Reverse Bias is a special condition in semiconductor devices. A semiconductor diode is a P-N junction diode. One part of it is made of P-type semiconductor material and the other half is made of N-type semiconductor material. A diode is made by diffusing both P and N semiconductors through a special process. The semiconductor diode is used in two ways in the circuit,
- In Forward Bias
- In Reverse Bias
In this article, we will learn about what is diode, what is reverse bias meaning, the workings of reverse bias, and others in detail.
What is Diode?
Full name of the diode is Semi Conductor Diode Valve. Diode is a unidirectional device that has two ends, one end of which is called an anode, and the other end is called a cathode. It works to convert AC into DC. It can also be said that in this the current flows only in one direction and not in the other direction. Because DC flows in only one direction.
Type of Diode
What is Reverse Bias Meaning?
If any external electrical energy source like a battery is connected to a P-N Junction Diode in such a way that the (+ve) positive terminal of the battery gets connected to the N-region and the P-region gets connected to the (-ve) negative terminal of the battery, then such Biasing is called Reverse Bias. As shown in the circuit diagram below.
When P-N Junction Diode is reverse biased. Then the negative terminal of the battery attracts holes from the P-region and the positive terminal attracts electrons from the N-region.
As electrons and holes move away from the junction, the width of the depletion layer increases. Barrier Potential also increases as the depletion layer width increases.
Process of escape of charge carrier holes and electrons towards the terminals of the battery occurs for a very short time. The polarity of the barrier potential generated at the junction is the same as the externally applied reverse voltage.
Due to Barrier Potential becoming too high, the bonds of some atoms in the P and N region are broken due to which some electrons and holes are generated, these charge carriers are called Minority Charge Carriers because their number is very less. Due to these charge carriers, a weak electric current starts flowing in the diode which is called Reverse Current. Reverse current is very less. It is so rare that we can neglect it.
Number of minority charge carriers generated in reverse bias depends on the temperature of the junction in a certain range of applied external reverse voltage and not on the externally applied voltage.
If the temperature between the junction is kept constant, then the reverse current remains constant for a certain range of applied external reverse voltage, which is called Reverse Saturation Current.
Working Of Reverse Bias
When the diode is connected in reverse bias, then in this condition, due to the negative end of the battery being connected to P type material, all the holes will collect at the negative end of the battery and the positive end of the battery will be N. Due to being connected to the type material, all the electrons will collect at the positive end of the battery. In this way, both holes and electrons move away from the junction and increase the thickness of the junction barrier. As a result no current flows across the junction.
Current Flow in Reverse Bias
It is seen that in a reverse-biased diode, some current flows through the depletion region. This current is called leakage current. Leakage current is dependent on minority current carriers, the minority carriers are electrons in the P type material and holes in the N type material. The following figure shows how current carriers react when a diode is reverse biased.
Current Flow in Reverse Bias
Avalanche Breakdown in Reverse Bias
This type of breakdown occurs in the presence of high electric field. When we apply a high electric field in the reverse biased state, the electrons start gaining higher kinetic energy. These electrons begin to break other covalent bonds and create more hole-electron pairs. These pairs begin to cross the decay region and contribute to a higher reverse biased current. Bond breakage is an irreversible process, and the p-n junction is completely destroyed after avalanche breakdown.
Learn more about, Zener Diode
What is PN Junction Diode?
Connection of p-type and n type semiconductors using a special technique results in the development of PN Junction at that point where the two types of semiconductor connect with each other. However, the nature of a device developed after junction is similar to that of diode valve. Therefore, this is termed as P-N Junction Diode.
When the two types of impure semiconductors are blended, charge carriers (free electrons and holes) begin to travel through diffusion at the junction. Holes emerge from the P-Type semiconductor and travel into the N- Type one, electrons come out from the latter diffusing to enter the former. After diffusion, the charge carriers reverse their charges. In this manner a thin neutral layer is created on either side of the junction. In this neutral layer, there are no charge carriers of any type. This middle layer has no net charge and is called the Depletion Layer. Width of this layer is about 10-6m.
Diffusion of charge carriers causes an increase in positively charged holes for the n-region and negatively charged electrons with the p-region near to junction. This leads to the formation of a potential difference across the junction, which is known as Barrier Potential. Consequently, an internal electric field arises at the junction whose direction points from n-region to p-region. Some time later, this domain gets so powerful that charge carrier diffusion halts.
Reverse Bias p-n Junction
In reverse bias condition p-n junction diode, the positive terminal attracts the electron away from the junction in N side and the negative terminal attracts the holes away from the junction in P side.
Zener Diode in Reverse Bias
Zener diode is a special type of diode that works in the reverse bias condition. If zener diode is attached to reverse bias condition the breakdown voltage of the diode increases and at the point of avalanche breakdown the normal current flows across the conductor.
Graph of Diode in Reverse Bias
Draw a graph between VR and IR, taking VR on the negative x-axis and negative IR on the y-axis. The graph obtained is known as the reverse bias characteristic curve.
