Varactor diode or varicap diode is a semiconductor diode that significantly changes its junction capacitance with respect to the applied reverse bias voltage. Many semiconductor PN junction diodes are designed differently to exhibit specific electrical properties. One such semiconductor diode is a varactor diode or varicap diode. It is specifically designed to exhibit unique junction capacitance characteristics. These diodes are useful in RF circuits for tuning signals. This is why a class of varactor diodes is also called tuner diodes.
Example of varactor diodes.
It should be noted that the desired operation of varactor diode is achieved only in reverse bias condition. Why? We'll find out later in this article. In the reverse bias condition, the internal capacitance of the diode changes significantly with respect to the applied reverse voltage. This is why, rather than voltage-current, voltage-capacitance is the important characteristic of a varactor diode.
What is a varactor diode or varicap diode?
Varactor diode is a semiconductor diode whose internal capacitance changes with respect to the applied reverse voltage. It is a voltage-controlled device that acts as a variable semiconductor capacitor. It is also called varicap diode, tuning diode, tuning diode, voltcap or voltage variable capacitance.
Varactor diodes are shown in electronic circuits by many symbols. The most popular symbols to indicate a varactor diode in a circuit are shown below.
Varactor diode symbol.
Varactor diode construction
Varactor diodes are designed in such a way that they have a very narrow depletion region. It is just a semiconductor diode with p-type and n-type layers. The n-type layer is connected to a gold-plated molybdenum pin through a table frame. This works as the cathode of the diode. The p-type layer is connected to another gold-plated molybdenum pin via gold wire. This works as the anode of the diode. Obviously, the polarity of a diode is really important, and that of a varactor diode is even more important, as it must essentially only work in reverse bias. Polarity is often indicated by distinguishing the cathode by a dot, or a different lead structure, or a band.
Varicaps are available in through-hole and SMD packages. Through-hole packages usually have a cathode indicated by a dot or a distinct lead structure. SMD packages have a cathode indicated by a dot or stripe at one end. Varactor diodes come in Transistor Outline (TO), Diode Outline (DO), Small Outline Diode (SOD), and Small Outline Transistor (SOT) packages. In some packages, three terminals can be seen, only two of which need to be connected. Varactor diodes designed for low frequency operation are made of silicon and those designed for high frequency operation are made of gallium arsenide.
One of the unique characteristics of varactor diodes is their doping. Unlike conventional semiconductor diodes (signal and power diodes) that have uniform doping throughout the p-type and n-type layer, in varactor diodes, the concentration of impurity atoms is lower near the junction, which gradually increases in the rest of the layer P. type and n-type layers. This type of doping is done to reduce the length of the depletion region.
How the varicap diode works
A varactor diode is expected to function as a variable capacitor. A capacitor is an electrical device that stores charge when a voltage is applied to it. As the voltage is reversed, the capacitor discharges the stored charge. When a capacitor is charging, current flows through it creating an electrostatic field across it. When a capacitor discharges through a load or during the negative cycle of an AC signal, it lets current flow through it in the reverse direction until its plates are electrically neutralized. The capacitance of a capacitor is nothing more than a measurement of the charge it can store in relation to the applied charging voltage. The capacitor is capable of storing charge due to opposition to the flow of current through it by its dielectric medium situated between two conducting surfaces.
A diode has transition and diffusion capacitance. Capacitance is shown by a diode because the depletion region is devoid of free charge carriers that act as a dielectric medium. The layer of negatively charged ions in the n-type material and the layer of positively charged ions in the n-type material act as conductive surfaces where charge is stored when there is no bias or when reverse bias voltage is applied.
A varactor diode is designed to have a narrow depletion region. Due to the unique doping pattern and physical structure of the diode, the length of the depletion region changes drastically upon application of the reverse bias voltage. The capacitance or charge storage capacity of a capacitor device is inversely proportional to the distance between its conducting surfaces and directly proportional to the area of the conducting surfaces. A varactor diode is designed to have a large junction area and a narrow depletion region. This is achieved by the mesa structure of the diode at the cathode end and the unique doping pattern. In the unbiased condition, the diode has greater capacitance. In the reverse bias condition, as the reverse bias voltage increases, the length of the depletion region increases and the junction capacitance of the diode decreases. When forward biased, the depletion region gradually disappears with the forward voltage and the diode enters the conducting state. Therefore, to operate a varactor diode as a voltage-controlled variable capacitor, it must be connected in reverse bias.
Features of Varactor diode
Unlike conventional diodes that have voltage-current as the most important characteristics, for varactor diodes voltage-capacitance is the most important electrical characteristic. Varicaps are designed to have a high capacitance variation with respect to the applied reverse voltage. The typical voltage-capacitance plot of a varactor diode is shown below.
Voltage-capacitance characteristics of varactor diode
Varator diode types
Varactor diodes are classified as Abrupt Varactor Diodes and Hyperabrupt Varactor Diodes. In abrupt varactor diodes, doping is increased linearly from the depletion region to the diode terminals. As a result, the capacitance decreases almost linearly with increasing reverse voltage. In hyperabrupt varactor diodes, doping is increased nonlinearly from the depletion region to the diode terminals. As a result, the capacitance changes exponentially with respect to the reverse voltage.
Varactor Diode Specifications
Some of the important electrical characteristics of a varactor diode are given below.
- Reverse current: This is the reverse leakage current of the varicap through its junction for a specified reverse voltage.
- Capacitance Ratio: This is the ratio of the minimum capacitance to the maximum capacitance of the varactor diode. A smaller value of this ratio shows a larger capacitance range.
- Diode Capacitance: This is the minimum capacitance for a specified AC reverse voltage and frequency.
- Junction Operating Temperature: This is the maximum range of junction temperatures that the varactor can withstand.
- Power Dissipation: This is the maximum power dissipated by the diode in its conducting state for a specified ambient temperature. It is also an indication of the maximum voltage and current that the varactor can tolerate.
Advantages of varactor diodes
Varactor diodes have several advantages. These are non-mechanical variable capacitors. They can be easily used in a semiconductor circuit. These are small and compact. These diodes produce less noise compared to other types of diodes.
Applications of varactor diodes
Varactor diodes are used in tuning circuits or for frequency modulation. Some of the notable applications of varicap are below.
- FR Filters
- Automatic frequency controllers
- High frequency radios
- Bandpass filters
- Voltage Controlled Oscillators
- Harmonic generators
- Frequency Multipliers
Varactor Diode Reference
Following is a list of some popular varactor diodes with notable specifications.
