A diode is a two-terminal semiconductor device that allows electric current to flow primarily in one direction while strongly restricting current in the opposite direction. Its behavior depends on the polarity of the voltage applied across its terminals. In forward bias, a diode offers relatively low resistance and permits current to flow, whereas in reverse bias, it presents very high resistance and allows only a small leakage current. In this article, we will explore what a diode is, how it works, and its circuit symbol.
What is a Diode?
A diode is a two-terminal semiconductor device that allows electric current to flow mainly in one direction. Because of this unidirectional current flow, diodes are widely used for rectification, switching, voltage regulation, protection, and signal processing.
A diode is generally made from semiconductor materials such as silicon or germanium. Its operation is based on a PN junction, which is formed by joining P-type and N-type semiconductor materials. Depending on the applied voltage, the diode can either allow current to flow or oppose the flow of current.
When the anode is more positive than the cathode, the diode is forward-biased and conducts current. When the cathode is more positive than the anode, the diode is reverse-biased and normally blocks current, allowing only a very small leakage current.
One of the most common applications of a diode is AC-to-DC conversion. In a rectifier circuit, diodes allow the appropriate portions of an AC waveform to pass, producing a pulsating DC output that can then be filtered and regulated for use in electronic circuits.
Diodes are available in different types, each designed for specific applications. Common examples include PN junction diodes, Zener diodes, Schottky diodes, LEDs, photodiodes, PIN diodes, varactor diodes, and tunnel diodes.
Diode Symbol
The above diagram shows the standard symbol for a diode, which consists of two terminals known as the anode and cathode. The anode is represented by an arrowhead that indicates the direction of the conventional current flow when the diode is forward-biased. The opposite end is the cathode.
For a detailed explanation of the symbols used for different types of diodes, see our guide to Diode Symbols and Their Types.
Diode Construction
A diode is a semiconductor device generally made from two types of semiconductor materials such as silicon or germanium. It consists of two semiconductor regions, P-type and N-type, joined together to form a PN junction. The two terminals of the diode are called the anode and cathode.
When the anode is at a higher potential than the cathode, the diode is forward-biased and allows current to flow through it with a relatively low forward voltage drop. When the cathode is at a higher potential than the anode, the diode is reverse-biased and normally blocks current, allowing only a small leakage current.
The bar in the standard diode symbol represents the cathode, while the opposite side represents the anode.
Working of a Diode
A diode is formed by joining P-type and N-type semiconductor materials. The P-type region contains holes as the majority charge carriers and electrons as minority carriers. In contrast, the N-type region has electrons as majority carriers and holes as minority carriers.
When the P-type and N-type materials are joined, a difference in carrier concentration causes majority carriers to move across the junction by diffusion. Electrons from the N-region move toward the P-region, while holes from the P-region move toward the N-region. Near the junction, these carriers recombine, leaving behind fixed charged ions. This creates a region with very few mobile charge carriers, known as the depletion region. The electric field developed across this region forms a potential barrier that controls the movement of charge carriers through the diode.
The operation of a diode depends on how an external voltage is applied across the junction. There are two basic operating conditions:
Unbiased Diode
When P-type and N-type semiconductor materials are joined to form a PN junction and no external voltage is applied, the junction is said to be unbiased.
The two regions contain different types and concentrations of majority charge carriers. The P-type side has a high concentration of holes, whereas the N-type side contains a large number of free electrons. Because of this concentration difference, carriers naturally tend to move toward the region where their concentration is lower.
Therefore, holes initially move from the P-side toward the N-side, while electrons move from the N-side toward the P-side. This movement is known as diffusion.
As electrons enter the P-type region, they recombine with holes near the junction. Similarly, holes entering the N-type region recombine with electrons. This carrier recombination leaves behind immobile charged atoms near the junction. The N-side develops exposed positive donor ions, while the P-side develops exposed negative acceptor ions.
The region surrounding the junction consequently becomes depleted of most mobile charge carriers. This region is known as the depletion region.
The fixed ions in the depletion region produce an internal electric field. This field opposes the continued diffusion of majority carriers. As a result, a potential difference develops across the junction, commonly called the built-in potential or barrier potential.
At equilibrium, the electric field prevents unrestricted diffusion of majority carriers across the junction. The PN junction therefore establishes its own potential barrier even when no external voltage is connected.
For a typical PN junction at room temperature, the barrier potential is approximately 0.7 V for silicon and 0.3 V for germanium.
