The condition of the diode is to give the forward voltage, and is greater than the diode voltage! 0.7v is the positive guide voltage of the silicon tube (the germanium tube is about 0.3v), and the voltage at both ends of the diode is basically unchanged.
1. The diode plus external forward voltage (plus the reverse voltage is not conductive).
2. The forward voltage must be greater than the dead zone voltage of the diode. Diode voltage dead zone: with the forward voltage, at the start of a positive feature, forward voltage is very small, not enough to overcome the p-n junction electric field inside the block effect, forward current is almost zero, this period is called dead zone. The forward voltage that does not make the diode conduction is called the dead zone voltage.
3. When the forward voltage is greater than the dead zone voltage, the electric field in the PN junction is overcome, and the diode is conducting the guide, and the current increases rapidly with the increase of the voltage. In the normal current range, the terminal voltage of the diode is almost constant, and this voltage is called the forward voltage of the diode. When the positive voltage on both ends of the diode exceeds a certain value, the internal electric field is quickly weakened, and the characteristic current is rapidly increasing, and the diode is guiding through.