Forward Characteristics

The forward V-I characteristic is obtained by forward-biasing the diode through a current-limiting resistor R, then varying the DC supply and recording the forward voltage V (x-axis) against the resulting forward current I (y-axis). Three inferences follow from the resulting curve. First, at room temperature a significant forward current only begins once V reaches roughly the barrier potential -- this is called the threshold, cut-in, or KNEE voltage $V_{th}$, about 0.3 V for germanium and 0.7 V for silicon; below it the current is negligible, but beyond it current rises sharply for only a small further rise in voltage. Second, the curve is clearly NOT a straight line but an exponential -- the diode does not obey Ohm's law. Third, the forward resistance $r_f$ is defined not as a fixed ratio $V/I$ but as the SLOPE of the curve, $r_f = \Delta V/\Delta I$, i.e. the ratio of a small change in voltage to the resulting small change in current -- so the diode's resistance is itself different at every point on the curve, decreasing as forward current increases. If the applied forward voltage is pushed beyond the diode's rated value, the resulting large current can destroy the junction through overheating -- this is diode breakdown, and the voltage at which it happens is the breakdown voltage; a diode should always be operated well between the threshold voltage and the breakdown voltage.

Worked example (ideal diode, forward bias). An ideal diode (zero barrier voltage, behaves as a closed switch when forward biased) is connected in series with a 5 $\Omega$ resistor across a 15 V supply, with the diode's polarity such that it is forward biased. Since the ideal diode contributes no voltage drop, Ohm's law applies directly to the resistor: $I = V/R = 15/5 = 3$ A. …