes thermistors useful for measuring temperature in various applications, from automotive to household appliances.

Resistance and Temperature: Practical Implications

Understanding how temperature affects resistance has important practical implications for the design and operation of electronic devices.

Mitigating Heat in Electronics

As electronic devices operate, they generate heat. If this heat is not effectively managed, it can raise the temperature of the device’s components, leading to increased resistance and decreased performance. Understanding how resistance changes with temperature can help in the design of effective cooling systems for electronic devices.

Temperature Compensation Circuits

In some electronic devices, it is important to maintain a consistent output regardless of changes in temperature. Temperature compensation circuits use components with known resistance-temperature characteristics to adjust the output of the device as the temperature changes, ensuring steady performance.

Sources & further reading

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