What is a variable-capacitance (varicap) diode?

A varicap diode is a product that utilizes depletion layer capacitance characteristics generated when reverse voltage is applied to a PN junction. The depletion layer works as a gap between electrodes. Capacity varies as the depletion layer changes according to the applied voltage.

A variable-capacitance diode is a type of pn junction diode. Unlike typical pn junction diodes, variable-capacitance diodes are used in regions where the pn junction is reverse-biased but does not enter breakdown.

Fig. 1 p-n junction diode I-V curve and usage range of variable capacitance diode
Fig. 1 p-n junction diode I-V curve and usage range of variable capacitance diode

a pn junction diode, a depletion region is formed around the pn junction as shown in the following band diagram (Fig. 2). In a pn junction diode, a depletion region is formed around the pn junction as shown in the following band diagram (Fig. 2). The depletion region has high electrical resistance like an insulator due to the depletion of mobile carriers. In this state, the depletion region behaves like a capacitor. The depletion region widens in proportion to reverse voltage.
Variable-capacitance diodes are designed to exploit this voltage-dependent capacitance of the depletion region.

Capacitance is expressed as follows:

C＝K x ( VR + Φ )-n

K: Device-specific constant determined by dopant concentration, junction area, etc.
Φ: Diffusion potential

There are several types of pn junctions including linearly graded, abrupt, and hyper-abrupt junctions. Variable n in the above equation depends on the type of junction. n is 1/3, 1/2, or 1 or greater for linearly graded, abrupt, and hyper-abrupt junctions, respectively. The rate of change in capacitance increases with the value of n. Typical pn junction diodes such as switching diodes have an oblique junction whereas variable-capacitance diodes have an abrupt or hyper-abrupt junction.

Fig. 2 Band diagram of a pn junction
Fig. 2 Band diagram of a pn junction
Fig. 3 Classification of pn junctions (dopant profiles)
Fig. 3 Classification of pn junctions (dopant profiles)

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