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Diode

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 Sub Topics A diode is a dispositive made of a semiconductor material, which has two terminals or electrodes that act like an on off switch. When it is on, it acts as a short circuit and passes all current. When it is off, it behaves like an open circuit and passes no current. Depending on the polarity it works. Normally diodes are working in forward bias except zener diode. All details about the diodes and the characteristics of the same are discussed in the following section.

What is a Diode?

Diode is a simplest solid state component used in an electrical circuit. A diode is a two electrode device that acts as a one way conductor. The most basic type of diode is the p-n junction diode, which is nothing more than a p-n junction with a lead connected to each of the semiconductor materials. When forward biased, a p-n junction diode conducts. When reverse biased, it effectively blocks the flow of current.
The schematic symbol for the p-n junction diode is shown in the figure. The n type material is called the cathode and the p type material is called the anode.

Diode Characteristics

The V-I characteristics of a semiconducting diode both in the forward bias and reverse bias conditions are expressed by the universal diode equation also referred to as the Shockley's diode equation

$I_{D}$ = $I_{0}(e^{\frac{V_{0}}{\eta V_{T}}}-1)$

where  $I_{D}$ is the diode current, $I_{0}$ is the reverse saturation current, $\eta$ = 1 for germanium and silicon, $\eta$ = 2 for silicon at relatively low current, $V_{T}$ is the volt equivalent of temperature

High voltage characteristics: Figure shows high voltage plate (anode characteristics when anode current is measured in mA. Keeping temperature of cathode constant T1, when plate voltage increased from zero volt in steps and corresponding value of plate current ip measured in mA. The variation occurs as shown in curve OAB if the cathode temperature is increased from T1 to T2. The curve coincide at low voltage but saturation current increases as shown by portion CD of characteristics.

Low voltage characteristics: The plate current is practically zero at zero plate voltage. If we measure plate current using a micro ampere plate current of the order of few micro ampere occurs. This is due to the fact that even at zero plate voltage a few electrons may have sufficient KE to reach the plate and constitute a plate current of few micro ampere. From the figure, the plate current becomes exactly zero at a particular negative value Vc of plate voltage, called cut off voltage. The maximum kinetic energy of emitted electron is related to Vc as Kmax = eVc

How does a Diode Work

When electric current is passed through the diode it emits a large number of electrons. These emitted electrons may be accelerated or retarded by applying positive or negative potential to anode relative to cathode.
1. Anode at zero potential relative to cathode: The emitted electrons do not have sufficient kinetic energy so as to reach the anode. However, a few electrons may reach the anode on account of their kinetic energy, constituting negligible current. The emitted electrons accumulate near the cathode and form a cloud of electrons. This is known as space charge. At a certain stage the number of electrons forming the space charge becomes constant for a given operating temperature. This space charge becomes a source of electrons that can be attracted to the plate, if it is at a positive potential.
2. Anode at negative potential relative to cathode: The emitted electrons are repelled back due to retarding potential of anode. For a particular negative potential of anode plate current may be zero.
3. Anode at positive potential relative to cathode: The electrons constituting the space charge are attracted to the plate. This flow of electrons from cathode to plate is known as plate current. These electrons flow through the external circuit and finally return to the cathode, thus making up the supply of electrons lost by emission. On increasing the plate potential more electron will gain sufficient kinetic energy so as to reach anode, hence the plate current increases.

Types of Diodes

There are many diode types each with its own operating characteristics and applications. The various diode types are easily identified by name, circuit application and schematic symbol.

Tunnel diode: It is used in forward bias and it shows the negative resistance characteristic in forward bias.

Avalanche photo diode: It is used in reverse bias due to incident photo energy electron hole pair will be generated and it causes avalanche break down.

Rectifier diodes: Common junction diodes are often referred to as rectifier diodes. They are used in many applications such as the construction of rectifiers, which convert alternating current into direct current. They can be obtained in a variety of case sizes and styles.

Zener diodes: These diodes are closely related to p-n junction diodes. They are constructed to take advantages of reverse current. Zener diodes find wide applications for controlling voltage in all types of circuits.

Light emitting diodes: These are semiconducting devices which produce visible light when a current passes through the diode. LED can provide monochromatic lights.

Schottky diodes: The diodes which are specially manufactured to solve fast switching problem are known as Schottky diodes. Its construction is different than the conventional p-n junction diode. It consists of a metal to semiconductor junction . These diodes are also called schottky barrier diodes, surface barrier diodes or hot carrier diodes.

Laser diode: It is a semiconducting laser in which the p-n junction is act as a active medium. This is almost similar to the light emitting diode. It has wide rage of applications like fibre communications, laser pointers, bar code reading etc.

P-N Junction Diode

The term diode used by itself, refers to the basic p-n junction diode. A p-n junction conducts when the n type material (cathode) is more negative than the p type material (anode).

Relating this characteristic, we can say that a diode conducts when the following conditions are met:

• The arrow points towards the more negative of the diode potentials; that is, the cathode is more negative than the anode.
• The voltage across the diode exceeds its barrier potential
• The diode conducts fully when the forward voltage across the component is approximately 0.7V (for Si) or 0.3V (for Ge)
A p-n junction diode is reverse biased when the n type material (cathode) is more positive than the p type material (anode). This causes the depletion layer to widen and block current. Relating this characteristic to the schematic symbol for the diode, we can make the following statement: A diode does not conduct when the symbol points to the more positive of the diode potentials.

Diode Applications

Some of the important applications are

• It helps to rectifying the voltage
• Act as a voltage reference
• Used to isolate the signals from the power supply
• It controls the magnitude of the signals
• Detecting and mixing the various signals