What Is a Varactor Diode?
A varactor diode is a diode whose capacitance can be varied by applying a reverse voltage.
They are also called variable capacitance diodes or varicaps. Generally, when a reverse voltage is applied to the PN junction of a diode, the depletion layer, which is the area without carriers such as electrons and holes, behaves like a capacitor.
A varactor diode actively utilizes the property that the capacitance, which acts as a capacitor, varies depending on the magnitude of the applied voltage.
Uses of Varactor Diodes
Varactor diodes are used in electronic components, such as electronic tuning circuits and voltage controlled oscillators (VCOs). Electronic tuning circuits and voltage controlled oscillators (VCOs) are used in devices that receive radio waves, such as radios, televisions, telecommunications equipment, and mobile communication devices, such as smartphones.
In order for those devices to receive signals at specific frequencies, the capacitance of the capacitors in the receiver and the impedance value of the coil must be adjusted. For this purpose, varactor diodes are used, whose capacitance can be controlled by voltage.
Principle of Varactor Diodes
The principle of varactor diode is to utilize as capacitance the carrier-free depletion layer that is created when a reverse voltage is applied to the PN junction of the diode. Also, to actively use the characteristic that the value of the capacitance depends on the reverse voltage value. In an ordinary diode, forward current flows when a bias voltage is applied in the forward direction, and no current flows when a bias voltage is applied in the reverse direction.
When the voltage is applied in the opposite direction, holes move from the P-type semiconductor part to the electrode and electrons move from the N-type semiconductor part to the electrode, forming a depletion layer with no carriers in the PN junction part. Since there is no charge in the depletion layer and charge is generated on both sides of the depletion layer, the diode behaves like a capacitor.
If the absolute value of the applied voltage in the reverse direction is large, the thickness of the depletion layer increases by that amount, resulting in a smaller equivalent capacitance. A varactor diode is a diode whose capacitance changes by varying the value of the applied reverse voltage. This capacitance change characteristic is effectively utilized.
Other Information on Varactor Diodes
1. LC Resonance Circuit Using Varactor Diodes
Since the capacitance value of a varactor diode can be varied by the bias applied voltage, it is possible to form an LC resonance circuit with an inductor and adjust its resonance frequency.
Using an LC resonant circuit that can be varied by the voltage value, a voltage-controlled oscillator (VCO), for example, incorporates a resonant circuit in a Colpitts oscillator.
A voltage-controlled oscillator (VCO) is a part of the PLL (Phased Lock Loop), which is a frequency adjustment circuit for mobile communications. It is a very important circuit for cellular communication applications between base stations and mobile terminals. As a tuning circuit, the LC resonant circuit is also used in FM modulation applications, such as radio.
2. Etymology and Breakdown Voltage of Varactor and Varicap
The word “varactor” in varactor diode stands for “variable reactor,” meaning a variable reactance, and in the case of varactor diodes, it specifically refers to a capacitive variable reactance. Varicap, on the other hand, is an abbreviation for “variable capacitor” and literally means variable capacitance.
While both varactor and varicap are abbreviations for variable capacitance, it should be noted that the diode bias on the reverse side, where capacitance can be varied, also has a breakdown voltage (breakdown voltage). Check the electrical specifications of the varactor diode you plan to use carefully, and use a voltage within the usable range of the reverse side.
The breakdown voltage (breakdown voltage) is also called the Zener voltage, and diodes that actively use this region are also called Zener diodes. Zener diodes are often used in protection circuits to prevent overvoltages from being applied to the circuitry of the device, or in circuits for constant voltage generation.