Structure and characteristics of varactor diodes

The volt ampere characteristic curve of a varactor diode is the same as that of a regular diode, except that it operates in the reverse bias region. As a reverse bias diode, its junction capacitance is the capacitance of the depletion layer. Therefore, the depletion layer can be regarded as a parallel plate capacitor with a dielectric between two conductive plates. The size of the junction capacitance is related to the magnitude of the reverse bias, and the larger the reverse bias, the smaller the junction capacitance; On the contrary, 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 inductor, as shown in Figure 1.

Figure 1 Formation and equivalent circuit of variable capacitance diode capacitor

The turtle in the equivalent circuit is a variable junction capacitor, which can be approximated as the total capacitance of a variable capacitance diode. It includes junction capacitance, shell capacitance, and other distributed capacitance; Rs， For series resistance, it includes PN junction resistance, lead resistance, and wiring resistance; Ls is the lead inductance. Cj, Rs, and Ls are all functions of reverse bias voltage, and the relationship between Cj and reverse bias voltage Vd is exactly the variable capacitance relationship we want to use, which can usually be expressed by the following equation:

In the formula: C0- PN junction capacitance Vd at zero bias - reverse bias V0- contact potential, silicon varactor diode is 0.50-0.75V

Figure 2 shows the PN junction capacitance characteristic curve of a varactor diode, and the relationship between junction capacitance and reverse bias is nonlinear. In order to overcome nonlinearity, correction networks, high bias voltages, and multiple loops are often used in applications.

Figure 2 Capacitance characteristic curve of varactor diode

The junction capacitance of a general diode is very small. Due to the special structure of a varactor diode, its variable junction capacitance is very large and can be used in circuits like a capacitor.