Diode: Definition, Symbol, and Types of Diodes
Key Learnings:
|
Diode Definition |
A diode is a component that
restricts the direction of electric current flow, primarily allowing current
to pass in one direction. |
|
Symbol and Orientation |
The diode symbol indicates the direction of conventional
current flow, from the anode to the cathode. |
|
PN Junction Role |
The PN junction in diodes forms a depletion zone that serves as a
barrier to current in the reverse direction. |
|
Forward vs. Reverse Bias |
In forward bias, diodes conduct electricity when the
external voltage exceeds the barrier potential, while in reverse bias, they
block current, expanding the depletion region. |
|
Practical Importance |
Understanding diode functionality is crucial for a wide range of
applications in electronics, from basic circuits to complex digital systems. |
What is a Diode?
A diode (also known as p-n junction diode) is an electronic component with two terminals that allows current to flow in only one direction (forward) when operating within a specific voltage range. In ideal cases, a diode would have zero resistance in the forward direction and infinite resistance in the reverse direction.
However, in practical applications, diodes cannot achieve perfect zero or infinite resistance. Instead, they have very low resistance in the forward direction to conduct current and very high resistance in the reverse direction to block it. This makes a diode function like a valve in an electrical circuit.
The most commonly used diodes are semiconductor diodes. These diodes conduct electricity when the forward voltage exceeds a certain threshold, a state known as "forward bias." In the reverse direction, they exhibit high resistance, a state known as "reverse bias," effectively preventing current flow.
A diode is considered "forward biased" when it conducts current in the forward direction. Conversely, it is "reverse biased" when connected in a circuit to block current in the reverse direction, exhibiting high resistance.
However, a diode can only block current in the reverse direction up to a certain voltage. If the reverse voltage exceeds this specified limit, known as the "reverse breakdown voltage," the diode will start to conduct electricity in the reverse direction. This breakdown causes the diode to allow current flow even in the high resistance direction, which is why, in practice, diodes have high resistance in the reverse direction rather than infinite resistance.
A PN junction forms the fundamental structure of a semiconductor diode, behaving like a short circuit when forward biased and an open circuit when reverse biased. The word "diode" comes from "di-ode," signifying a device with two terminals. Diodes are crucial in many electronics projects, such as those found in the Arduino starter kits, Rectifier circuits etc.
Symbol of Diode
The symbol for a diode is shown below. The arrowhead indicates the direction of conventional current flow when the diode is forward biased. This means that the anode is connected to the p-type material and the cathode is connected to the n-type material.
To create a PN Junction diode, a p-type semiconductor is attached with a n-type semiconductor. The joining side of these two semiconductor crystals create a junction barrier, hence creating PN junction.
The p-type material is made by doping the silicon/germanium semiconductor with trivalent material. Conversely, n-type material is made by doping the semiconductor with pentavalent material.
The terminal connected to p-type material is referred as Anode. Similarly, the terminal connected to n-type material is referred as Cathode.
Working Principle of Diode
The working principle of a diode is based on the interaction between n-type and p-type semiconductors. In an n-type semiconductor, there is a high concentration of free electrons and a low concentration of holes.
Essentially, the n-type semiconductor is characterized by its abundance of free electrons and scarcity of holes.
Unbiased Diode
Now if we see what will happen when a n-type region and a p-type region is connected to form the PN junction crystal. Here the concentration of holes in the p-type region is high and it is low in the n-type region. This creates a concentration difference between both sides and due to this, majority carriers diffuse from one side to another.
Conversely, in the n-type region the concentration of free is high and in p-type region it is low. It creates a concentration difference between the regions and due to this, free electrons from n-type region start to diffuse in the p-type region.
As free electrons from the n-type region diffuse into the p-type region, they recombine with holes present in the p-type region, resulting in uncovered negative ions. Similarly, holes from the p-type region diffusing into the n-type region recombine with free electrons in the n-type region, leading to uncovered positive ions in the n-type region.
As a result, a layer of negative ions forms in the p-type side and a layer of positive ions forms in the n-type region along the junction between these two semiconductor types. These layers of uncovered ions create a region in the middle of the diode where no charge carriers are present because all charge carriers recombine within this area. Due to the absence of charge carriers, this region is known as the depletion region.
Once the depletion region is formed, the electric field across it prevents further diffusion of charge carriers from one side to the other in the diode.
The layer of uncovered positive ions in the n-type side repels the holes in the p-type side, while the layer of uncovered negative ions in the p-type side repels the free electrons in the n-type side. This creates a potential barrier across the junction, effectively preventing further diffusion of charge carriers.
When the reverse voltage across a diode exceeds a safe limit, the increased electrostatic force and kinetic energy of minority charge carriers can cause covalent bonds to break. This leads to a substantial generation of free electron-hole pairs within the diode, and this process accumulates over time.
Forward Biased Diode
So far we saw, the diode remains open due to junction barrier when it is unbiased. Now let's see what will happen if excite the diode with forward bias (i.e. positive voltage).
The diode is said to be forward biased when the p-type terminal (Anode) is connected with positive terminal of the source and n-type (Cathode) is connected in the negative terminal of the source.
Now if we increase the voltage slowly from zero.
Initially, no current flows through the diode because the majority charge carriers do not experience enough influence from the external electric field to cross the depletion region. As mentioned, the depletion region acts as a potential barrier that prevents the majority charge carriers from moving across.
This potential barrier is known as the forward bias potential barrier. Majority charge carriers begin crossing this barrier only when the externally applied voltage across the junction exceeds the potential of the forward bias barrier. In silicon diodes, this barrier potential is approximately 0.7 volts, while for germanium diodes, it is around 0.3 volts.
Reverse Biased Diode
We saw that the diode conducts current if the forward voltage is above its barrier potential. Now we will see what will happen if the diode is reverse biased.
A diode is said to be reverse biased if it's n-type terminal (Cathode) is connected to the positive terminal of the source and p-type terminal (Anode) is connected in the negative side of the source.
Under that conditions, due to the electrostatic attraction of the negative potential from the source, holes in the p-type region are pushed further away from the junction, resulting in more uncovered negative ions at the junction.
Similarly, the free electrons in the n-type region are pushed away from the junction towards the positive terminal of the voltage source, leaving more uncovered positive ions at the junction.
As a result of this effect, the depletion region widens. This state of a diode is known as reverse bias. During reverse bias, majority carriers do not cross the junction but instead move away from it. This mechanism effectively blocks the flow of current through the diode.
As mentioned earlier, there are always some free electrons in the p-type semiconductor and some holes in the n-type semiconductor. These opposite charge carriers within a semiconductor are known as minority charge carriers.
During reverse bias, the holes in the n-type side find it easier to cross the reverse-biased depletion region because the electric field across the depletion region aids minority charge carriers in crossing. As a result, a small current flows through the diode from the positive to the negative side. This current, known as reverse saturation current, is very small in magnitude due to the limited number of minority charge carriers present in the diode.
Increasing the reverse voltage beyond safe limits causes electrostatic forces and kinetic energy to break covalent bonds, generating many free electron-hole pairs in a cumulative process that can damage the diode.
The large number of generated charge carriers can contribute to a substantial reverse current in the diode. If this current is not controlled by an external resistor connected to the diode circuit, it could potentially lead to permanent damage to the diode.
V-I Characteristic curve of diode
Types of Diode
The types of diode include:
- Zener diode
- PN junction diode
- Tunnel diode
- Varactor diode
- Schottky diode
- Photodiode
- PIN diode
- Laser diode
- Avalanche diode
- Light emitting diode
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