- The varactor diodes are one of the type of PN junction diode whose internal capacitance can be changed by applying the river by voltage. So these diodes are also known as the vericap diodes and the term vericap refer to the variable capacitance. Similarly the term varactor refers to the variable reactance.
- So, in this diode we can change the applied reverse bias voltage its capacitance or in other words it’s reactance will change and this property of the varactor diode is very useful in many application. Particularly when we want to change the capacitance in the circuit using the external applied voltage then this varactor diode are very useful. So because of this property they are used in the RF communication system and the symbols indicate that it is the PN junction diode one but unlike the normal rectifier diode it is intended to be used as a capacitor. So to understand working of varactor diode first of all let’s see how we can check the capacitance of the parallel plate capacitor.
- So for a parallel plate capacitor the capacitance can be given as C=eA/d, where A is the area of the plates while e is the permittivity of the dielectric material and this is the difference between the two plates or the thickness of the dielectric materials. So for the fixed value of the e and A, if we change the distance between the two plates or in other words if we change the thickness of the dielectric medium then the capacitance of the capacitor will change. So with the same concept the capacitance of the reactor diode is varied.
- Now as said the varactor diode is also one kind of PN junction diode. You must be aware that in the PN junction diode the deflection region gets formed at the junction where this p-type and n-type material meets with each other. So this p-type material contains excessive amount of holes while the n-type material contains the electrons.
On the other add if you see it is depletion region then it is depleted of the charger carriers. So in a way this PN junction diode behave like capacitor means year this p-type and n-type region act like conducting plates while the deflection region act like a dielectric medium and know that for the PN junction diode as we change applied reverse bias voltage than the width of the depletion region changes. That means as we increase the applied reverse bias voltage in the width of the deflection region will also increase and as the depletion region with increased then from this equation we can say that value of the capacitor and will reduce. That means we can say that this capacitance is inversely proportional to the applied reverse biased voltage and if we see the exact relation then it can be given by the following expression, where v is the applied reverse bias voltage and PF is a forward voltage or the built-in world voltage of the diode. and here this Co represent the capacitance of the diode wheneven it is unbiased.