In this note, we are going to learn about a topic called “What is PN Junction Diode and its working?“, and its working as well. Welcome to Poly Notes Hub, an one stop solution for engineering notes for diploma engineering students.
Author Name: Arun Paul.
What is PN Junction Diode ?
When a p-type semiconductor is suitably joined to an n-type semiconductor, it is called a PN Junction Diode. Below, shows the construction and Symbol of the PN Junction Diode. Most semiconductor devices contain one or more PN Junctions. The PN Junction is of great importance because it is in effect, the control element for semiconductor devices. A thorough knowledge of the formation and properties of pn Junction can enable the reader to understand the semiconductor devices.
What is PN Junction ?
A PN junction, sometimes referred to as a semiconductor junction, is an essential part of semiconductor devices including transistors and diodes. Two distinct kinds of semiconductor materials are joined together to create it. Learn More about ON Junction.
Here is an explanation:
P-Type Semiconductor: This area has been purposefully doped with impurities to introduce positive charge carriers, sometimes known as “holes.” These substances frequently include Group III elements like boron from the periodic table.
N-Type Semiconductor: Materials that introduce negative charge carriers, often electrons, are added to this region. These substances frequently include elements from Periodic Table Group V, including phosphorus.
Above, we attached an image where you can easily understand where the PN Junction exist
About Depletion Layer in PN Junction
At the instant of pn-junction formation, the free electrons near the junction in the n region begin to diffuse across the junction into the p region where they combine with holes near the junction. The result is that the n region loses free electrons as they diffuse into the junction. This creates a layer of positive charges(pentavalent ions) near the junction. As the electrons move across the junction, the region loses holes as the electrons and holes combine. The result is that there is a layer of negative charges(trivalent ions)near the junction.
These two layers of positive and negative charges form the depletion region (or depletion layer). The term depletion is because, near the junction, the region is depleted (i.e. emptied)of charge carriers (free electrons and holes) due to diffusion across the junction. It may be noted the depletion layer is formed very quickly and is very thin compared to the region and the p region. For clarity, the width of the depletion layer is shown exaggerated.
What is Barrier Potential ?
An electric field is created across the junction as a result of the depletion region’s development, which prevents additional charge carrier flow. This electric field generates a potential difference called the “barrier potential” or “junction potential.”
The barrier potential of a pn Junction depends upon several factors including the type of semiconductor material, the amount of doping, and temperature. The typical barrier potential is approximately:
For Germanium 0.3 V and for Silicon 0.7 V
Types of Biasing of PN Junction Diode
There are two types of biasing in PN Junction diode
- Forward Biasing
- Reverse Biasing
Forward Biasing
When external d.c. voltage applied to the junction is in such a direction that it cancels the potential barrier, thus permitting current flow, it is called forward biasing.
To apply forward bias, connect the positive terminal of the battery to the p-type and the negative terminal to n-type as shown in Figure.
As the potential barrier voltage is very small (0.1 to 0.3 V), therefore, a small forward voltage is sufficient to eliminate the barrier. Once the potential barrier is eliminated by the forward voltage, junction resistance becomes almost zero and a low resistance path is eliminated for the entire circuit. Therefore, current flows in the circuit. This is called forward current. With forward bias to pn Junction, the following points are worth noting:
- The potential barrier is reduced and at some forward voltage (0.1 to 0.3 V), it is eliminated.
- The junction offers low resistance (called forward resistance ) to current flow.
- Current flows in the circuit due to the establishment of a low resistance path. The magnitude of the current depends upon the applied forward voltage.
Reverse Biasing
When the external d. c. voltage applied to the junction is in such a direction that the potential barrier is increased, it is called reverse biasing.
To apply reverse bias, connect the negative terminal of the battery to the p-type and the positive terminal to the n-type as shown in Figure.
The applied reverse voltage establishes an electric field that acts in the same direction as the field due to the potential barrier. Therefore, the resultant field at the junction is strengthened and the barrier height is increased. The increased potential barrier prevents the flow of charge carriers across the junction. Thus, a high-resistance path is created, the following points are worth noting:
- The potential barrier is increased.
- The junction offers very high resistance (called reverse resistance) to current flow.
- No current flows in the circuit due to the establishment of a high resistance path.