Forward-Biased Diode
A PN junction is forward biased when the P-type region or anode is connected to the positive terminal of an external voltage source and the N-type region or cathode is connected to the negative terminal.
The applied voltage produces an electric field that opposes the built-in electric field of the junction. Consequently, the depletion region becomes narrower and the potential barrier is reduced.
At a small applied voltage, only a limited current flows because the junction barrier still restricts the movement of majority carriers. As the forward voltage is increased, the barrier becomes sufficiently reduced for a large number of majority carriers to cross the junction.
Once appreciable conduction begins, holes move from the P-side toward the N-side and electrons move from the N-side toward the P-side. This carrier movement produces a substantial forward current.
The commonly used approximate forward voltage values are:
- Silicon diode: approximately 0.7 V
- Germanium diode: approximately 0.3 V
Beyond the normal conduction region, a relatively small increase in forward voltage can produce a significant increase in current. Therefore, an external resistor or another suitable current-limiting arrangement is generally used to keep the diode current within its rated value.
In the forward-biased state, the diode therefore behaves as a low-resistance path for current.
Reverse-Biased Diode
A PN junction is reverse biased when the P-type region or anode is connected to the negative terminal of the external source and the N-type region or cathode is connected to the positive terminal.
The applied electric field now supports the electric field already present in the depletion region. As a result, majority carriers are attracted away from the junction. Holes move toward the negative terminal, while electrons move toward the positive terminal.
This movement exposes additional fixed ions near the junction and causes the depletion region to become wider. The potential barrier also increases, making it difficult for majority carriers to cross the junction.
Consequently, a reverse-biased diode blocks almost all majority-carrier current. However, a small current can still exist because both semiconductor regions contain a small number of minority charge carriers.
These minority carriers are swept across the depletion region by the electric field, producing a very small reverse current. Under normal reverse-bias conditions, this current is known as the reverse saturation current and is generally much smaller than the forward current.
If the reverse voltage continues to increase and reaches the diode’s specified breakdown voltage, the reverse current can rise very rapidly. Depending on the junction and voltage level, breakdown may occur through Zener breakdown or avalanche breakdown.
During avalanche breakdown, energetic carriers can generate additional electron-hole pairs through collisions within the semiconductor. This multiplication process can cause a large increase in reverse current.
Breakdown itself does not necessarily mean immediate failure; some specially designed devices, such as Zener diodes, are intended to operate in this region. However, an ordinary diode can be damaged if excessive reverse current is allowed to flow.
Therefore, the reverse voltage and current of a conventional diode should always remain within the manufacturer’s specified ratings.
Types of Diodes
The followings are types of diodes.
- Light Emitting Diode
- Laser diode
- Avalanche diode
- Zener diode
- Schottky diode
- Photodiode
- PN junction diode
Light Emitting Diode (LED)
When an electric current flows between the electrodes of a light emitting diode, it produces light. In other words, light is generated only when a sufficient amount of forward current passes through it. Although there are frequency levels that do not allow visibility, many diodes can produce light that is not visible to the naked eye. LEDs come in various colors, and some can even emit three colors at once. The color of light depends on the energy gap of the semiconductor used.
Laser Diode
Laser diode is a particular type of diode that produces coherent light, which is commonly used in CD drives, DVDs, and laser devices. While these diodes are more expensive than LEDs, they are more affordable than other laser generators. The only drawback is that they have a limited lifespan.
Avalanche Diode
This particular diode operates in reverse bias and uses the avalanche effect. When the voltage drop remains constant and is not affected by current, the avalanche breakdown occurs. Because of their high sensitivity, these diodes are often used for photodetection.
Zener Diode
Zener diodes are a type of diode that are highly useful as they can provide a stable reference voltage. These diodes are operated in reverse bias and break down when a certain voltage is applied. When the amount of electrical current flowing through a resistor is restricted, it produces a consistent voltage output. Therefore, Zener diodes are widely used in power supplies to provide a reference voltage.
Schottky Diode
Schottky diodes possess a lower forward voltage as compared to other silicon PN junction diodes. This voltage drop can be observed when there is a low current, and the voltage range falls between 0.15 and 0.4 volts. Schottky diodes are constructed differently from other diodes to achieve this performance. As a result, they are commonly used in rectifier applications.
Photodiode
A photodiode is a specialized type of diode that can detect even small amounts of current flow resulting from light. This makes it an extremely useful tool for light detection. Photodiodes are typically reverse-biased and are commonly used in solar cells and photometers. They can also generate electricity.
