Diode I vs. V

 

We all learned Ohms law at some point: V=IR. This means for a resistor, the current is just V/R or that there is a linear relationship between I and V. (That is, plotting I vs. V for a given resistor would give you a straight line with positive slope 1/R.)

This linear relationship between I and R does not hold for semiconductor devices, as the above graphs for I vs. V of a Silicon diode show. In fact, the relationship is roughly exponential for a moderate range of currents (below 20 milliamps). Eventually as the current through the diode rises, the voltage levels off and the graph becomes more like an S. Then past a certain current, the diode sizzles and you need a new diode.

So, basically the idea of a diode is to severely restrict until a certain threshhold voltage across it is reached (roughly .6 V for a silicon diode). Once that threshhold is reached, the voltage drop across the diode stays relatively constant across a wide range of currents, as long as you don't exceed the diode's maximum current. (Or put a voltage directly across it.)