Hey Guys! Our main aim is to keep you updated with credible information relating to engineering and technology so you keep coming back for what we have to offer. Today, I’m going to unlock the details on the Introduction to PN Junction. This is a junction formed when two different types of semiconductor material i.e. P-type and N-type are joined together. This process is a building block for the development of the diode.
The P-type material is formed by the addition of impurity with trivalent doping material like Boron, while the N-type material is formed by the addition of pentavalent doing material like Antimony. It is important to note that the overall charge of both N-type and the P-type material is electrically neutral unless both are joined together.
In this tutorial, I’ll discuss each and everything related to the PN junction i.e. how it is formed and used in the construction of the different electronic device. Let’s dive in and explore what is this about and everything you need to know.
Introduction to PN Junction
- The PN junction is a junction formed together by P-type semiconductor and N-type semiconductor material. The junction plays an important role in the development of the diode which is the building block of most of the semiconductor devices like transistors, BJTs, solar cells and integrated circuits.
- The PN junction is generated when one side of the junction is doped with acceptor impurity (trivalent) in P-region and another side is doped with donor (pentavalent) impurity in N-region.
- The region where both N-type and P-type material are joined together is called junction, also known as the boundary of the semiconductor.
- The majority charge carriers in the P region are holes, while majority charge carriers in N-region are electrons.
- When the junction is formed, the holes present in the P region are diffuse into the N-region, leaving behind the negative charge in the P region which then recombines with electrons, resulting negatively charged ions in the P-region.
- Similarly, electrons present at the N-region diffuse into the P-region leaving behind the positive charge in the N-region and recombine with the holes, creating positively charged ions in the N-region.
- As the junction is formed each region of silicon crystal becomes depleted from major charge carriers around the junction. This region is known as depletion or space charge region. The width of this region highly affects the current flowing between the junction.
- Under normal conduction, when there is no voltage applied across the PN junction, the junction is said to have in an equilibrium state. The potential difference formed in that state is called built-in potential which is 0.7 V for silicon semiconductors and 0.3 V for germanium semiconductors.
- The depletion region formed around the junction where diffusion of holes and electrons occurs will generate an electric field that acts as a shield between the electron-hole diffusion.
- When there is a lot of diffusions occur, it will create a point where no more diffusion is possible due to the electric field generated across the junction.
- At this point, the junction will behave as a barrier which prevents further diffusion. This point is known as a potential barrier.
- External voltage more than this potential barrier is required to put the junction in the operating condition where it starts conducting.
Forward Bias Condition
- Voltage biasing is known as how the voltage is applied at the PN junction. If a positive voltage is applied at the P-region and a negative voltage is applied at the N-region, it makes the junction forward biased which exhibits maximum conductivity and low resistance.
- When a battery is connected this way, both charge carriers i.e. electrons in the N-region and holes in the P-region will attract toward the junction, that will reduce the width of the depletion region.
- The positive voltage applied at the P-region will repel the holes, similarly, a negative voltage applied at the N-region will repel the electrons.
- With forward bias voltage, the electrons present in the N-region will push toward the P-region, similarly, holes in the P-region will continue to diffuse into the N-region as long as the forward bias voltage is applied across the junction.
- The diffusion of these charge carriers is highly dependent on the intensity of the voltage applied. A small change in the applied voltage will change the frequency of diffusion across the junction.
- The length at which electron passes the PN junction and reach the P-region before they recombine with holes is called diffusion length. Diffusion length is highly dependent on the intensity of the forward bias voltage applied across the junction.
- So, we can conclude that, when PN junction is forward biased, both electrons and holes will push towards the junction, the depletion region will be very thin, resistance is low, and there is a significant amount of current flowing across the junction.
- In the forward biased condition, electrons in the N region can easily cross the junction and join the holes in the P region and same is true for holes present in the P region.
Reverse Bias Condition
- If PN junction is biased in such a way, its P-side is connected with a negative voltage while N-side is connected with a positive voltage, it will constitute a reverse biased condition where no current flow between the junction, resulting maximum resistance. In this condition, a voltage across the cathode will be relatively larger than the anode side.
- Now, P-region is connected with the negative side and N-region is connected with the positive side, in this condition charge carriers (holes in P-region and electrons in N-region) will push away from the depletion region, resulting the width of the depletion region to increase.
- The width of depletion region behaves in proportion with the reverse bias voltage applied across the junction.
- When reverse bias voltage increases, it will increase the depletion region, which stops the current flow between the junction and exhibits maximum resistance at that point.
- In this condition, PN junction behaves as an insulator.
- When a reverse biased voltage is increased above a certain level, it allows the depletion region to break, resulting in the flow of current between the PN junction. This is called the breakdown region which is not harmful to diodes as long as the current flowing remains within the limit and prevents the device from overheating.
Characteristic Curve of Silicon Diode
Following figure shows the I vs V characteristic curve of a silicon diode.
- The majority carriers in the P-region cross the junction and recombine with the majority carriers in the N-region and same is true for the majority carriers present in N-region.
- This process will mask the static positive state charge in the N-region and static negative space charge in the P-region. The area where this space charge is created is known as depletion region.
- We can see from the curve when the forward bias voltage is increased, it will increase the current flowing through the diode. Forward bias voltage above 0.7 V is required to allow the diode starts conducting.
- Under reverse bias condition, a diode will show a different behavior and exhibits no or very little current flowing through the diode.
- The small current flowing under reverse bias condition is known as leakage current. Germanium semiconductors come with more leakage current as compared to silicon semiconductors which makes them (germanium semiconductors) less suitable for the development of electronic devices.
- The reverse bias voltage won’t affect the leakage current as long it remains within a certain limit.
- When reverse bias voltage increase too much, it will break the depletion region, causing the significant amount of current flowing through the diode.
- High resistance is added to the diode in order to avoid diode from reaching this breakdown condition.
- We can fabricate diodes with different voltage ratings based on our needs and demands, however, diodes with high reverse voltage ratings are preferable.
- NPN transistor is the perfect example of silicon diode where two silicon diodes are joined together back to back.
Difference between Depletion Region, Potential Barrier, and Built-in Potential
Some people get confused between these three terms. Let’s find out the main difference between them.
- Depletion Region. Region depleted with main charge carriers is known as depletion region.
- Thin depletion region, a function of forward biasing, predicts there is a considerable amount of current flowing through the junction.
- Similarly, thick depletion region means no or very little current flowing through the junction which is the result of reverse biasing.
- Potential Barrier. When diffusion of electrons and holes occurs across the junction, it develops electronic field around the junction which resists the further diffusion of electrons and holes.
- In this case depletion region acts as a barrier. This barrier is known as a potential barrier.
- Built-in Potential. When there is no external biasing applied across the PN junction the junction is said to have in an equilibrium state.
- The potential difference generated across the junction in the equilibrium state is called built-in potential.
- In the thermal equilibrium state, built-in potential is equal to the potential across the depletion region.
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