What is a Varactor Diode?

G.W. Poulos
G.W. Poulos
Man holding computer
Man holding computer

A varactor diode, more commonly referred to as a varicap, is an electronic semiconductor device very closely related to a standard diode but with certain capabilities similar to a capacitor. The difference between a varactor and a standard diode is that a standard diode is designed to minimize the device’s capacitance while a varactor diode is designed to use and exploit capacitance. Varactor diodes find common use in parametric electronics, such as parametric amplifiers and other tuning circuits that can be varied by a change in voltage.

All diodes are constructed of two pieces of different materials fused together with a lead attached to each end. One material is negatively charged, called the cathode, and the other is positively charged, called the anode. When a diode is first created, these two materials exchange electrons where they meet, creating an area within the diode called the depletion layer, which has no favored charge. Creating the two different charges in the diode is accomplished by adding a negatively or positively charged material to the matrix of the diode. In a normal diode, this charged material is added as uniformly as possible across the widths of the cathode and anode to allow the diode to change states as quickly and at as low a voltage as possible.

If a negative voltage is applied to the anode lead of a diode, the potential of the negative voltage combines with the potential of the negative cathode. Once this force is strong enough to bridge the depletion layer, it will easily travel across the positively charged anode of the diode, and the diode will conduct electricity. A diode in this state is said to have a forward bias.

When the same voltage is applied to the cathode lead of a diode, the depletion layer widens from both sides of the anode, with the positive anode sandwiched between the two negative forces creating a capacitive electric field inside the diode. At first this field only slows the flow of electrical current passing through the diode, but as the voltage increases and the field builds in strength, the field will eventually become strong enough to block electrical current from passing through the diode completely. A diode in this state is said to have a reverse bias.

Most diodes are designed to pass as quickly as possible through the area when current still flows and a capacitive electric field exists. A varactor diode, however, is designed to operate within this zone. Unlike a standard diode, the charged material in a varactor is added in a gradient pattern, which varies the strength of the charge across the width of the cathode and anode. The depletion layer of a diode constructed in this manner varies in width in proportion to the voltage applied to it. As a result, the diode can act as a variable capacitor with the capacitive field, decreasing as more voltage is applied and increasing as less voltage is applied.

The varactor diode can be used in place of a more expensive and difficult to install device such as a variable capacitor. Varactor diodes, however, are not always used simply to reduce cost and ease manufacturing processes. Varactors are often used in applications such as television or radio tuners where their abilities allow them to lock onto a channel or station’s frequency very quickly.

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