The voltage-current (V-I) characteristics of a diode describe the relationship between the voltage applied across its terminals (u) and the current flowing through it (I), which can be represented as I=f(U). The V-I characteristics of the 2CP12 (common type silicon diode) and 2AP9 (common type germanium diode) are explained as follows:
(1)Forward Characteristics.
The first quadrant of the diode's V-I characteristic curve represents the forward characteristics, indicating the behavior of the diode under forward bias conditions. In the initial part of the forward characteristics, when the forward voltage is very small, the external electric field is not strong enough to overcome the internal electric field that hinders the majority carriers. As a result, the forward current is nearly zero in this region, which is known as the dead zone of the diode's V-I characteristic curve. The corresponding voltage is called the dead zone voltage. The dead zone voltage is approximately 0.5V for silicon diodes and 0.2V for germanium diodes.
When the forward voltage exceeds a certain value, the internal electric field weakens significantly, leading to a rapid increase in forward current and diode conduction. This region is known as the forward conduction region. Once the diode enters forward conduction, even a slight variation in forward voltage causes a significant change in forward current. Therefore, the forward characteristic curve of a diode is steep. Consequently, when a diode is in forward conduction, the voltage drop across the diode is relatively small and remains almost constant. Typically, the voltage drop across a silicon diode is around 0.7V, while that of a germanium diode is around 0.3V. To prevent excessive current flow that could damage the diode, it is common practice to connect a current-limiting resistor in series when using a diode with a high applied voltage.
(2)Reverse Characteristics.
The third quadrant of the diode's V-I characteristic curve represents the reverse characteristics, indicating the behavior of the diode under reverse bias conditions. Within a certain range of reverse voltage, the reverse current is very small and remains relatively constant. This region is known as the reverse cutoff region. The reverse current is generated by the drift motion of minority carriers. At a given temperature, the number of minority carriers remains essentially constant, resulting in a constant reverse current that is independent of the reverse voltage magnitude. Therefore, this reverse current is commonly referred to as the reverse saturation current.
(3)Reverse Breakdown Characteristics.
As the reverse voltage continues to increase beyond a certain value, the reverse current in the diode suddenly increases. This phenomenon is known as reverse breakdown. During reverse breakdown, a significant amount of reverse current flows through the PN junction, which can lead to PN junction damage. Hence, ordinary diodes should be protected from reaching the breakdown region. However, voltage regulator diodes (Zener diodes) are designed to operate in the breakdown region intentionally because although the current varies significantly, the voltage remains nearly constant. It is this characteristic that allows Zener diodes to function as voltage regulators.
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