(0.7v for the silicon tube and 0.3v for the germanium tube).
The positive characteristic is in the electronic circuit, the positive pole of the diode is connected to the high potential end, the negative pole is connected to the low potential end, the diode will lead to the pass, this connection mode is called forward bias. It must be noted that, when the forward voltage applied to both ends of the diode is very small, the diode still fails to guide and the forward current through the diode is very weak. Only when the forward voltage reaches a certain value (which is called the "threshold voltage", the germanium tube is about 0.2v, and the silicon tube is about 0.6v), the diode can be directly connected. The voltage at both ends of the diode is basically unchanged (the germanium tube is about 0.3v and the silicon tube is about 0.7v), which is called the "positive pressure drop" of the diode.
Reverse characteristic in electronic circuit, diode on the low voltage side of the anode, cathode in the high voltage side, now almost no current flows through the diode, the diode is by the state, the connection way, known as reverse biased. When the diode is in the reverse bias, there will still be a faint reverse current flowing through the diode, known as the leakage current. When the reverse voltage on both ends of the diode increases to a certain value, the reverse current will increase sharply, and the diode will lose the single directional conduction characteristic, which is called the breakdown of the diode.