Now in this equation the value of n depends on the doping profile of the varactor diode. So, based on the doping profile there are two type of a varactor diodes.
The first one is the abrupt varactor diode. So, this is the doping profile of the abrupt varactor diode so as you can see in this p-type and n-type regions, the doping concentration is uniform. But at the junction there is an abrupt change in the doping profile. So for this type of junction the value of n=1/2, so usually the normal PN junction diodes have this abrupt junction.
On the other end this is the profile of the hyper-abrupt varactor diode and as you can see on both sides, the doping concentration reduces with the distance and for this type of doping profile the value of n=2. So from the expression we can say that for the hyper-abrupt junction as the value of this reverse bias voltage increases, then there will be more change in the capacitance.
So in general any diode can be used as a variable capacitor but the reactor diode are optimise and manufactured in a such a way that they provides the more change in the capacitance with applied reverse bias voltage.
So here this Rr is the reverse resistance of the diode and this Rs is the ohmic resistance. Now typically the value of reverse resistance is it megaohms because whenever we reverse bias this diode then only leakage current or the reverse saturation current will flow through the diode.
And that current can be represented by this reverse resistance so for the varactor diode, to minimise this leakage current or the reverse saturation current this resistance should be as high as possible.
So this is the equivalent circuit of the reactor diode at the low frequencies but whenever it is operated at the high frequencies then we also need to consider the parasitic capacitance and inductance so this is is the equivalent circuit of director diode at high frequency.
So now let’s see some of the important specification of the varactor diode which we need to consider while selecting the diode for specific application and the first and foremost important specification are the capacitance range and capacitance ratio.
So if you see the data sheet of any varactor diode then they used to specify the value of the capacitance at the different voltages. For example the reverse voltage changes from 1.2V to 8V. Then the value of the capacitance changes from around 450 pF to 25 pF. So basically this parameter is given an idea in what range we can change the value of the capacitance for the given diode.
The second important specification is the capacitance ratio and this is commonly expressed as Cx/Cy, where x and y are two end of the voltage range over which the measuring the capacitance. For example, over here the value of x=1.2V, while the value of y=8V.
And it defines which change the voltage then how much change can occur in the capacitance value that mean if the ratio is more than it define that, as we change the voltage then there will be a more change in the capacitance value.
For the hyper abrupt varactor diode is it possible to achieve the ratio which is more than 10. While for the abrupt diodes usually it is used to be in the range of 2 to 3. So or the better tunability of the circuit the large capacitance ratio are desirable.
Another specification is the leakage current or the reverse current. So as I mentioned, for the varactor diode the reverse current or the leakage current should be as minimum as possible.
Similarly the another important specification is the breakdown voltage. Because this varactor diode are operated in the reverse bias condition. So while operateing this diode we need to make sure that the applied reverse bias voltage does not exceed this breakdown voltage because if it is exceeds then it can damage the diode.
Then the another important parameter in the Q-factor. So whenever this varactor diode are used in the tuning circuit or in the filters, then for the sharp response the diode with the high Q-factor should be selected. That means for the good selectivity the value of this Q-factor should be as high as possible.
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