nonOhmic

Nonohmic describes electrical components whose current does not flow in direct proportion to voltage, i.e., they do not obey Ohm's law. In nonohmic devices the resistance is not constant but varies with the operating conditions such as voltage, current, temperature, or time.

Consequently, their I-V characteristic is nonlinear. A device may be nonohmic over a broad range of voltages or only in particular regions. The slope of the I-V curve, known as the differential or dynamic resistance (dV/dI), changes with the operating point.

Common nonohmic devices include diodes (junctions, LEDs, Zener diodes), which exhibit exponential or threshold-type I-V behavior; transistors (BJTs, MOSFETs) in active regions; thermistors (NTC and PTC) whose resistance changes with temperature; varistors and many switches; and temperature- or light-dependent resistors. Incandescent lamps are also nonohmic because filament resistance increases with temperature as it lights.

In circuit design, nonohmic behavior is modeled with device equations such as the diode equation or transistor equations, and dynamic resistance is used in small-signal analysis. Understanding nonohmic behavior is essential for rectification, amplification, sensing, and switching.