A PN-junction diode is formed when a p-type semiconductor is fused to an n-type semiconductor creating a potential barrier voltage across the diode junction The effect described in the previous tutorial is achieved without any external voltage being applied to the actual PN junction resulting in the junction being in a state of equilibrium. However, if we were to make electrical connections at the ends of both the N-type and the P-type materials and then connect them to a battery source, an additional energy source now exists to overcome the potential barrier.
The effect of adding this additional energy source results in the free electrons being able to cross the depletion region from one side to the other. The behaviour of the PN junction with regards to the potential barrier’s width produces an asymmetrical conducting two terminal device, better known as the PN Junction Diode.
A PN Junction Diode is one of the simplest semiconductor devices around, and which has the characteristic of passing current in only one direction only. However, unlike a resistor, a diode does not behave linearly with respect to the applied voltage as the diode has an exponential current-voltage ( I-V ) relationship and therefore we can not described its operation by simply using an equation such as Ohm’s law.
If a suitable positive voltage (forward bias) is applied between the two ends of the PN junction, it can supply free electrons and holes with the extra energy they require to cross the junction as the width of the depletion layer around the PN junction is decreased.
By applying a negative voltage (reverse bias) results in the free charges being pulled away from the junction resulting in the depletion layer width being increased. This has the effect of increasing or decreasing the effective resistance of the junction itself allowing or blocking current flow through the diode.
Then the depletion layer widens with an increase in the application of a reverse voltage and narrows with an increase in the application of a forward voltage. This is due to the differences in the electrical properties on the two sides of the PN junction resulting in physical changes taking place. One of the results produces rectification as seen in the PN junction diodes static I-V (current-voltage) characteristics.
There are two operating regions and three possible “biasing” conditions for the standard Junction Diode and these are:
Zero Bias – No external voltage potential is applied to the PN junction diode.
Reverse Bias – The voltage potential is connected negative, (-ve) to the P-type material and positive, (+ve) to the N-type material across the diode which has the effect of Increasing the PN junction diode’s width.
Forward Bias – The voltage potential is connected positive, (+ve) to the P-type material and negative, (-ve) to the N-type material across the diode which has the effect of Decreasing the PN junction diodes width.
Zero Biased Junction Diode
When a diode is connected in a Zero Bias condition, no external potential energy is applied to the PN junction. However if the diodes terminals are shorted together, a few holes (majority carriers) in the P-type material with enough energy to overcome the potential barrier will move across the junction against this barrier potential. This is known as the “Forward Current” and is referenced as IF Likewise, holes generated in the N-type material (minority carriers), find this situation favourable and move across the junction in the opposite direction. This is known as the “Reverse Current” and is referenced as IR. This transfer of electrons and holes back and forth across the PN junction is known as diffusion, as shown below:
The potential barrier that now exists discourages the diffusion of any more majority carriers across the junction. However, the potential barrier helps minority carriers (few free electrons in the P-region and few holes in the N-region) to drift across the junction. Then an “Equilibrium” or balance will be established when the majority carriers are equal and both moving in opposite directions, so that the net result is zero current flowing in the circuit. When this occurs the junction is said to be in a state of “Dynamic Equilibrium“. The minority carriers are constantly generated due to thermal energy so this state of equilibrium can be broken by raising the temperature of the PN junction causing an increase in the generation of minority carriers, thereby resulting in an increase in leakage current but an electric current cannot flow since no circuit has been connected to the PN junction.
Reverse Biased PN Junction Diode
When a diode is connected in a Reverse Bias condition, a positive voltage is applied to the N-type material and a negative voltage is applied to the P-type material. The positive voltage applied to the N-type material attracts electrons towards the positive electrode and away from the junction, while the holes in the P-type end are also attracted away from the junction towards the negative electrode. The net result is that the depletion layer grows wider due to a lack of electrons and holes and presents a high impedance path, almost an insulator. The result is that a high potential barrier is created thus preventing current from flowing through the semiconductor material.
Forward Biased PN Junction Diode
When a diode is connected in a Forward Bias condition, a negative voltage is applied to the N-type material and a positive voltage is applied to the P-type material. If this external voltage becomes greater than the value of the potential barrier, approx. 0.7 volts for silicon and 0.3 volts for germanium, the potential barriers opposition will be overcome and current will start to flow. This is because the negative voltage pushes or repels electrons towards the junction giving them the energy to cross over and combine with the holes being pushed in the opposite direction towards the junction by the positive voltage. This results in a characteristics curve of zero current flowing up to this voltage point, called the “knee” on the static curves and then a high current flow through the diode with little increase in the external voltage as shown below:
The application of a forward biasing voltage on the junction diode results in the depletion layer becoming very thin and narrow which represents a low impedance path through the junction thereby allowing high currents to flow. The point at which this sudden increase in current takes place is represented on the static I-V characteristics curve above as the “knee” point.
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A PN junction diode is a semiconductor device that allows current to flow in one direction only. It is made up of two semiconductor materials, one with a positive charge (p-type) and one with a negative charge (n-type). The p-type material is made by adding impurities to a semiconductor material, such as silicon, that have an excess of positive charge carriers (holes). The n-type material is made by adding impurities to a semiconductor material that have an excess of negative charge carriers (electrons).
