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Why are some conductors non ohmic ?

The term “non-ohmic” refers to the behavior of certain materials that do not obey Ohm’s Law, which states that the current through a conductor is directly proportional to the voltage across it, given a constant temperature. In other words, in non-ohmic conductors, the relationship between voltage and current is not a straight line. This behavior can be attributed to various factors that affect the conductivity of the material. Here’s a detailed explanation:

  1. Temperature Dependency:
    • One common reason for non-ohmic behavior is the dependence of conductivity on temperature. In many materials, especially semiconductors and some electrolytes, the conductivity changes significantly with temperature. As the temperature increases, the mobility of charge carriers (electrons or ions) may change, affecting the overall conductivity and causing a deviation from Ohm’s Law.
  2. Semiconductors:
    • Semiconductors, such as silicon and germanium, are classic examples of non-ohmic conductors. Unlike metals, where charge carriers (electrons) have high mobility, the conductivity of semiconductors is highly sensitive to temperature and the concentration of charge carriers. Increasing the temperature or altering the doping level can lead to non-linear changes in conductivity.
  3. Light Dependency (Photoconductivity):
    • Some materials exhibit non-ohmic behavior when exposed to light. This phenomenon is known as photoconductivity. In photoconductive materials, the presence of light increases the number of charge carriers, leading to a non-linear relationship between voltage and current. This effect is commonly observed in certain types of photoresistors and photodiodes.
  4. Ionically Conductive Materials:
    • Electrolytes, which are ionically conductive materials, can also display non-ohmic behavior. The movement of ions in these materials is influenced by factors such as ion concentration and the presence of impurities. Changes in these parameters can lead to variations in conductivity that do not adhere strictly to Ohm’s Law.
  5. Non-Uniform Cross-Section:
    • In some cases, a non-uniform cross-sectional area of the conductor may result in non-ohmic behavior. Variations in cross-section can lead to uneven current distribution and affect the overall resistance of the material.
  6. Threshold Voltage in Semiconductor Devices:
    • In semiconductor devices like diodes and transistors, the voltage-current relationship is highly non-linear. These devices exhibit a threshold voltage below which little current flows, and above which there is a rapid increase in current. This behavior is crucial for their function in electronic circuits.

In summary, conductors exhibit non-ohmic behavior when certain factors, such as temperature, light exposure, material properties, and structural variations, lead to deviations from the linear relationship between voltage and current prescribed by Ohm’s Law. Understanding and characterizing non-ohmic behavior are essential for designing and analyzing electronic components and systems where these materials are employed.

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