What is a Semiconductor Diode?

What is a Semiconductor Diode?

A semiconductor diode is a two-terminal electronic component that allows electric current to pass in one direction but blocks it in the other. They are the silent superheroes of electronics and are used in almost all electrical devices.

When a voltage is applied to the diode, its depletion region’s electric field weakens and holes and free electrons interact with each other. The result is current flow.

P-N Junction

The P-N junction has two different types of impurity ions on each side. When these ions combine together they form electron-hole pairs. These pair of oppositely charged ions are responsible for current flow in the diode. The electric field created by these ions causes the depletion region to become narrower with forward bias.

In the zero bias or thermal equilibrium state, the junction potential is equal on both sides. This allows the majority charge carriers (holes in the n-side and electrons in the p-side) to diffuse across the junction without much resistance.

When an external circuit is connected to the semiconductor diode, a positive voltage on the n-side attracts holes away from the junction and towards the positive electrode. Similarly, a negative voltage on the p-side attracts electrons away from the junction and towards the negative electrode. The result is that a layer of free charge, known as the depletion zone, forms around the junction and behaves like an insulator.

As the applied voltage increases, the electric field in the depletion zone becomes stronger. This helps the electrons in the n-side to overcome the coulomb barrier and conduct across the junction. This is referred to as the Zener effect or the avalanche breakdown process. At a critical voltage the depletion zone breaks down and current flows through the diode.

Depletion Region

When a negative voltage is applied across the semiconductor diode it pushes or repels electrons towards the junction from the negative terminal (the cathode). This, combined with holes dc to ac power inverter being pushed in the opposite direction away from the junction by the positive voltage (the anode), results in current flowing through the diode. This is called reverse bias.

This increase in current through the diode occurs because of a buildup of charge close to the junction. This charge is created by electrons crossing over to fill vacancies in the p-type region. The resulting concentration of negative charge near the junction is called the depletion region.

It is this zone of negative charge that stops more electrons from combining with vacancies and jumping across the junction to the n-type side. It is this layer of charge that gives the semiconductor diode its high resistance characteristics.

This depletion region also creates an electric field that acts to widen the depletion zone and strengthen its barrier against further electrons crossing over. This effect is what increases the drift component of current through the p-n junction under reverse bias, and decreases the diffusion component. The difference between these two current components is known as the built-in voltage or junction potential.

Forward Bias

When a positive voltage is applied to the p-n junction, the potential barrier decreases. This makes it easier for electrons to move through the diode and flow into the n region. The electrons then recombine with the holes that were pushed by the negative voltage into the p-n junction. This process is called Forward Current.

The electric field of the applied voltage reduces the built-in potential between the p and n regions and decreases the width of the depletion region. As the voltage increases, the majority carrier density of the n-type material increases. The electrons in the n-type material will move into the valence band of the p-type material, lowering their energy levels and decreasing the width of the depletion region.

This behavior makes a semiconductor diode a very useful device. It is used in many electronic circuits to switch current between two points. It can also be used to convert electrical energy into light (e.g. LEDs). A current-limiting resistor is often connected in series with the diode to prevent overheating. The direction in which the external voltage is applied to a diode determines its characteristics. This is why it is important to understand how a semiconductor responds to different types of voltages, known as Biasing. If you don’t know this, you might end up with a circuit that does not work or even damages the diode.

Reverse Bias

When a negative voltage is applied to the diode, it creates a potential Microprocessor barrier across the p-n junction. This potential barrier slows down the electrons and holes crossing the junction. The current that flows in this condition is called the reverse current. Initially the reverse current is small but as the external voltage increases it becomes larger.

The positive side of the external voltage pushes the free electrons in the n region towards the p-type material. This causes more positive impurity ions to be created in the depletion layer and widens it. The negative side of the external voltage pulls the holes in the p-type region and this decreases the width of the depletion layer. Eventually, the electrons and holes cross over the junction and recombine. The diode starts conducting a substantial amount of current. This point on the characteristic curve is called the knee voltage.

A diode is used in many circuits, from protecting signals to power domains and to limiting transient voltage events. They are also commonly used as isolators, voltage-controlled oscillators, amplification devices and sensors. They are also found in solar cells, electronic switches and lighting applications. It is a very simple but useful device!