The Varactor Diode
The term varactor is a shortened form of variable reactor, referring to the voltage-variable capacitance of a reverse-biased p-n junction.
The Varactor Diode:
- If the p-n junction is abrupt, the capacitance varies as the square root of the reverse bias Vr.
- 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 one-third.
- 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 one-half.
- 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 Nd(x) given by Gxm, 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 Vr-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 Cj vs. Vr dependencies available by choosing doping profiles, varactor diodes can be designed for specific applications.
- For some high-frequency applications, varactors can be designed to exploit the forward-bias charge storage capacitance in short diodes.