Unlike a resistor, the amount of current through a diode will depend upon 'which way round' we apply the voltage.

When the voltage is applied this way round it tends to pull the free electrons and holes apart, and increases the height of the energy barrier between the two sides of the diode. As a result it is almost impossible for any electrons or holes to cross the depletion zone and the diode current produced is virtually zero. A few lucky electrons and holes may happen to pick up a lot of thermal (kinetic) energy. This gives them enough 'go' to cross the barrier, hence the reversed biassed current is not zero, just very, very small.

When the voltage is applied this way round it tends to push the electrons and holes towards the junction. It also reduces the height of the energy barrier and reduces the width of the depletion zone. These effects make it easier for free electrons and holes with modest amounts of thermal (kinetic) energy to cross the junction. As a result, we get a sizeable current through the diode when we apply a forward bias voltage.

If you look up diodes in a physics book you'll probably find an explanation which finishes up telling you that current through a diode varies exponentially with the applied voltage. The shape of this exponential curve depends upon various factors which include a 'fiddle factor' called the saturation current.

There are two problems with this result. One is that the equation is fairly complicated and quite difficult to use for analysing some circuits. The second problem is that this equation is usually wrong! The reason for this is that the actual current/voltage relationship depends upon the detail of how the diode was made - the choice of materials, doping, etc. Being simple souls who like a quiet life, electronic engineers deal with these problems by simplifying things and using whichever of the following three models of the diode suits them.

Vd represents the height of the diode barrier when no voltage is applied. This means that the energy required for an electron (or hole) to be able to cross the barrier is eVd.

Whichever simple model we adopt, the basic diode maxim is therefore:
Forward bias - all current, almost no volts.
Reverse bias - all volts, almost no current.

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