As with PN Junction, diodes have unidirectional conductivity. Typical volt-ampere characteristic curves of silicon diodes (fig). In the diode plus positive voltage, when the voltage value is small, the current is very small; when the voltage exceeds 0.6V, the current starts to increase exponentially, usually called the diode open voltage; When the voltage reaches about 0.7V, the diode is in a fully conductive state, usually called the voltage as the diode's conduction voltage, expressed by the symbol UD.
For germanium diodes, the open voltage is 0.2V, and the conduction voltage UD is about 0.3V.
In the diode with a reverse voltage, when the voltage value is small, the current is minimal, its current value is reverse saturation current is. When the reverse voltage exceeds a value, the current starts to increase sharply, called the reverse breakdown, called the voltage diode for the reverse breakdown voltage, with symbolic UBR said. Different types of diode breakdown voltage UBR value varies considerably from decades to thousands of volts.