Field and Potential in P-N Junction - Biasing

• Introduction to P-N junction
• Biasing the P-N junction
• Forward biasing
  • Definition
  • Working principle
  • Characteristics of forward biased P-N junction
• Reverse biasing
  • Definition
  • Working principle
  • Characteristics of reverse biased P-N junction

Forward Biasing of P-N Junction

• