Varactor Diode Definition

Varactor diode definition
Varactor diode is a p-n junction diode whose capacitance is varied by varying the

reversevoltage. Before going to varactor diode, let’s first take a look at the capacitor.
What is a capacitor?
A capacitor is an electronic component that stores electrical energy or electric charge in the form of
an electric field. The basic capacitor is made up of two parallel conductive plates separated by a
dielectric. The two conductive plates acts like electrodes and the dielectric acts like an insulator.
The conductive plates are good conductors of electricity so they easily allow electric current
through them. On the other hand, a dielectric is poor conductor of electricity so it does not allow
electric current through it but it allows electric field or electric force.
The positive and negative charges accumulated on both plates exert attractive force on each other.
This attractive force between the plates is nothing but the electric field between the plates.
We know that the capacitance is the ability to store electric charge. So at both plates, the charge is
stored. Thus, there exists a capacitance at both the plates.
What is varactor diode?
The term varactor is originated from a variable capacitor. Varactor diode operates only in reverse
bias. The varactor diode acts like a variable capacitor under reverse bias.
Varactor diode is also sometimes referred to as varicap diode, tuning diode, variable reactance
diode, or variable capacitance diode.
Varactor diode construction
The varactor diode is made up of the p-type and n-type semiconductor. In the n-type
semiconductor, free electrons are the majority carriers and holes are the minority carriers. So the
free electrons carry most of the electric current in n-type semiconductor. In the p-type
semiconductor, holes are the majority carriers and free electrons are the minority carriers. So the
holes carry most of the electric current in p-type semiconductor.
When a p-type semiconductor is in contact with the n-type semiconductor, a p-n junction is formed
between them. This p-n junction separates the p-type and n-type semiconductor.
At the p-n junction, a depletion region is created. A depletion region is a region where mobile
charge carriers (free electrons and holes) are absent.
The depletion region is made up of positive and negative ions (charged atoms). These positive and
negative ions does not move from one place to another place.
The depletion region blocks free electrons from n-side and holes from p-side. Thus, depletion
region blocks electric current across the p-n junction.
Varactor diode symbol
The symbol of a varactor diode is shown in the below figure. The circuit symbol of the varactor
diode is almost similar to the normal p-n junction diode.
Two parallel lines at the cathode side represents two conductive plates and the space between these
two parallel lines represents dielectric.
Photodiode
one component of reverse current in a diode is the flowof minority carriers. These carriers exist
because thermal energy keeps dislodgingvalence electrons from their orbits, producing free
electrons and holes in theprocess. The lifetime of the minority carriers is short, but while they exist,
theycan contribute to the reverse current.
When light energy bombards a pn junction, it can dislodge valenceelectrons. The more light striking
the junction, the larger the reverse current in adiode. A photodiode has been optimized for its
sensitivity to light. In this diode,a window lets light pass through the package to the junction. The
incoming lightproduces free electrons and holes. The stronger the light, the greater the numberof
minority carriers and the larger the reverse current.
Figure shows the schematic symbol of a photodiode. The arrowsrepresent the incoming light.
Especially important, the source and the seriesresistor reverse-bias the photodiode. As the light
becomes brighter, the reverse current increases. With typical photodiodes, the reverse current is in
the tens of microamperes.
OPTOCOUPLER
An optocoupler (also called an optoisolator) combines an LED and a photodiodein a single

