The volt-ampere characteristic refers to the relationship between the voltage u applied across the diode and the current flowing through the diode, that is, I=f(U). The volt-ampere characteristics of 2CP12 (ordinary silicon diode) and 2AP9 (ordinary germanium diode).

(1) Forward characteristics
The first quadrant of the diode volt-ampere characteristic curve is called the forward characteristic, which indicates the working condition of the diode when a forward voltage is applied. At the beginning of the forward characteristic, since the forward voltage is very small, the external electric field is not enough to overcome the obstruction of the internal electric field on the majority carriers, and the forward current is almost zero. This area is called the forward diode volt-ampere characteristic curve

Dead zone, and the corresponding voltage is called the dead zone voltage. The dead zone voltage of the silicon tube is about 0.5V, and the dead zone voltage of the germanium tube is about 0.2V.

When the forward voltage exceeds a certain value, the internal electric field is greatly weakened, the forward current increases rapidly, and the diode is turned on. This area is called the forward conduction zone. Once the diode is forward-conducting, any slight change in the forward voltage will cause the forward current to change significantly, and the forward characteristic curve of the diode is very steep. Therefore, when the diode is forward-conducting, the forward voltage drop on the tube is not large, and the change in the forward voltage drop is very small. Generally, it is about 0.7V for silicon tubes and about 0.3V for germanium tubes. Therefore, when using a diode, if the applied voltage is large, a current-limiting resistor should generally be connected in series in the circuit to avoid excessive current that burns out the diode.

(2) Reverse characteristics
The third quadrant of the diode voltage characteristic curve is called the reverse characteristic, which indicates the working condition of the diode when a reverse voltage is applied. Within a certain reverse voltage range, the reverse current is very small and does not change much. This area is called the reverse cutoff region. This is because the reverse current is formed by the drift motion of minority carriers; at a certain temperature, the number of minority carriers is basically unchanged, so the reverse current is basically constant and has nothing to do with the magnitude of the reverse voltage, so it is usually called the reverse saturation current.

(3) Reverse breakdown characteristics
When the reverse voltage continues to increase to a certain value, the reverse current in the diode will suddenly increase. We call this the reverse breakdown of the diode. This characteristic is shown in segment D of Figure 1.2.6. When reverse breakdown occurs, the PN junction has a large reverse current, which will cause damage to the PN junction in severe cases. Therefore, ordinary diodes should avoid breakdown, but Zener diodes must be in a breakdown state because in the breakdown region, although the current changes greatly, the voltage can remain basically unchanged. It is precisely by utilizing this characteristic that the Zener diode can play a role in voltage stabilization.