Graph of Diode in Reverse Bias
Forward Bias Vs Reverse Bias
One of the key difference between reverse bias and forward bias is,
- Forward biasing typically takes place in a standard diode when there is sufficient voltage across the device that allows electrons to freely flow, whereas reverse values describing negative voltages deny natural electron movement.
- Reverse-biased diode voltage does not lead to a considerable flow of current. This feature is also valuable for rectifying AC. Other applications of this characteristic range from electronic signal control.
Advantages and Disadvantages of Reverse Bias
Various advantages and disadvantages of reverse bias condition in any diode are added below,
Advantages of Reverse Bias
Advantages of reverse bias in any diode are,
- Reduction in Current Flow: The main benefit of the reverse bias is that it greatly decreases current flow through a semiconductor device. It can be effective in managing and controlling functionality of electronic devices.
- Barrier Broadening: In reverse-biased diode the depletion zone increases in width. It also increases the barrier potential, limiting charge carriers to move across. This characteristic is an important feature of diodes and transistors.
- Breakdown Voltage: The method of reverse bias is used to investigate and determine the breakdown voltage in a semiconductor device. The breakdown voltage is the highest reverse bias that can occur without causing a rapid increase in current, leading to device failure.
- Zener Diode Operation: A zener diode requires reverse bias for its proper functioning. Zener diodes are operated in the reverse breakdown, which allows almost unchanging voltage over a diode regardless of variations of the current.
Disadvantages of Reverse Bias
Disadvantages of the reverse bias in any diode are,
- Leakage Current: Under the reverse bias, even though a very small amount of current which is called leakage current may sometimes pass through the semiconductor device. This could lead to power dissipation and its effect on the device’s performance.
- Avalanche Breakdown: In certain instances, reverse bias voltage can result in avalanche breakdown if it is too high. This is a transient peak in current owing to the generation of pairs from collisions at this depletion region resulting into destruction of device.
- Time-Dependent Effects: Moreover, long-term exposure to reverse bias conditions in high temperatures can result in time-dependent effects that lower the device’s semiconductor reliability over a period.
- Recovery Time: Some of the semiconductor devices that have been subjected to reverse bias may take some time for recovery before they are able revert back into their normal operating state. While recovering, the device might not work as planned.
Applications of Reverse Bias
Some applications of reverse bias are,
- Zener Diode: Zener diodes are specifically designed to operate in reverse bias. These are employed as voltage regulators in electronic circuits, maintaining a nearly constant voltage across the diode over a wide range of currents.
- Photodiode: Reverse bias is applied to the photodiode to create a depletion region that increases sensitivity to light. When photons strike the photodiode, they generate electron-hole pairs, creating a photocurrent that can be measured.
- Avalanche Photodiodes: Similar to regular photodiodes, avalanche photodiodes use reverse bias to induce avalanche breakdown, resulting in high sensitivity to low levels of light. These are often used in applications where high speed and low light detection are important, such as fiber optic communications.
- Varactor Diodes: Varactor diodes, also known as voltage-variable capacitors, are reverse-biased to take advantage of the change in capacitance with applied voltage. They find applications in voltage-controlled oscillators, phase-locked loops, and frequency modulators in communication systems.
- Avalanche Transistor: Some transistors, such as avalanche transistors, are designed to operate in reverse avalanche breakdown. This feature is used in some high-frequency applications, such as RF (radio frequency) amplifiers.
- Breakdown Voltage Testing: Reverse bias is often used in testing semiconductor devices to determine their breakdown voltage, an important parameter in understanding device reliability and performance limits.
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Frequently Asked Questions (FAQs)
Why is Reverse Bias Important in Zener diode?
Reverse bias is important in Zener diodes because it allows them to operate in the breakdown region, while maintaining a constant voltage across the diode. This characteristic is used in voltage regulation circuits.
How does Reverse Bias Enhance Performance of Photodiode?
Reverse bias widens the depletion region in photodiodes, increasing their sensitivity to light. This enhances the generation of electron-hole pairs in response to photons, leading to measurable photocurrent.
What is Avalanche Breakdown?
Avalanche breakdown is a phenomenon where carriers gain enough energy from the applied electric field to cause further impact ionization, leading to a rapid increase in current.
Why is Reverse Bias applied to Varactor Diode?
Varactor diodes are reverse-biased to take advantage of the change in capacitance with applied voltage. This property is used in electronic circuits where voltage-controlled capacitance is required, such as in frequency-modulation applications.
How does Reverse Bias affect Breakdown Voltage of a Semiconductor Device?
Reverse bias increases the width of the depletion region and barrier capability in a semiconductor device. Prolonged exposure to reverse bias can cause breakdown, where the voltage causes a significant increase in current flow.
What is Forward and Reverse Bias?
- Forward Bias occurs when the voltage across a diode permits the natural flow of current
- Reverse Bias occurs when the voltage across the diode in the opposite direction of current flow.
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