Q.A silicon power diode has $V_j$ (the drop across the p+ n- junction) of 0.4 V, $R_{ON}$ (ohmic drop) in the drift region of 0.002 $\Omega$ and $I_F$ = 75 A. Determine $V_{AK}$.

The V-I characteristics of a power diode describe how its current depends on the applied voltage. Like a signal diode, the current follows the Shockley diode equation

where $I_S$ is the reverse saturation current, $V$ the applied voltage, $q = 1.6\times10^{-19}\ \text{C}$ the electron charge, $k = 1.381\times10^{-23}\ \text{J K}^{-1}$ Boltzmann's constant and $T$ the absolute temperature. Both the forward and reverse curves shift with junction temperature.

When a power diode conducts in the forward direction, its anode-to-cathode voltage has two parts: the drop $V_j$ across the p+ n- junction, and the ohmic drop across the resistive drift region. Writing the ohmic term as $R_{ON} I_F$ gives

Because this ohmic term rises linearly with forward current, the forward V-I characteristic of a power diode is straighter (more linear) than that of an ordinary signal diode — a key distinguishing feature.

In numerical problems the thermal voltage $V_T = kT/q$ is evaluated at the stated temperature, and the exponential term makes the forward current rise steeply for a small increase in forward voltage. These relationships let you calculate the forward current or the forward drop for a given operating point, a common II PUC Electronics exam problem.