Diode
A diode is a semiconductor device that allows current to flow in one direction while blocking it in the opposite direction. The flow of current through a diode depends on its biasing (forward or reverse)
1. Forward Current (IF_F)
When the diode is forward-biased (the anode is more positive than the cathode), current flows through it.
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Mechanism:
1.Forward Biasing: o When a voltage is applied across the diode such that the p-type side (anode) is positive and the n-type side (cathode) is negative, the depletion region at the p-n junction narrows.
o This allows charge carriers (electrons from the n-side and holes from the p- side) to move across the junction.
2.Threshold Voltage (VT_T): o For current to flow significantly, the applied voltage must exceed the diode's threshold voltage.
For Silicon diodes, VT_T ≈ 0.7 V.
For Germanium diodes, VT_T ≈ 0.3 V.
3.Current Flow: o Once the threshold voltage is exceeded, current increases exponentially with the applied voltage. This relationship is governed by the diode equation: I=IS(eqVkT−1)I = I_S \left( e^{\frac{qV}{kT}} - 1 \right)
where: ISI_S = reverse saturation current (a small leakage current)
qq = charge of an electron (1.6×10−191.6 \times 10^{-19} C)
VV = applied voltage
kk = Boltzmann constant (1.38×10−231.38 \times 10^{-23} J/K)
TT = temperature in Kelvin.
Characteristics:
•Current increases rapidly with voltage beyond the threshold.
•The diode exhibits low resistance in this mode.
2. Reverse Current (IR_R)
When the diode is reverse-biased (the cathode is more positive than the anode), a very small current flows through it, called reverse current or leakage current.
Mechanism:
1.Reverse Biasing: o The p-n junction widens, increasing the depletion region.
o This prevents the majority carriers from crossing the junction.
2.Reverse Saturation Current (ISI_S): o A small current flows due to the minority charge carriers (holes in the n-region and electrons in the p-region).
o ISI_S is temperature-dependent and increases with an increase in temperature.
3.Breakdown Region: o If the reverse voltage exceeds a critical value called the breakdown voltage (VB_B), the diode conducts large current in reverse.
Zener Breakdown: Occurs in Zener diodes at low reverse voltages due to quantum tunnelling.
 Avalanche Breakdown: Happens at higher reverse voltages due to carrier multiplication.
Characteristics:
•Reverse current is very small compared to forward current.
•The diode exhibits high resistance in this mode, except during breakdown.
3. Total Current in a Diode
The net current through the diode is the sum of forward and reverse currents. Under normal operation, either forward or reverse current dominates based on the biasing.
Key Points:
•Ideal Diode: In theory, current flows perfectly in one direction, with no leakage or resistance in forward bias and infinite resistance in reverse bias.
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•Real Diode: Has small leakage current in reverse bias and a finite threshold voltage in forward bias.
•Temperature Effect: Both forward and reverse currents increase with temperature due
to increased carrier activity.
Applications:
•Rectifiers: Convert AC to DC.
•Voltage Regulators: Zener diodes maintain a stable voltage.
•Clamping/Clipping Circuits: Shape or limit voltage waveforms.
•Switching: Used in digital circuits for high-speed operation.
Vacuum Diodes
A vacuum diode is an electronic device that consists of two electrodes (an anode and a cathode) enclosed in a vacuum-sealed envelope. Its operation is based on the thermionic emission of electrons from the heated cathode, which are then attracted to the positively charged anode.
1. Characteristic Curves of Vacuum Diodes
The characteristic curve of a vacuum diode illustrates the relationship between the anode current (IAI_A) and the anode voltage (VAV_A).
Regions of the Characteristic Curve:
1.Saturation Region: o At low anode voltages, the anode current increases linearly with the anode voltage.
o Electrons emitted from the cathode are fully collected by the anode.
