Is the v-i relationship of a diode linear or exponential?

The voltage-current (V-I) relationship of a diode is exponential. Specifically, it follows the Shockley diode equation, which indicates that the current through the diode increases exponentially with an increase in voltage, particularly in the forward bias region. In reverse bias, the current remains very small, approaching zero, until breakdown occurs. Thus, the behavior is distinctly nonlinear.