We have studied three-terminal devices like Bipolar transistor, Darlington transistor, Field-effect transistors in detail in previous sessions. Let us concentrate on the most common two-terminal device: the diode. Diodes are small two-terminal electronic components that are designed to conduct current in one direction. There are many types of diodes available: Light Emitting Diode, Laser diode, Avalanche diode, Zener diode, Schottky diode, Photodiode, and PN junction diode.
P-N junction diodes are also called rectifier diodes. P-N junction diodes are made of two layers, in which one layer is doped with P-type semiconductor material and the other layer with N-type semiconductor material. The combination of both P and N-type layers forms a junction known as the P-N junction. Hence, the name P-N junction diode.
Let us discuss in detail the special purpose P-N junction diodes.
Special Purpose P-N Junction Diodes
Special Purpose P-N Junction Diodes includes:
- Zener Diode
- Light-Emitting Diode
- Photodiode
- A solar cell or photovoltaic devices
Light-emitting diodes, photodiodes, and photovoltaic devices are known as optoelectronic junction devices.
Zener Diode
Zener diode is a special type of semiconductor diode that allows current to flow in the reverse direction. It is a unique diode that is designed to conduct in the reverse direction when a certain specified voltage is reached. The first person to describe the electrical properties of Zener Diode is Clarence Melvin Zener.
The Zener diode features a heavily doped p-n junction that allows the current to flow in either forward or reverse direction. Zener voltage is also known as knee voltage. It is the voltage that is sufficient for the reverse breakdown condition which allows the diode to conduct in the reverse direction. Zener Effect is a type of electrical breakdown that occurs in a reverse-biased condition of the Zener diode.
The Circuit symbol of the Zener diode is as shown below.
In forward-biased conditions, a Zener diode behaves like a regular diode. In reverse biased conditions, a small leakage current is created and is circulated in the device. When the reverse voltage increases up to the fixed breakdown voltage (Vz), the current starts flowing through the diode.
Two types of breakdowns seen in Zener Diode are:
- Avalanche Breakdown
- Zener Breakdown
The avalanche breakdown occurs in Zener diodes as well as rectifiers at a sufficiently high reverse voltage. Zener breakdown occurs at low reverse voltages.
The V-I characteristics are explained in the figure below. The V-I characteristics of a Zener diode is explained in two phases:
(i) Forward Characteristics
(ii) Reverse Characteristics
Light-Emitting Diode (LED)
Light-Emitting Diode is an electric component that emits light when the electric current passes through it. Light-Emitting Diode (LED) is a specific type of diode having similar characteristics as the p-n junction diode. When electricity passes through the LED, the electrons recombine with holes emitting light in the process.
The symbol of the diode is shown below. The diode is encased with a clear cover to view the emitted light. The LED can be seen in the picture below.
When the LED is forward biased, the electrons flow from the N-type to the P-type. Holes are moved from p to n layer. Due to forward bias, at the boundary, the concentration of the minority carriers increases. At the junction, majority carriers recombine with excess minority carriers. When the minority carriers recombine with the majority carriers, energy is released in the form of photons. Photons with energy less or equal than the bandgap are emitted. The intensity of the light increases when the current reaches maximum.
The VI-characteristic of a LED is as shown in the figure below.
LEDs can emit red, green, orange, yellow, and blue light. LEDs are widely used as decorative lamps, used in television remote, optical communication, automotive, and display devices.
Light-Emitting Diodes require less power to operate. They adapt to instant on-off switching capability. They are economical and offer long-life durability.
Photodiodes
Photodiodes are special p-n junction diodes operated in reverse bias. They are mainly designed for detecting optical signals. Photodiodes feature a transparent window which allows light to fall on it. Photodiodes are also addressed with various names like a light detector, photodetector, and photo-sensor. Silicon, Germanium and Indium gallium arsenide are some of the photodiode materials. The symbol of the photodiode is shown below.
The real-time image of the photodiode is as shown below.
Photodiodes are made of photosensitive semiconducting material to absorb the light which falls on it. Photodiodes are operated under reverse bias conditions below the breakdown voltage.
When the energy of the photons which strike (hv) is greater than the energy gap (Eg) of the semiconductor material in the photodiode, electron-hole pairs are created near the depletion region.
Electrons and holes are separated before they combine in the presence of an electric field. When electric current passes through it, electrons are accumulated in the n-side and holes in the p-side. EMF is generated when electrons and holes move in the photodiode. current flows through the device when external load is connected. The magnitude of the photocurrent generated is directly proportional to the intensity of incident light.
In reverse bias condition, as the intensity of the light increases, the current also increases. Photodiodes are used in logic circuits, solar cell panels, detection circuits, character recognition circuits and optical communication. Photodiodes are used in smoke detectors and fire detectors.
Solar Cell
Solar cells are also known as photovoltaic cells. A solar cell is an electrical device that converts optical radiation into electricity by the photovoltaic effect. The solar cell is as shown in the figure below.
Solar power or solar energy is the main source for operating devices like solar cookers, water heaters, solar lamps and many more devices. Let us know the working of the key element, the solar cell.
Solar cells feature two or more layers of semiconductors. One layer contains positive charge and the other layer contains negative charge. As we know sunlight consists of tiny packets of energy known as photons. When the sunlight strikes the cell, photos are either transmitted, reflected, or absorbed.
This process takes place in three steps:
- Generation
- Separation
- Collection
When the photons are absorbed by the layer containing negative charges, the energy of the photon gets transferred to an electron in an atom of the cell. These electrons escape to the outer shell of the atom, when the energy increases. The freed electron moves to the positive layer by establishing a potential difference between the positive and the negative layer. When the two layers are connected to an external circuit, the electron flows through the circuit, creating a current.
Solar cells are renewable solar sources of energy and do not create any pollution in the atmosphere. Maintenance and repair of solar cells is negligible. They are economical and can be installed in any place.
|
Related links |
|
Stay tuned with BYJU’S for more such interesting articles. Also, register to “BYJU’S – The Learning App” for loads of interactive, engaging Physics-related videos.
Frequently Asked Questions – FAQs
What is a P-N Junction Diode?
P-N junction diodes are also called rectifier diodes. These are formed by the junction known as the P-N junction created by P-type semiconductor and N-types semiconductor material.
What are the various types of diodes?
Various types of diodes are:
Where are LASER diodes used?
State true or false: Zener diode is designed to operate under reverse bias conditions in the breakdown region.
Name the optoelectronic junction devices
Optoelectronic junction devices includes:
Why shouldn’t you touch electrical equipment with wet hands? Find out answer to this question by watching the video.
Comments