2.Space-Charge Limited Region: o As the anode voltage increases, the current becomes limited by the space charge created by electrons near the cathode.
o The anode current follows the Child-Langmuir Law: IA=4ϵ092emVA3/2d2I_A = \frac{4 \epsilon_0}{9} \sqrt{\frac{2e} {m}} \frac{V_A^{3/2}}{d^2}
where: ϵ0\epsilon_0: Permittivity of free space
ee: Electron charge
mm: Electron mass
dd: Distance between cathode and anode
VAV_A: Anode voltage
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3.Thermionic Emission Limited Region: o At very high anode voltages, the current saturates because the cathode emits electrons at its maximum rate, governed by the Richardson-Dushman equation: J=AT2e−Ï•/kTJ = A T^2 e^{-\phi / kT}
where: JJ: Current density
AA: Richardson constant
 TT: Cathode temperature
Ï•\phi: Work function of the cathode material
kk: Boltzmann constant
Planar Vacuum Diode
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In a planar vacuum diode, the cathode and anode are parallel plates separated by a vacuum. The potential distribution in the diode is a key aspect of its operation.
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Potential Distribution:
1.Vacuum Conditions: o The vacuum ensures no medium obstructs electron motion, except for the space charge effect.
2.Electron Emission and Acceleration: o Electrons are emitted thermionically from the cathode and accelerate toward the anode due to the electric field.
3.Space-Charge Effects: o The electron cloud near the cathode modifies the potential distribution.
o The potential decreases non-linearly from the cathode to the anode to due the repulsion among electrons (space charge).
Gas Diodes
A gas diode is a type of electronic device that operates similarly to a vacuum diode but contains a small amount of gas, such as argon, neon, or mercury vapor, instead of a complete vacuum. The presence of gas significantly affects the operation of the diode, introducing new phenomena such as ionization and gas discharge. These properties make gas diodes useful in applications like voltage regulation, rectification, and surge protection.
1. Construction of Gas Diodes
A typical gas diode consists of:
•Cathode: The negative electrode, often made of a material that facilitates electron emission.
•Anode: The positive electrode.
•Gas-filled envelope: A sealed chamber containing a small amount of inert gas or a gas mixture at low pressure.The design is similar to a vacuum diode but with a gas-filled environment, enabling ionization effects during operation.
Gas Diodes operation
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The operation of a gas diode involves the interaction between electrons, gas atoms, and ions within the diode.
Forward Bias: • When the anode is positive relative to the cathode:
o Electrons are emitted from the cathode and accelerated toward the anode.
o The gas in the diode becomes ionized as high-energy electrons collide with gas atoms.
o Ionization creates positive ions and free electrons, which contribute to the conduction process.
Reverse Bias:•When the anode is negative relative to the cathode:
o Very little current flows because the electric field repels the electrons emitted by the cathode.
o In reverse bias, gas diodes exhibit high resistance until the reverse breakdown voltage is reached.
Applications of Gas Diodes
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1.Voltage Regulation: o Gas diodes are used in voltage regulators because of their stable breakdown voltage.
o Example: Neon lamps and gas-filled Zener diodes.
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2.Surge Protection: o Gas diodes protect electronic circuits from voltage spikes by conducting large currents during overvoltage conditions.
o Example: Gas discharge tubes in power line protection.
3.Rectification: o Used in high-power rectifiers for converting AC to DC in industrial applications.
4.Indicators: o Neon lamps, which are a form of gas diodes, are widely used as visual indicators in electrical circuits.
Characteristics of Gas Diodes
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The current-voltage (I−VI-V) characteristics of gas diodes differ significantly from those of vacuum diodes:
1.Non-linear Behaviour : o Below the breakdown voltage, the diode exhibits high resistance.
o After breakdown, the current increases rapidly, often with a visible glow discharge.
2.Stable Breakdown Voltage: o The breakdown voltage is relatively stable, depending on the type of gas and pressure.
3.Negative Resistance: o Seen in the I−VI-V curve after breakdown.
4.Thermal Effects: o Conducting ionized gas generates heat, which can affect performance and requires heat dissipation mechanisms.
6. Advantages of Gas Diodes
•High breakdown voltage.
•Stable operation in high-voltage applications.
•Low cost and simple construction.
•Visible glow discharge for operational indication.
7. Disadvantages of Gas Diodes
•Slower response time compared to solid-state diodes.
