26.7: Non-ohmic Devices
In most substances, the current flow is proportional to the voltage applied to it. A simple relationship between the values of current, voltage, and resistance is known as Ohm's law. Nonohmic devices do not exhibit a linear relationship between voltage and current. One such device is the semiconducting circuit element known as a diode. A diode is a circuit device that allows current flow in only one direction.
Consider a simple circuit consisting of a battery, a diode, and a resistor. A diode consists of two terminals: an anode (positive terminal) and a cathode (negative terminal). When the anode is connected to the negative potential and the cathode is connected to the positive potential, the diode is said to have reverse bias. With reverse bias, the diode has an extremely large resistance and there is very little current flow—essentially zero current through the diode and the resistor. As the voltage applied to the circuit increases, the current remains essentially zero until the voltage reaches the breakdown voltage and the diode conducts the current. The breakdown voltage is defined as the largest reverse voltage that can be applied without causing an exponential increase in the leakage current in a diode. When the battery and the potential across the diode are reversed, making the anode positive and the cathode negative, the diode conducts and current flows through the diode if the voltage is greater than 0.7 V. The resistance of the diode is close to zero. A graph can be plotted between current and voltage, which shows that the voltage and the current do not have a linear relationship.