Varactor Diode Structure

The word varactor comes from variable reactor, it is a device whose reactance can be varied in a controlled manner with a bias voltage. The symbol for this diode is

It is used widely in amplifier and harmonic generator, few examples are

-mixing

-detection

- variable voltage tuning

Let the doping distribution be

The abrupt doing profile is achieved by epitaxy or ion-implantation, where m=0

In the varactor diode one side is heavily doped and on the other side the doping and the impurity concentration decreases with distance as shown in Fig 11.1[http://www.vias.org/feee/varactor_diodes.html]

[http://www.vias.org/feee/varactor_diodes.html]

where m is a number < 0 like -5/3, -3/2,-1 etc

Poission's equation in the n-side is given by

Applying the boundary conditions

and (applied voltage) (built-in voltage), where W is the depletion width, we get the capacitance as

Where A is the area of junction and

Therefore the slope of this variation is given by

The variation of C vs. V is shown in Fig 11.2

If then (Hyperabrupt junction), when then (abrupt junction)and when m=0 then S=1/2(graded junction). This is depicted in Fig 11.2

The larger the value of S, larger is the variation of with biasing.

The equivalent circuit of varactor diode is shown in Fig 11.3

Where, is the junction capacitance, is the series resistance, is the parallel equivalent resistance of the generation-recombination current, diffusion current, and the surface leakage current.

Both decreases with reverse bias voltage and increases with reverse bias voltage.

The Quality factor Q of the varactor

The maximum Quality Factor therefore is

The variation of Q with frequency is shown in Fig 11.4

Therefore major considerations for a varactor diodes are (i) Capacitance, (ii) Voltage, (iii) Variation of capacitance with voltage, (iv) Maximum working voltage, (v) Leakage current and at high frequencies for a good varactor diode the equivalent circuit can be represented with lumped elements as