•Limited to low-frequency applications.
•Susceptible to aging effects, as gas ionization may degrade over time.
8. Common Examples
1.Neon Lamps: Used for indication and voltage stabilization.
2.Gas Discharge Tubes (GDTs): Used for surge protection.
3.Mercury Arc Rectifiers: Used in industrial power conversion.
Crystal Diodes
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A crystal diode is a semiconductor device that allows current to flow in one direction while blocking it in the opposite direction. It is a fundamental component of electronic circuits and is also known as a p-n junction diode. Crystal diodes are widely used in rectification, signal processing, and voltage regulation. Crystal diodes form the backbone of modern electronics, enabling key functionalities in circuits ranging from power supplies to high-speed communication systems. Their simplicity, reliability, and efficiency make them indispensable components.
1. Construction of a Crystal Diode
1.P-N Junction: o A crystal diode is made by joining p-type (positively charged holes as majority carriers) and n-type (negatively charged electrons as majority carriers) semiconductor materials.
2.Crystal Structure: o The materials are typically silicon or germanium, which have a crystalline lattice structure.
o Silicon diodes are more common due to their higher thermal stability and lower leakage current compared to germanium diodes.
3.Terminals: o Anode (P-side): Connected to the positive terminal in forward bias.
o Cathode (N-side): Connected to the negative terminal in forward bias.
4.Encapsulation: o The diode is enclosed in a small, protective casing with leads for external connections.
2. Working Principle of a Crystal Diode
The operation of a crystal diode depends on the behavior of the p-n junction under forward and reverse bias conditions.
Forward Bias:•When the p-side is connected to the positive terminal and the n-side to the negative terminal:
1.The depletion region at the junction narrows, reducing the potential barrier. 2.Electrons from the n-side and holes from the p-side move across the junction.
3.A current flows through the diode due to the recombination of charge carriers.
Reverse Bias:•When the p-side is connected to the negative terminal and the n-side to the positive terminal:
1.The depletion region widens, increasing the potential barrier.
2.Majority carriers are blocked, and only a small leakage current flows due to minority carriers.
3.If the reverse voltage exceeds a critical value (breakdown voltage), a large reverse current flows, possibly damaging the diode.
3. Characteristics of Crystal Diodes
The current-voltage (I−VI-V) characteristics of a crystal diode are non-linear and can be divided into three regions:
1.Forward Bias Region: o Current increases exponentially with voltage once the threshold voltage is exceeded.
 Silicon diode: Threshold voltage ≈ 0.7 V.
Germanium diode: Threshold voltage ≈ 0.3 V.
2.Reverse Bias Region: o A small reverse leakage current flows until the breakdown voltage is reached.
3.Breakdown Region: o At high reverse voltages, the diode conducts heavily due to breakdown mechanisms:
 Zener Breakdown: Quantum tunneling in heavily doped diodes.
Avalanche Breakdown: Carrier multiplication in lightly doped diodes.
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4. Applications of Crystal Diodes
1.Rectification :o Converts AC to DC in power supply circuits (e.g., half-wave and full-wave rectifiers).
2.Clipping and Clamping Circuits: o Used to limit voltage levels or shift signal baselines.
3.Voltage Regulation: o Zener diodes are specialized crystal diodes for maintaining stable voltages.
4.Signal Demodulation: o Used in radio receivers to extract audio signals from modulated carrier waves.
5.Switching: o Acts as a fast electronic switch in digital circuits.
5. Types of Crystal Diodes
1.Silicon Diodes: o High thermal stability.
o Low leakage current.
o High forward voltage drop (~0.7 V).
2.Germanium Diodes: o Lower forward voltage drop (~0.3 V).
o Higher leakage current .
o Less thermally stable.
3.Schottky Diodes: o Formed using a metal-semiconductor junction.
o Very fast switching speed and low forward voltage drop ).
4.Zener Diodes: o Designed to operate in reverse breakdown for voltage regulation.
5.Light Emitting Diodes : o Emit light when forward biased due to electron-hole recombination.
6. Advantages of Crystal Diodes
•Small size and lightweight.