package. Figure 5-30 shows an optocoupler. It has an LED on the inputside and a photodiode on the
output side. The left source voltage and the series resistor set up a current through the LED. Then
the light from the LED hits the photodiode, and this sets up a reverse current in the output circuit.
This reverse current produces a voltage across the output resistor. The output voltage then equals
the output supply voltage minus the voltage across the resistor.
When the input voltage is varying, the amount of light is fl uctuating. This means that the output
voltage is varying in step with the input voltage. This is why the combination of an LED and a
photodiode is called an optocoupler. The device can couple an input signal to the output circuit.
Other types of optocouplers use phototransistors, photothyristors, and other photo devices in their
output circuit side.
The key advantage of an optocoupler is the electrical isolation betweenthe input and output circuits.
With an optocoupler, the only contact between theinput and the output is a beam of light. Because
of this, it is possible to have aninsulation resistance between the two circuits in the thousands of
megohms. Isolationlike this is useful in high-voltage applications in which the potentials of thetwo
circuits may differ by several thousand volts.
The Schottky Diode
Schottky diodes are high-current diodes used primarily in high-frequency and fast-switching
applications. They are also known as hot-carrier diodes. The term hot-carrier is derived from the
higher energy level of electrons in the n region compared to those in the metal region. A Schottky
diode symbol is shown in Figure 3-49. A Schottky diode is formed by joining a doped
semiconductor region (usually n-type) with a metal such as gold, silver, or platinum. Rather than a
pn junction, there is a metal-to-semiconductor junction, as shown in Figure 3-50. The forward
voltage drop is typically around 0.3 V because there is no depletion region as in a pn junction diode.
The Schottky diode operates only with majority carriers. There are no minority carriers and thus no
reverse leakage current as in other types of diodes. The metal region is heavily occupied with
conduction-band electrons, and then-type semiconductor region is lightly doped.
When forward-biased, the higher energy electrons in the n region are injected into the metal region
where they give up their excess energy very rapidly. Since there are no minority carriers, as in a
conventional rectifier diode, there is a very rapid response to a change in bias. The Schottky is a
fast-switching diode, and most of its applications make use of this property. It can be used in high-
frequency applications and in many digital circuits to decrease switching times. The LS family of
TTL logic (LS stands for low-power Schottky) is one type of digital integrated circuit that uses the
Schottky diode.
The PIN Diode

The pin diode consists of heavily doped p and n regions separated by an intrinsic (z) region, as
shown in Figure (a). When reverse-biased, the pin diode acts like a nearly constant capacitance.
When forward-biased, it acts like a current-controlled variable resistance. This is shown in Figure
(b) and (c). The low forward resistance of the intrinsic region decreases with increasing current.
The forward series resistance characteristic and the reverse capacitance characteristic are
shown graphically in Figure for a typical pin diode.
The pin diode is used as a decontrolled microwave switch operated by rapid changes in
bias or as a modulating device that takes advantage of the variable forward-resistance
characteristic. Since no rectification occurs at the pn junction, a high-frequency signal can
be modulated (varied) by a lower-frequency bias variation. A pin diode can also be used in
attenuator applications because its resistance can be controlled by the amount of current.
Certain types of pin diodes are used as photo-detectors in fiber-optic systems.
What is a varistor?
A varistor is a voltage dependent resistor (VDR). The resistance of a varistor is variable

and depends on the voltage applied. The word is composed of parts of the words “variable resistor”. Their resistance
decreases when the voltage increases. In case of excessive voltage increases, their resistance drops dramatically. This
behavior makes them suitable to protect circuits during voltage surges. Causes of a surge can include lightning strikes
and electrostatic discharges. The most common type of VDR is the metal oxide varistor or MOV.
Definition
Varistors are nonlinear two-element semiconductors that drop in resistance as voltage increases. Voltage
dependent resistors are often used as surge suppressors for sensitive circuits.
Packages
Here are some examples of different packages which are often encountered. The block packages are used for higher
power ratings.
Disc Block
Characteristics
A voltage dependent resistor has a nonlinear varying resistance, dependent on the voltage applied. The impedance is
high under nominal load conditions, but will sharply decrease to a low value when a voltage threshold, the
breakdown voltage, is exceeded. They are often used to protect circuits against excessive transient voltages. When
the circuit is exposed to a high voltage transient, the varistor starts to conduct and clamps the transient voltage to a
safe level. The energy of the incoming surge is partially conducted and partially absorbed, protecting the circuit.
The most common type is the MOV, or metal oxide varistor. They are constructed of a sintered matrix of zinc oxide
(ZnO)
Applications
The nonlinear characteristic of the varistor make them ideal for use as surge protector devices. Sources of high
voltage transients can for example be lightning strikes, electrostatic discharges or inductive discharge from motors or
transformers. They are for example often used in surge protector power strips. Special types with a low capacitance
protect communication lines. These VDRs are useful for a wide variety of applications that can include:
 Telephone and other communication line protection

 Radio communication equipment transient suppression
 Surge protector power strips
 Cable TV system surge protectors
 Power supply protection
 Microprocessor protection
 Electronics equipment protection
 Low voltage board level protection
 Transient voltage surge suppressor (TVSS)
 Car electronics protection
 Industrial high energy AC protection

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Varactor diode definition

Varactor diode is a p-n junction diode whose capacitance is varied by varying the

An optocoupler (also called an optoisolator) combines an LED and a photodiodein a single

The PIN Diode

A varistor is a voltage dependent resistor (VDR). The resistance of a varistor is variable

 Telephone and other communication line protection

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