P-N Junction Diode
The P-N junction diode, also known as a rectifier diode, is an electronic component that is predominantly used in the rectification process. It is composed of semiconductor material, with two layers of semiconductors, one doped with P-type material and the other with N-type material. The junction formed between these layers is called the P-N junction, giving the diode its name.
The P-N junction diode conducts current in the forward direction and blocks it in the reverse direction.
Characteristics of Diode
The characteristics of a diode can be better understood by considering its operation in three distinct regions.
- Forward-biased diode
- Reverse-biased diode
- Zero biased diode
Forward-biased Diode
When a diode is forward-biased and conducting current, a small voltage drop occurs across it. For silicon diodes, this voltage drop is 690mV, and for germanium diodes, it is 300mV. The p-type material has a positive potential energy, while the n-type material has a negative potential energy.
Reverse-biased Diode
When the voltage of a battery isapplied in the opposite direction, a diode is considered to be reverse-biased. In the case of silicon diodes, the reverse current is -20μA, whereas for germanium, it is -50μA. The potential energy is negative across the p-type material and positive across the n-type material. If the reverse voltage is increased beyond a certain point, the diode begins to conduct in reverse, shown by a sharp downward turn in the graph.
Zero-biased Diode
When a diode is zero-biased, no external voltage is applied to it, and the voltage potential across the diode is zero.
Ideal Diodes
An ideal diode is a theoretical semiconductor device that permits current to flow freely in the forward direction while completely preventing current flow in the reverse direction. It is commonly used as a basic model for understanding diode switching and rectification.
When an ideal diode is forward biased, it behaves like a short circuit, offering zero resistance to current flow and having zero voltage drop across its terminals.
When the diode is reverse biased, it behaves like an open circuit. Therefore, no current flows through the diode, while the applied reverse voltage appears across its terminals.
Applications of Semiconductor Diodes
Semiconductor diodes are widely used in electrical and electronic circuits because of their ability to control the direction of current flow. Different types of diodes are designed for specific functions, such as rectification, voltage regulation, switching, signal processing, light emission, and protection.
Some important applications of semiconductor diodes are given below:
- AC-to-DC Rectification: Rectifier diodes are used in power supplies to convert alternating current (AC) into pulsating direct current (DC).
- Clipping Circuits: Diodes are used in clipper circuits to remove or limit selected portions of an input waveform without significantly affecting the remaining signal.
- Clamping Circuits: Diodes in clamper circuits shift the DC level of an AC waveform while maintaining its overall shape.
- Voltage Regulation: Zener diodes are commonly used to maintain a relatively constant voltage and provide voltage stabilization in electronic circuits.
- Reverse Current Protection: Diodes prevent unwanted current from flowing in the reverse direction and are therefore used to protect circuits from reverse-polarity or back-current conditions.
- Switching Applications: Switching diodes can rapidly change between conducting and non-conducting states, making them useful in digital and high-speed electronic circuits.
- Logic Circuits: Diodes can be used to implement basic logic functions and diode-based switching networks.
- Signal Demodulation: Diodes are used in detector circuits to recover information from modulated signals, particularly in simple AM demodulation systems.
- Overvoltage Protection: Special diode arrangements, including Zener and avalanche diodes, can protect sensitive components by limiting excessive voltage.
- Light Emission: Light-emitting diodes (LEDs) convert electrical energy into light and are widely used in indicators, displays, lighting, and communication systems.
- Light Detection: Photodiodes convert incident light into an electrical current and are used in optical sensors, light meters, remote-control receivers, and communication systems.
- High-Frequency Applications: Tunnel diodes and other specialized semiconductor diodes can operate at high frequencies and are used in oscillators, microwave circuits, and high-speed electronic systems.
- Medical Applications: Semiconductor diodes are used in medical and healthcare equipment for sensing, signal processing, optical detection, and light-based diagnostic or therapeutic systems.
- Data Storage and Output: LEDs, photodiodes, and other diode-based devices are used in optical data systems, displays, indicators, and data transmission equipment.
Conclusion
A diode is a two-terminal semiconductor device that allows current to flow mainly in one direction while blocking it in the opposite direction. Made primarily from silicon or other semiconductor materials, diodes are widely used for rectification, switching, voltage regulation, protection, and signal processing. Different types of diodes, such as Zener, Schottky, LED, and PIN diodes, are designed for specific applications, making the diode an essential component in modern electronic circuits.
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