Structure and Characteristics of Varactor Diodes

The V-I characteristic curve of a varactor diode is the same as that of a regular diode, but it operates in the reverse bias region as a reverse-biased diode. Its junction capacitance corresponds to the capacitance of the depletion layer, so the depletion layer can be viewed as a parallel plate capacitor with a dielectric between two conductive plates. The magnitude of the junction capacitance is related to the reverse bias voltage—the higher the reverse bias, the smaller the junction capacitance; conversely, the larger the junction capacitance. The equivalent circuit of a varactor diode is equivalent to a variable capacitor connected in series with a resistor and an inductor, as shown in Figure 1. diode

Figure 1: Formation and Equivalent Circuit of Varactor Diode Capacitance

In the equivalent circuit, Cj represents the junction capacitance, which can be approximated as the total capacitance of a varactor diode, including junction capacitance, housing capacitance, and other distributed capacitances; Rs is the series resistance, comprising PN junction resistance, lead resistance, and wiring resistance; Ls is the lead inductance. Cj, Rs, and Ls are all functions of reverse bias voltage, where the relationship between Cj and reverse bias voltage Vd is the varactor characteristic we intend to utilize, typically expressed by the following formula:diode

In the equation: C0 – PN junction capacitance at zero bias, Vd – reverse bias voltage, V0 – contact potential (0.50~0.75V for silicon varactor diodes)

Figure 2 shows the capacitance characteristic curve of the PN junction in a varactor diode, where the junction capacitance exhibits a nonlinear relationship with reverse bias voltage. To mitigate this nonlinearity, measures such as correction networks, high bias voltage, and multi-loop circuits are often employed in practical applications.diode

Figure 2 Varactor Diode Capacitance Characteristic Curve

The junction capacitance of a regular diode is generally very small. Due to its unique structure, the variable junction capacitance of a varactor diode is quite large, allowing it to be used in circuits like a capacitor.diode