Diodes

A  diode functions as the electronic version of a one-way valve. By restricting the
direction of movement of charge carriers, it  allows an electric current to flow in one
direction, but blocks it in the opposite direction.
A diode's current-voltage, or I-V, characteristic can be approximated by two regions of
operation. Below a certain difference in potential between the two leads, the diode can
be thought of as an open (non-conductive) circuit. As the potential difference is
increased, at some stage the diode will become conductive and allow current to flow, at
which point it can be thought of as a connection with zero (or at least very low)
resistance. In a typical semiconductor p-n diode, conventional current can flow from the
p-doped side to the n-doped side, but not in the opposite direction. When the diode is
reverse-biased, the charge carriers are pulled away from the center of the device, creating
a depletion region. More specifically, the transfer function is logarithmic, but so sharp
that it looks like a corner.