When there is
no voltage applied to the diode, some
of the free electrons in the n-type layer migrate to fill the holes
in the the p-type layer. (This occurs without any external voltage
applied. Press the start button restart the animation.) The movement
of the electrons creates a voltage difference between the two layers
known as the barrier voltage.
If a negative
voltage is connected to the anode, it forces more electrons into
the p-type layer, raising the barrier voltage and cutting off the
flow of current. (Press the blue button on the animation for a demonstration.)
If a positive
voltage of greater than 0.6 volts is applied to the anode, the barrier
voltage is overcome and electrons will flow through the diode. (Press
the red button on the animation.)