Varactor diodes are the one type of pn junction in which by applying reverse bias voltage we change the internal capacitance of the diode. So, these diodes are also known as the varicap diodes. The term varicap refers to the variable capacitance. Similarly the term varactor refers to the variable reactance. The varactor diode was first developed in 1961 by Pacific Semiconductor Subsidiary. So, in this diode, as we change the applied reverse bias voltage, its capacitance or in other words its reactance will change. This property of the varactor diode is very useful in many applications. Particularly when we want to change the capacitance in the circuit using the external applied voltage the frequency of the circuit will change. So, because of this property they are used in the RF communication systems and this is the symbol of the varactor diode.
The symbol indicates that, it is the pn junction diode but unlike the normal rectifier diode it is intended to be used as a capacitor. All the diodes can exhibits the variable capacitance but the variations are manufactured to exploit the effect and increase the capacitance variation. Varactor diode is more sensitive towards the depletion region.
Working of the Varactor diode:
So, to understand the working of this varactor diode, first of all we have to know that how we can change the capacitance of the parallel plate capacitor. So for a parallel plate capacitor, the capacitance can be given as:
C = (∈A)/d
A is the Area of the plates while, ε is the permittivity of the dielectric material and d is the distance between the two plates or the thickness of the dielectric material. So, for the fixed value of the ε and A, if we change the distance between the two plate or in other words, if we change the thickness of the dielectric medium then the capacitance of the capacitor will change. When we increase the distance between the two plates then the capacitance will decrease and when we decrease the distance between the two plates the capacitance will increase. Similarly when we increase the area of the plates the capacitance will increase and when we decrease the area of the plates the capacitance will decrease.
So, with the same concept, the capacitance of the varactor diode is varied. Now, as I said, the varactor diode is also one kind of pn junction diode. The depletion region gets formed at the junction where this p-type and the n-type materials meet with each other. So, this p-type material contains excessive amount of holes while the n-type material contains the electrons. In P type when trivalent impurity is added in silicon means the atoms whose valance electrons are three and silicon consist of four electrons. When these two atoms combine it will form 7 electrons so there will be deficiency of one electron due to which hole will be created. Similarly in N type material when penta valaent element are added means the atom which have 5 valance electrons in silicon then there will be one extra electron and it will be called N type material. The holes in the p type material will attract the electrons in the N type material. Due to which some electrons will migrate from N type material to P type material due to which depletion region will be formed and the region will be neutral.
On the other end, if you see this depletion region, then it is depleted of the charge carrier. So, in a way this pn junction diode behaves like a capacitor. That means here this p-type and n-type regions acts like a conducing plates while the depletion region acts like a dielectric medium. As we know that the dielectric material act like insulator and neutral atom will also act like an insulator. We will apply the reverse bias voltage to the varactor diode by connecting the battery positive terminal to the n type material and negative terminal to the P type material. We know that for the pn junction diode, as we change the applied reverse bias voltage then the width of the depletion region changes because the positive terminal of the battery will attract the electrons and the negative terminal will attract the holes due to which the depletion region will be increase. The depletion region will become thick. That means as we increase the applied reverse bias voltage, then the width of the depletion region will also increases means that the depletion region will become thicker. As we know that when we increase the voltage the width of the depletion region will increase so we can say that there is direction relation between the voltage and the width of the depletion region.
The capacitance will decrease when we increase the voltage and vice versa. So there will inverse relation between the voltage and capacitance.
As the depletion region width increases, then from this equation we can say that the value of the capacitance will reduce. That means we can say that, this capacitance is inversely proportional to the applied reverse voltage and if we see the exact relation, then it can be given by the following expression.
C= C0/(1+|V/Vf |)n
Where V is the applied reverse bias voltage, and Vf is the forward voltage or the built-in voltage of the diode. And here this Co represents the capacitance of the diode whenever it is unbiased. 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 types of 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 when the value of n= 1/2 for this type of junction. But at the junction there is an abrupt change in the doping profile.
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 rapidly with the distance. For this type of doping profile the value of n =2 So, from this expression we can say that for the hyper-abrupt junction as the value of this reverse bias voltage increases, then there will be a more change in the capacitance. So, in general, for the normal pn junction diode, if we see the curve of the capacitance verses the applied reverse bias voltage then it will look like this.
That means in general any diode can be used a variable capacitor. But the varactor diodes are optimized and manufactured in such a way that it provides the more change in the capacitance with the applied reverse bias voltage.
So, now let’s see the equivalent circuit of the varactor diode. So, if we apply the reverse bias voltage, then in that condition the equivalent circuit of the varactor diode will look like this:
So, here Rr is the reverse resistance of the diode and Rs is the ohmic resistance. Now, typically the value of this Reverse resistance is in Mega Ohms. Becuase 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 represeted 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. For lower frequencies the equivalent circuit of the varactor diode will be such that:
But whenever it is operated at the high frequencies then we also need to consider the parasitic capacitance and inductance. So, this is the equivalent circuit of the varactor diode at high frequencies.
Important specification of the Varactor diodes:
Alright, so now let’s see some of the important specifications of the varactor diodes which we need to consider while selecting the diode for specific application. The first and foremost important specifications are the capacitance range and capacitance ratio. So, if you see the datasheet of any varactor diode then they used to specify the value of the capacitances at the different voltages. For example, as shown in the datasheet, as the reverse voltage changes from 1.2 V to 8V, then the capacitance changes from around 450pF to 25 pF. So, basically this parameter gives an idea, in what range we can change the value of the capacitance for the given diode.
Then the second important specification is the capacitance ratio. The capacitance ratio can be expressed as:
The capacitance can be measured at the ends which are x and y that represents the voltage range. For example over here, the value of x = 1.2 V, while the value of y = 8V and it defines, as we change the voltage then how much change can occur in the capacitance value. That means if this ratio is more, then it defines that, as we change the voltage then there will be a more change in the capacitance value. For the hyper abrupt varactor diodes, it is 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, for the better tunability of the circuit, the large capacitance ratios are desirable. Another specification of the varactor diode is the reverse current or the leakage current. 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 these varactor diodes are operated in the reverse bias, condition. So, while operating thee diodes we need to make sure that, the applied reverse bias voltage does not exceed this breakdown voltage. Because if it exceeds the breakdown voltage the varactor diode will be damage.
Q-factor is another important parameter of the varactor diode. So, whenever the varactors diodes are used in the tuning circuits or in the RF fliters, then for the sharp response, the diodes 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. So, these are the some of the performance parameters for the varactor diodes.
Applications of the Varactor diodes:
The tuning circuits consist of varactor diode as I mentioned earlier. For example, as shown in figure, here the varactor diode is employed in the tuning circuit.
So, for the given tuning circuit, the resonant frequency
fr= 1/(2π √(LC’))
Where C’= CT + Cc
So, here CT is the capacitance of the varactor diode, while Cc is the coupling capacitor. The value of the capacitor can be change by changing the applied reverse bias voltage. So, in this way, we can tune this circuit to the particular frequency.
So, these varactor diodes are used in the FM receivers as well in other communication equipments where the circuit needs to be tuned electronically. Apart from that they are used in the RF fliters, as well as in the voltage control oscillators. Some of the applications are:
- Self-adjusting bridge circuits
- FM radio and TV receiver
- Tuning of LC resonant circuit in microwave frequency multipliers
- Adjustable band pass filter
- Very low noise microwave parametric amplifiers
So, these are the some of the applications of the varactor diode.