When the p-type and n-type materials are brought into contact, a junction is formed. At the junction, the positive charge carriers in the p-type material are attracted to the negative charge carriers in the n-type material. This attraction causes the charge carriers to diffuse across the junction. The diffusion of charge carriers creates a region at the junction where there are no charge carriers. This region is called the depletion region.
The depletion region has a high resistance to current flow. However, if a voltage is applied to the diode with the p-type material connected to the positive terminal of the voltage source and the n-type material connected to the negative terminal, the depletion region will narrow. This narrowing of the depletion region allows current to flow through the diode. This is called forward bias.
If a voltage is applied to the diode with the p-type material connected to the negative terminal of the voltage source and the n-type material connected to the positive terminal, the depletion region will widen. This widening of the depletion region prevents current from flowing through the diode. This is called reverse bias.
The current-voltage characteristics of a PN junction diode are shown in the following graph. The graph shows that the current through the diode increases rapidly with a small increase in voltage when the diode is in forward bias. However, the current through the diode is very small when the diode is in reverse bias.
PN junction diodes are used in a variety of applications, including rectifiers, clippers, clampers, voltage regulators, Light detectors, photodiodes, solar cells, zener diodes, tunnel diodes, and Schottky diodes.
Rectifiers are used to convert alternating current (AC) to direct current (DC). Clippers are used to remove unwanted portions of an AC or DC signal. Clampers are used to hold an AC or DC signal at a certain level. Voltage regulators are used to maintain a constant voltage level. Light detectors are used to detect light. Photodiodes are used to convert light into an electrical signal. Solar cells are used to convert sunlight into electricity. Zener diodes are used to regulate voltage. Tunnel diodes are used to switch signals at high frequencies. Schottky diodes are used to rectify high-frequency signals.
What is a transistor?
A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor’s terminals changes the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.
What is a capacitor?
A capacitor is an electronic component that stores electrical energy in an electric field. It is a passive two-terminal electrical component consisting of two Conductors separated by an insulating dielectric. The conductors are usually plates of Metal, separated by a thin layer of an insulator such as glass, ceramic, or plastic.
What is an inductor?
An inductor is an electrical component that stores energy in a Magnetic Field. It is a passive two-terminal electrical component consisting of a coil of wire. When current flows through the coil, it creates a magnetic field around the coil. The magnetic field stores energy, which can be released back into the circuit when the current is stopped.
What is a resistor?
A resistor is an electrical component that resists the flow of current. It is a passive two-terminal electrical component that dissipates electrical energy as heat. The resistance of a resistor is measured in ohms.
What is an integrated circuit?
An integrated circuit (IC), also known as a microchip, is a small electronic circuit that contains many tiny transistors, resistors, and capacitors. Integrated circuits are used in almost all electronic devices, from computers to cell phones to cars.
What is a microprocessor?
A microprocessor is a computer processor that incorporates the functions of a computer’s central processing unit (CPU) on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains all, or most of, the central processing unit functions of a computer, and is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and provides results as output.
What is a memory?
Memory is an electronic device that stores information. It is a physical device that can be used to store data, programs, and other information. Memory is used in computers, cell phones, and other electronic devices.
What is a hard drive?
A hard drive is a data storage device that uses magnetic storage to store and retrieve digital information. It is a non-volatile storage device, which means that it can retain data even when the power is turned off. Hard drives are used in computers, laptops, and other electronic devices.
What is a solid-state drive?
A solid-state drive (SSD) is a data storage device that uses flash memory to store data. It is a non-volatile storage device, which means that it can retain data even when the power is turned off. SSDs are used in computers, laptops, and other electronic devices.
A network is a group of computers that are connected together so that they can share information and Resources. Networks can be local area networks (LANs), which are networks that are limited to a small area, such as a home or office, or wide area networks (WANs), which are networks that are spread over a large area, such as a city or country.
The internet is a global system of interconnected computer networks that use the standard Internet protocol suite (TCP/IP) to serve billions of users worldwide. It is a network of networks that consists of millions of private, public, academic, business, and government networks, of local to global scope, that are linked by a broad array of electronic, wireless, and optical networking technologies. The Internet carries an extensive range of information resources and Services, such as the inter-linked hypertext documents and applications of the World Wide Web (WWW), electronic mail, telephony, and file sharing.
Sure, here are some MCQs without mentioning the topic PN Junction Diode:
A semiconductor diode is a two-terminal electronic component that allows current to flow in one direction only.
A semiconductor diode is made of a semiconductor material, such as silicon or germanium, with two regions of opposite polarity.
The P-region of a semiconductor diode has an excess of positive charge carriers, while the N-region has an excess of negative charge carriers.
When a voltage is applied to a semiconductor diode, the P-region and N-region are attracted to each other, and current flows through the diode.
If the voltage is applied in the opposite direction, the P-region and N-region are repelled from each other, and no current flows through the diode.
A semiconductor diode is used in many electronic devices, such as rectifiers, amplifiers, and oscillators.
Here are some more MCQs:
What is the name of the region in a semiconductor diode that has an excess of positive charge carriers?
What is the name of the region in a semiconductor diode that has an excess of negative charge carriers?
When a voltage is applied to a semiconductor diode in the forward direction, what happens?
When a voltage is applied to a semiconductor diode in the reverse direction, what happens?
What are some of the electronic devices that use semiconductor diodes?
