Different types of diodes with factors and uses

There are different types of diodes depending on the frequency used, their function and construction, their application, etc.

Depending on Junction Diodes

Junction diodes are common P-N junction diodes but differ in construction. There are three types of junction diodes,

Rectifier Diode

These diodes are common P-N junction diodes, which allow current to flow only in one direction and stop in the other direction. These diodes are used to convert the alternating current into DC in the rectifier circuit.

We can see the same rectifying diode with metal projection. Which is added to the diode to reduce the heat distribution which can sometimes affect the diode. This type of metal projection is called a heat sink. These help in improving the performance of the diode and the diode is able to withstand higher powers without being affected.

Half wave rectifier and full wave rectifier circuits use these diodes. These rectifier circuits are used in the power supply sections of many circuits where there is a need to convert alternating input current to direct current for that circuit applications.

Zener Diode

It is a special type of diode that allows current to flow not only in the forward direction but also in the opposite direction. A normal diode, when operated in reverse bias, becomes damaged if a reverse current above a certain value is passed through it. This “fixed value” is called the breakdown voltage.

Zener diode has a very low breakdown voltage. But this diode allows reverse current to pass through it. When once this breakdown voltage is exceeded. That breakdown voltage is called the Zener voltage. This is a controlled breakdown that does not damage the diode when a reverse current above the Zener voltage passes through the Zener diode.

In its reverse bias, a Zener diode exhibits a controlled breakdown voltage and this allows current to flow keeping the value of the voltage across the Zener diode close to the Zener breakdown voltage value. This value of the Zener breakdown voltage of any Zener diode is chosen for certain applications.

The avalanche diode is another diode that has similar properties to a Zener diode. Avalanche breakdown at the P-N junction occurs when the voltage drop is constant and independent of the current. This avalanche diode is used for photodetection.

V-I Characteristics of a Zener diode

The V-I characteristics of a Zener diode are common to any diode when it is operated in forward bias. But it becomes very important to consider the reverse bias operation of the Zener diode.

The point where the bent in reverse bias operation is shown is the Zener breakdown voltage, after which the diode allows high reverse current through it. This Zener voltage is indicated by V_Z. This incredible quality of the Zener diode has made it one of the most reliable and has many applications.

Applications of Zener diode

There are many applications of this diode like –

• It is mostly used as a voltage regulator.
• Transistor biasing provides the fixed reference voltage in the circuit.
• For peak clipping or limiting in wave shaping circuits.
• As a surge protector in many circuits.
• To protect the meter against damage from accidental applications.

Switching Diode

It is a common single P-N junction diode specially designed for switching purposes. This diode can clearly display two states of high and low resistance which can be used alternately.

The junction capacitance of this diode is designed to be very low to minimize other effects. The switching speed is designed to be very high. When the diode has a high resistance, it acts as an open switch and it acts as a closed switch when it has a low resistance. This transition occurs at a faster rate in switching diodes than in any normal diode.

Applications of switching diode

These have many applications like –

• It is used in high-speed rectifying circuits.
• It is used in ring modulators.
• It is used in radio frequency receivers.
• It is used as a reverse polarity protector.
• It is used for both general purpose and high-speed switching applications.

Depending on Special Purpose Diodes

There are some diodes that are designed to serve certain purposes. There are many types like Transient Voltage Suppression Diode, Gold Doped Diode, Super Barrier Diode, Point Contact Diode, Peltier Diode, etc. But apart from these, there are some major diodes, which have found many applications.

Varactor Diode

A junction diode has two potentials on either side where the depletion region can act as a dielectric. Therefore a capacitance exists. A Varactor diode is a special case diode that operates in reverse bias, where the junction capacitance varies. Varactor diode is also called Vari Cap or Volt Cap.

If the reverse voltage is applied, the width of the dielectric region increases, thereby reducing the junction capacitance. When the reverse voltage decreases, the width of the dielectric decreases, increasing the capacitance. If this reverse voltage is completely zero, the capacitance will be at its maximum.

Various symbols are used for the Varactor diode which represents its function.

Although all diodes have this junction capacitance, the Varactor diode is primarily manufactured to exploit this effect and increase the variation in this junction capacitance.

Applications of Varactor diode

There are many applications of this diode like –

• It is used as a voltage variable capacitor.
• It is used in variable LC tank circuits.
• Used as automatic frequency control.
• Used as a frequency modulator.
• Used as RF phase shifter.
• Used as a frequency multiplier in local oscillator circuits.

Tunnel Diode

If the impurity concentration of the common P-N junction increases excessively, it forms a tunnel diode. It is also known as Esaki diode after its inventor.

When the impurity concentration in the diode is increased, the width of the depletion region decreases, forcing the charge carriers to cross the junction with some additional force. When this concentration is further increased, due to the reduced width of the depletion region and the increased energy of the charge carriers, they enter through the potential barrier rather than climb over it. This penetration can be understood as tunneling and hence the name tunnel diode.

