Diode: CBSE 12th Board Physics Explanation

A diode is an electrical component having two terminals that conducts electricity in one direction (assuming it functions at a specific voltage level). It only conducts electricity in one way when a particular minimum potential difference or voltage is supplied to its two terminals. It’s a two-terminal electrical component with low (preferably zero) resistance to current flow in one direction and high (ideally infinite) resistance in the other. A diode’s most common function allows electrical current to flow in one direction (called forward current in diodes) while blocking it in the other (reverse direction). If you’re studying this topic for NEET 2022, JEE Main 2022, or JEE Advanced 2022, don’t forget to look through the following syllabus for an overview. Locate an Aakash Coaching Institute near you for a greater grasp of diodes.

What is a Diode?

A diode is made up of two words: “Di” means “two,” and “Ode” means “electrodes”; therefore, a device or component with two electrodes is called a diode. A diode or a p-n junction diode is an important component in a circuit that transforms alternating current to direct current since it only allows electricity to travel in one direction (rectification). Current will flow if a larger external voltage is added in the opposite direction of the built-in potential. Therefore most diodes are deemed “on” because they have an external forward bias.

On the other hand, Diodes have very low resistance in one direction (to enable current to flow) and very high resistance in the other (to prevent current flow). The high resistance to current flowing in the reverse direction of the diodes is significantly decreased to an extremely low resistance when the reverse voltage across the forward-biased diode reaches a value known as the breakdown voltage. Because the holes and electrons reject one another and expand the depletion zone when the terminals are switched (reverse biased), no current flows through the p-n junction diode. Click here for a more detailed explanation of basic physics concepts.

Diode Structure

On one side of the diode, the silver line is the cathode terminal (the top of the triangle is the cathode). On the other side, the anode in the forward-biased situation is the arrowhead, which symbolises the direction of conventional current flow. 

The current passing through the diode has zero resistance in one direction to the cathode and anode but has a very high resistance in the other, causing the current to flow to zero. A white or black line on most diodes represents the cathode, where electrons will flow when the diode conducts.

Types of Diodes

The diode is forward-biased when the anode voltage is greater than the cathode voltage, and it conducts readily with a minimal voltage drop. The potential energy across the p-type material is positive, while the potential energy across the n-type material is negative.

The diode is reverse-biased when the cathode voltage is higher than the anode voltage. The voltage is depleted, and the potential energy across the p-type material is negative, while the potential energy across the n-type material is positive.

Silicon or germanium are the most common semiconductor materials used in diodes.

Also See: CBSE Class 12 Board Exam | CBSE Class 12 Board Exam Syllabus | CBSE Class 12 Physics SYllabus

Functions and Applications of a Diode

• Using a Diode as a Rectifier

The most common and vital application of a diode is to convert AC to DC.

A single or a set of four diodes is used in most power conversion applications.

Diodes are used to construct several sorts of rectifier circuits. The most popular rectifier circuits are half-wave, full-wave centre-tapped, and full-bridge rectifier circuits.

There are two ways to use a rectifier:-

• The Half Wave Rectifier is the simplest rectifier circuit, using a single-phase or multi-phase input power source. The diode conducts throughout the positive half of the cycle and does not conduct during the negative half. As a result, the diode converts AC to unidirectional pulsing DC. 
• The Full Wave Rectifier utilises two p-n junction diodes. The principle behind its working is that a junction diode has a very low forward and a very high reverse bias resistance.

Solar panels using diodes

Bypass diodes are diodes that are used to safeguard solar panels from damage. When a solar panel is defective, broken, or shaded by fallen leaves, snow, or other obstacles, the total output power falls, resulting in hot spot damage as the remainder of the cells’ current would pass via the shaded cell causing overheating. The bypass diode’s main purpose is to prevent the solar cells from this hot spot heating risk.

These diodes are connected in parallel with the solar cells, limiting the voltage across the defective cell while allowing current to pass from the good cells to the external circuit, eliminating the overheating problem.

Optoelectronic Junction Devices

Devices that can work on both light and electrical currents are known as optoelectronic devices. Semiconductor diodes in which photons produce carriers, a process known as photo-excitation, are a type of Optoelectronic Device.

The following are some examples of semiconductor diodes:-

Light Emitting Diode (LED): It’s a strongly doped forward-biased p-n junction diode that spontaneously converts electrical energy into light energy, such as infrared and visible light. 

Forward Biased LED

LEDs have the following benefits over traditional low-power incandescent bulbs:-

• There is no need to warm up because the movement is quick.
• It’s almost monochrome.
• Low operating voltage and power consumption, extended life, and toughness
• It’s faster and more efficient.

