Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Junction Properties
The Varactor Diode
The term varactor is a shortened form of variable reactor, referring to the voltagevariable capacitance of a reversebiased pn junction.
The Varactor Diode:
 If the pn junction is abrupt, the capacitance varies as the square root of the reverse bias V_{r}.
 In a graded junction, however, the capacitance can usually be written in the form
 For example, in a linearly graded junction the exponent n is onethird.
 Thus the voltage sensitivity of C, is greater for an abrupt junction than for a linearly graded junction.
 For this reason, varactor diodes are often made by epitaxial growth techniques, or by ion implantation.
 The epitaxial layer and the substrate doping profile can be designed to obtain junctions for which the exponent n is greater than onehalf.
 Such junctions are called hyperabrupt junctions.
 In the set of doping profiles shown in Fig, the junction is assumed p^{ }n so that the depletion layer width W extends primarily into the n side.

Three types of doping profiles on the n side are illustrated, with the donor distribution N_{d}(x) given by Gx^{m}, where G is a constant and the exponent m
is 0,1, or 3/2.  We can show that the exponent n in the above equ is 1/(m 2) for the p^{ }n junction.
 Thus for the profiles of Fig, n is 1/2 for the abrupt junction and 1/3 for the linearly graded junction.
 The hyperabrupt junction with m = 3/2 is particularly interesting for certain varactor applications, since for this case n = 2 and the capacitance is proportional to V_{r}^{2} .
 When such a capacitor is used with an inductor L in a resonant circuit, the resonant frequency varies linearly with the voltage applied to the varactor.
 Because of the wide variety of C_{j} vs. V_{r} dependencies available by choosing doping profiles, varactor diodes can be designed for specific applications.
 For some highfrequency applications, varactors can be designed to exploit the forwardbias charge storage capacitance in short diodes.