Tunnel diodes are low-power devices and must be handled with care as they are easily affected by heat and static electricity. Tunnel diodes have distinctive V-I characteristics that explain their function.

When the diode is in a forward-biased state, the current increases rapidly as the forward voltage increases and it rises to a peak point called the peak current, denoted by I_P. The voltage at this point is called the peak voltage, denoted by Vp. This point is represented by A in the above graph.

If the voltage is increased beyond Vp, the current begins to decrease. It decreases up to a point called the valley current, which is represented by I_V. The voltage at this point is called the valley voltage, denoted by V_V. This point is represented by B in the above graph.

If the voltage is increased further, the current increases as in a normal diode. For larger values of forward voltage, the current increases further beyond.

When the diode is in a reverse-biased state, the diode acts as an excellent conductor as the reverse voltage increases. Here the diode acts as a negative resistance region.

Applications of Tunnel diode

There are many applications for tunnel diodes:

• Used as a high-speed switching device.
• used as a memory storage device.
• Used in Microwave and Relaxation Oscillators.

Schottky Diode

It is a special type of diode in which the P-N junction is replaced by a metal-semiconductor junction. In a typical P-N junction diode the P-type semiconductor is replaced with a metal and the N-type material is joined to the metal. In this combination, there is no depletion region between them.

The metal used in this Schottky diode can be gold, silver, platinum or tungsten, etc. Also, for semiconductor materials other than silicon, gallium arsenide is mostly used.

Operation

When no voltage is applied or when the circuit is neutral, the energy level of electrons in an N-type material is lower than in metal. If the diode is forward biased, then in the N-type these electrons gain some energy and move forward with some higher energy. Hence these electrons are called hot carriers.

Advantages

There are many advantages of Schottky diodes –

• It is a unipolar device and hence no reverse current is generated.
• Its forward resistance is low.
• The voltage drop is very low.
• Correction is fast and easy with Schottky diodes.
• No depletion region exists and hence, there is no junction capacitance. So, the diode quickly switches to the OFF state.

Applications

There are many applications of Schottky diodes –

• used as a detector diode.
• used as a power rectifier.
• Used in RF Mixer Circuit.
• used in power circuits.
• Used as a clamping diode.

Depending on OptoElectronic Diode

These are diodes that work on light. The word “Opto” means light. There are types that conduct depending on the intensity of the light and others whose conduction gives some light. Each type has its own applications.

Some diodes conduct according to the intensity of light falling on them. There are two main types of diodes in this category. They are the photodiode and the solar cell.

Photo Diode

A photo diode, as the name implies, is a P-N junction that works on light. The intensity of the light affects the level of conduction in this diode. The photo diode consists of a P-type material and an N-type material with an intrinsic material or a depletion region in between.

This diode usually operates in reverse bias state. When light is focused on the depletion region, electron-hole pairs are formed and electrons flow. This conduction of electrons depends on the intensity of the focused light.

When the diode is connected with the supply in reverse bias, a small reverse saturation current flows due to thermally generated electron-hole pairs. Minority carriers cause current to flow in reverse bias, with the output voltage dependent on this reverse current. As the intensity of the light focused at the junction increases, the current due to minority carriers increases.

The photo diode is encapsulated in a glass package so that light can fall on it. To focus the light properly on the depletion region of the diode, a lens is placed over the junction.

Even when there is no light, a small amount of current flows which is called dark current. By changing the level of illumination, the reverse current can be changed.

Advantages of Photo diode

Photo diode has many advantages –

• Low noise
• High gain
• High speed operation
• High sensitivity to light
• Low cost
• Small size
• Long lifetime

Applications of Photo diode

There are many applications for photo diode

• Character detection.
• Objects can be detected (visible or invisible).
• Used in circuits that require high stability and speed.
• used in demodulation.
• used in circuit switching.
• used in encoders.
• Used in optical communication equipment.

Solar Cell

Light-dependent diodes include the solar cell, which is a common P-N junction diode, but its conduction is caused by a multitude of photons that are converted into a flow of electrons. It is similar to a photo diode but has another purpose to convert maximum incident light into energy and store it.

A solar cell has its own name and symbol indicating the storage of energy, although it is a diode. The feature of extracting and storing more energy is concentrated in the solar cell.

Construction of a Solar cell

A P–N junction diode with an intrinsic material in the extinction region is designed to be encapsulated in a glass. The maximum possible area for incident light is highlighted with thin glass on top so as to collect maximum light with minimum resistance.

When light is incident on a solar cell, the photons present in the light collide with the valence electrons. The electrons are activated to leave the parent atoms. Thus, a flux of electrons is generated and this current is directly proportional to the intensity of the light focused on the solar cell. This phenomenon is called the photo-voltaic effect.

