Non-ohmic Devices

A non-ohmic device does not exhibit a linear relationship between voltage and current, unlike ohmic conductors that follow Ohm's law. Diodes are common examples of non-ohmic devices. Their resistance changes with applied voltage, making their current-voltage relationship nonlinear. This behavior contrasts with materials where current flow is directly proportional to applied voltage.

In forward bias, the anode connects to positive potential and the cathode to negative potential. When the applied voltage exceeds 0.7 volts, the diode conducts current. As voltage increases, current increases significantly, but the voltage across the diode remains approximately 0.7 volts. The diode's resistance becomes nearly zero in this state.

Under reverse bias, the anode connects to negative potential and the cathode to positive potential. The diode exhibits extremely large resistance with negligible current flow. This blocking state persists until the applied voltage reaches the breakdown voltage, at which point exponential current increase occurs. Essentially zero current flows through the diode and circuit until breakdown.

Breakdown voltage is the largest reverse voltage a diode can withstand before experiencing exponential leakage current increase. Once this threshold is exceeded, the diode begins conducting in reverse bias. This voltage represents a critical operating limit for diodes in circuits. Understanding breakdown voltage prevents device damage and circuit failure.

Plotting current versus voltage for a diode produces a nonlinear curve, clearly showing that voltage and current lack proportional relationship. The graph displays a sharp increase in current above 0.7 volts in forward bias and minimal current in reverse bias until breakdown. This curved characteristic distinguishes non-ohmic devices from linear ohmic conductors.

In forward bias, the diode's resistance decreases as voltage increases, allowing current to rise while the voltage across it stabilizes near 0.7 volts. This occurs because the semiconductor material's internal structure changes with applied voltage. The diode essentially regulates voltage across itself while accommodating increasing electrical current through the circuit.

The anode is the positive terminal and the cathode is the negative terminal of a diode. Their polarity relative to applied voltage determines whether the diode conducts or blocks current. When anode is positive and cathode is negative, forward bias allows conduction. Reversing this polarity creates reverse bias, blocking current flow until breakdown voltage is reached.