I-V characteristics refer to the relationship between the voltage u applied across the diode and the current flowing through the diode, i.e., I = f(U). I-V characteristics of 2CP12 (general-type silicon diode) and 2AP9 (general-type germanium diode).
(1) Forward Characteristics The first quadrant of the diode's I-V characteristic curve is called the forward characteristic, which represents the diode's operation when a forward voltage is applied. At the beginning of the forward characteristic, because the forward voltage is very small, the external electric field is insufficient to overcome the resistance of the internal electric field to the majority carriers, and the forward current is almost zero. This region is called the dead zone of the diode's I-V characteristic curve, and the corresponding voltage is called the dead zone voltage. The dead zone voltage of a silicon diode is approximately 0.5V, and that of a germanium diode is approximately 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 conducts. This region is called the forward conduction region. Once a diode is forward-biased, even a slight change in the forward voltage will cause a significant change in the forward current, resulting in a very steep forward characteristic curve. Therefore, when a diode is forward-biased, the forward voltage drop across the diode is small, and the change in forward voltage drop is minimal, typically around 0.7V for silicon diodes and around 0.3V for germanium diodes. Therefore, when using diodes, if the applied voltage is large, a current-limiting resistor is generally connected in series in the circuit to prevent excessive current from burning out the diode.
(2) Reverse Characteristics The third quadrant of the diode's voltage-voltage characteristic curve is called the reverse characteristic, which represents the diode's operation when a reverse voltage is applied. Within a certain reverse voltage range, the reverse current is very small and does not change much; this region 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 remains essentially constant, so the reverse current is essentially constant and independent of the magnitude of the reverse voltage; therefore, 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 reverse breakdown. 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 can lead to damage to the PN junction in severe cases. Therefore, ordinary diodes should be protected from breakdown. However, Zener diodes must operate in the breakdown state because although the current changes significantly in the breakdown region, the voltage remains basically constant. It is by utilizing this characteristic that Zener diodes can play a voltage-regulating role.