•High efficiency and reliability.
•Fast response times for switching.
•Operates over a wide range of voltages and currents.
7. Disadvantages of Crystal Diodes
•Limited voltage and current handling capacity.
•Susceptible to damage from overheating or overvoltage.
•Non-linear characteristics require careful circuit design.
Junction Diode
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A junction diode is a semiconductor device that consists of a single p-n junction formed by doping a single piece of semiconductor material. It allows current to flow in one direction (forward bias) and blocks it in the other (reverse bias). The junction diode is the simplest form of a diode and is widely used in electronic circuits for rectification, switching, and signal processing. The junction diode is a vital semiconductor device that allows unidirectional current flow and exhibits distinct forward and reverse bias behaviors.
1. Construction of a Junction Diode
A junction diode is constructed from a semiconductor material, such as silicon or germanium. The construction involves two regions:
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1.P-Type Region: o Contains an abundance of holes (positive charge carriers) created by adding trivalent impurities (e.g., boron).
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2.N-Type Region: o Contains an abundance of electrons (negative charge carriers) created by adding pentavalent impurities (e.g., phosphorus).
3.P-N Junction: o The interface where the p-type and n-type regions meet.
o At the junction, electrons and holes diffuse across the boundary, leading to the formation of a depletion region.
4.Depletion Region: o A region around the p-n junction that is devoid of free charge carriers due to recombination.
o Contains immobile ions, creating an electric field and a potential barrier.
2. Working Principle of a Junction Diode
The operation of a junction diode depends on the biasing conditions:
Forward Bias:•The p-side is connected to the positive terminal, and the n-side to the negative terminal of an external voltage source.
•The applied voltage reduces the potential barrier at the p-n junction.
•When the applied voltage exceeds the threshold voltage (VTV_T):
o Silicon diode: VT≈0.7 VV_T ≈ 0.7 \, \text{V}.
o Germanium diode: VT≈0.3 VV_T ≈ 0.3 \, \text{V}.
•Charge carriers (electrons and holes) flow across the junction, resulting in a current.
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Reverse Bias:•The p-side is connected to the negative terminal, and the n-side to the positive terminal.
•The applied voltage increases the width of the depletion region and the potential barrier.
•Only a small leakage current flows due to minority carriers.
•If the reverse voltage exceeds the breakdown voltage (VBV_B), the diode conducts heavily (breakdown).
3. I-V Characteristics of a Junction Diode
The current-voltage (I−VI-V) characteristics of a junction diode are non-linear and can be divided into:
1.Forward Bias Region: o Below the threshold voltage: Current is negligible.
o Beyond the threshold voltage: Current increases exponentially.
2.Reverse Bias Region: o Small reverse leakage current flows until breakdown voltage.
o At breakdown voltage (VBV_B):
Zener Breakdown: In heavily doped diodes at low reverse voltages.
Avalanche Breakdown: In lightly doped diodes at high reverse voltages.
4. Applications of Junction Diodes
1.Rectification: o Converts AC to DC in power supplies (half-wave and full-wave rectifiers).
2.Clipping and Clamping Circuits: o Shapes waveforms by limiting or shifting voltage levels.
3.Voltage Regulation: o Zener diodes (specialized junction diodes) maintain a stable voltage.
4.Signal Demodulation :o Extracts audio signals from modulated carrier waves.
5.Switching: o Acts as a fast on/off switch in digital circuits.
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5. Types of Junction Diodes
1.Standard Diode: o Used in rectification and basic switching.
2.Zener Diode: o Operates in reverse breakdown for voltage regulation.
3.Schottky Diode: o Fast switching diode with a metal-semiconductor junction.
4.LED (Light Emitting Diode):o Emits light when forward-biased.
5.Photodiode: o Generates current when exposed to light (used in sensors).
6. Advantages of Junction Diodes
•Simple construction and operation.
•High efficiency and reliability.
•Wide range of applications in electronics.
•Small size and cost-effective.
7. Limitations of Junction Diodes
•Current flows in only one direction.
•Limited voltage and current handling capacity.
•Susceptible to damage due to overheating or overvoltage