The diode, as a zero-order approximation, can be considered to be a one-way current flow device that allows current to flow from the anode to the cathode but not in reverse.
As first-order model, the diode forward current is modeled as I=Is*exp(V/kt), and therefore has a logarithmic current vs. voltage characteristic. For currents in the range of >1uA in typical diode, this results in an apparent voltage threshold of around 0.5 to 0.7 volts.
Typical diodes have reverse breakdown voltages anywhere from 5 to 100 volts, depending on the application and construction of the diode. Reverse breakdown is the reverse-bias threshold above which the diode experiences avalanche multiplication of carriers in the depletion region.