Difference between a Photo diode and Solar cell

The photo diode works faster and focuses on switching rather than providing more power at the output. Because of this its capacitance value is low. Furthermore, according to its applications, the photo diode has a small area of incidence of light energy.

A solar cell focuses on delivering high output energy and storing the energy. It has a high capacitance value. The operation is a bit slow compared to the photo diode. According to the purpose of the solar cell, the area of incidence of light is larger than that of the photo diode.

Applications of Solar Cell

There are many applications for Solar cell –

• Science and Technology: It is used in Solar panels for Satellites, telemetry, Remote lighting systems etc.
• Commercial Use: It is used in solar panels to store electricity, portable power supplies, domestic uses such as cooking and heating using solar energy etc.
• Electronic: it is used in Watches, Calculators, Electronic Toys, etc.

LED (Light Emitting Diodes)

It is also a common PN junction diode except that instead of silicon and germanium, materials such as gallium arsenide and gallium arsenide phosphide are used.

Like a normal P-N junction diode, it is connected in the forward bias condition so that the diode conducts. Conduction occurs in an LED when free electrons in the conduction band combine with holes in the valence band. This process of recombination emits light. This process is called electroluminescence. The color of the emitted light depends on the spacing between the energy bands.

The materials used also affect the colors, with gallium arsenide phosphide emitting either red or yellow, gallium phosphide emitting either red or green and gallium nitrate emitting blue light. Whereas gallium arsenide emits infrared light. LED is mostly used in remote controls for invisible infrared light.

LEDs have a flat side and a curved side, the lead on the flat side is designed to be smaller than the other, to indicate that the smaller one is the cathode or negative terminal and the other is the anode or positive terminal.

As the electrons jump into the holes, the energy is spontaneously dissipated as light. LED is a current-dependent device. The intensity of the output light depends on the current through the diode.

Advantages of LED

There are many advantages of LED –

• High efficiency
• High speed
• High reliability
• Low heat dissipation
• Larger life span
• Low cost
• Easily controlled and programmable
• High levels of brightness and intensity
• Low voltage and current requirements
• Less wiring required
• Low maintenance cost
• No UV radiation
• Instant Lighting effect

Applications of LED

There are many applications for LED –

• In Displays: Especially used for seven segment display, Digital clocks, Microwave ovens, Traffic signaling, Display boards in railways and public places, Toys, etc.
• In Electronic Appliances: Stereo tuners, Calculators, DC power supplies, On/Off indicators in amplifiers, Power indicators, etc.
• Commercial Use: Infrared readable machines, Barcode readers, Solid state video displays, etc.
• Optical Communications: In optical switching applications, For optical couplings where manual assistance is not available, Information transfer through FOC, image sensing circuit, burglar alarm, in railway signaling techniques, door and other security control system,etc.

Laser Diode

The laser diode is another popular diode of its kind. It is an optical diode that emits light but with an excited process. The name LASER means amplification of light by stimulated emission of radiation.

Stimulated Emission

It is a P-N junction diode whose action starts when a beam of light is incident on it. As with a light beam, when photons are incident on an atom, the atom gets excited and it reaches a higher level which can be called higher energy level.

An atom when it shifts from a higher energy level to a lower energy level releases two photons that are similar in characteristics to the incident photon and in phase equal to that. This process is called stimulated emission. An atom can usually remain in this excited state for 10^-8 seconds.

Principle of Laser Diode

Whenever a photon is incident on an atom, that atom is excited from a lower energy state to a higher energy state and two photons are released in the process. In fact, an atom can usually remain in this excited state for 10^-8 seconds. So, to achieve amplification, during this exciting process, the atom is placed in another state called meta stable state which is below the higher energy level and above the lower energy level.

An atom can remain in this meta steady state for 10^-3 seconds. While the atom moves to a lower state, two photons are released. If there are more atoms in the excited state before the photons hit the atoms, we have a Lasing effect.

Advantages

There are many advantages of Laser diode –

• The power used by the laser diode is very low.
• High on/off switching speed.
• more compact.
• less expensive.
• they are cheaper than laser generators.
• Less likely to deliver electric shock.

Disadvantages

There are few disadvantages of Laser diode –

• More diverging rays and hence the quality is not so good.
• Their lifetime is less than that of LEDs.
• Possibility of damage during unstable power supply.

Applications

There are many applications of Laser diode –

• Used as pump-laser and seed-laser.
• Used in optical data storage devices.
• Used in laser printers and laser fax machines.
• Used in laser pointers.
• Used in bar-code readers.
• They are used in DVD and CD drives.
• Used in HD DVD and Blu Ray technology
• It has many industrial purposes like heat treatment, cladding, seam welding etc.
• Communication technology has many uses such as data linking and transmission.