Explanation of the volt ampere characteristics of diodes

Explanation of the volt ampere characteristics of diodes

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

(1) Positive characteristics

The first quadrant of the diode voltage characteristic curve is called the forward characteristic, which represents the working condition of the diode when an external forward voltage is applied. At the beginning of the forward characteristic, due to the small forward voltage, the external electric field is not sufficient to overcome the hindering effect of the internal electric field on the majority carriers, and the forward current is almost zero. This region is called the volt ampere characteristic curve of the forward diode

Dead zone, the corresponding voltage is called dead zone voltage. The dead zone voltage of silicon transistor is about 0.5V, and the dead zone voltage of germanium transistor is about 0 2V.

When the forward voltage exceeds a certain value, the internal electric field is greatly weakened, the forward current rapidly increases, and the diode conducts. This area is called the forward conduction region. Once a diode conducts in the forward direction, any slight change in forward voltage will cause a significant change in forward current, resulting in a steep forward characteristic curve of the diode. Therefore, when the diode is conducting in the forward direction, the forward voltage drop on the diode is not significant, and the change in forward voltage drop is very small. Generally, the silicon diode is o Around 7V, germanium diode is 0 About 3V. Therefore, when using diodes, if the applied voltage is high, it is generally necessary to connect a current limiting resistor in series in the circuit to prevent excessive current from burning out the diode.

(2) Reverse characteristic

The third quadrant of the diode voltage characteristic curve is called the reverse characteristic, which represents the working condition of the diode when a reverse voltage is applied. Within a certain range of reverse voltage, the reverse current is very small and does not change much, and this area is called the reverse cutoff region. This is because reverse current is formed by the drift motion of minority carriers; At a certain temperature, the number of minority carriers remains basically constant, so the reverse current is basically constant and independent of the magnitude of the reverse voltage, hence it is commonly referred to as the reverse saturation current.

(3) Reverse breakdown characteristic

When the reverse voltage continues to increase to a certain value, the reverse current in the diode will suddenly increase, and we call it reverse breakdown of the diode. This characteristic is shown in section D of Figure 1.2.6. When reverse breakdown occurs, there is a large reverse current in the PN junction, which can cause damage to the PN junction in severe cases. Therefore, ordinary diodes should be avoided from breakdown, but voltage regulators 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 voltage regulators can play a stabilizing role.