• Photodiode: A photodiode is a type of junction diode used to detect light signals. It is a photosensitive reverse-biased p-n junction designed so that light can fall on its junction. 

VI Characteristics of Photodiode.

When a reverse bias is applied, the current in a photodiode varies with the change in light intensity.

• Solar Cell: A solar cell is a p-n junction diode that turns sunlight into electricity. It works by utilising the fact that an electrical voltage is produced when light or other radiant energy strikes two different materials nearby (photovoltaic effect).

Photo Current through an illuminated p-n junction.

                 V-I Characteristics of a Solar Cell.

Silicon (Si), Germanium (Ga), and Arsenic (Ar) are some of the materials used to make solar cells (As).

• Zener Diode: A Zener diode is a strongly doped p-n junction diode that is reverse biased. It is intended to enable current flow “backwards” when a particular fixed reverse voltage, known as the Zener voltage, is attained since it is operated in the breakdown zone. 

VI characteristics of a Zener Diode.

Zener breakdown is the breakdown of a diode caused by the band to band tunnelling.

As a Voltage Regulator, in the Zener Diode, the current changes dramatically when the applied reverse voltage hits the Zener diode’s breakdown point. As a result, even though the current through the Zener diode varies over a wide range, a large change in the current can be produced by a nearly insignificant change in the reverse bias voltage, i.e. Zener voltage remains constant despite the current through the Zener diode varying over a wide range. 

Circuitry Depiction of the Zener Diode as a Voltage Regulator.

Conclusion

A diode has two terminals with a low resistance to current flow in one direction, with low resistance on one side and high resistance on the other, thereby confining current flow in one direction. Semiconductor diodes are two-terminal devices with a p-n junction in the middle and metallic contacts on both ends.

The following are some examples of semiconductor diode applications:

• LED (light-emitting diode) is a p-n junction diode that transforms electrical energy into light energy.
• Rectifier diode: A rectifier diode is a diode used to convert alternating current to direct current (A.C).
• The photodiode is a type of junction diode that detects light signals.
• Zener diode: In electrical systems, a Zener diode stabilises the current and voltage.

On diodes, HC Verma Solutions has a great deal of information.

FAQs

1. What is the main function of the diode? Clarify?

The primary function of a diode is to enable current to flow in just one direction and not the other. Anode (positive lead) and cathode (negative lead) establish the polarity of a diode (negative lead). When a positive voltage is given to the anode, most diodes enable the current flow. It functions as a current one-way switch as it permits current to flow freely in one direction while drastically restricting current flow in the other.

2. What is the basic mechanism of a photodiode?

A Photodiode is a junction diode built of a light-sensitive semiconductor. A photodiode is a p-n junction diode with a reverse biasing configuration.

When the light of an appropriate frequency is shone on the p-n junction of a photodiode, new electron-hole pairs are generated by absorbing the photons of that frequency. The intensity of light controls the number of charge carriers. Photodiodes are used to detect optical signals because of this feature.

3. How is a diode utilised as a Half Wave Rectifier?

When an AC voltage to be rectified is coupled to the primary coil of a step-down transformer. A p-n Junction Diode works as a Half-Wave Rectifier. The secondary coil is linked to the diode through resistors, and output is obtained.

The p-n junction is forward biased during the positive half cycle of the input AC. As a result, the resistance of the p-n junction drops and current flows. And we have output in the load. The p-n junction is reverse biased during the negative half cycle of the input AC. The p-n junction’s resistance is large, and the current does not flow. Therefore there is no output in the load, and the current flows in the same direction through the load resistance during the whole AC cycle.

4. What determines whether a diode is forward or reverse biased?

A diode is said to be forward-biased when voltage is put across it so that it permits current to flow. The p-type material has positive potential energy, whereas the n-type material has negative potential energy.

The diode is said to be reverse-biased when voltage is put across it so that current cannot flow through it. The potential energy across the p-type material is negative, while the potential energy across the n-type material is positive, indicating that the voltage has completely depleted.

5. What are bypass diodes?

Bypass diodes are diodes that protect solar panels from harm. When a solar panel is damaged, fractured, or shaded by falling leaves, snow, or other impediments, the total output power drops, creating hotspot damage because the remaining current of the cells passes via the shaded cell causing overheating. The bypass diode’s primary function is to protect solar cells from hot spot heating.

6. Why is photodiode operated in reverse bias?

When a photodiode is illuminated with light, the covalent bonds between electrons and holes are broken, resulting in an equal number of additional electrons and holes. In contrast, the fractional change in the minority charge carrier is much higher than the fractional change in the majority charge carrier. Since the fractional change in minority carrier current is substantially greater in reverse bias than forwarding bias, reverse bias is preferable. As a result, photodiodes are operated in reverse bias.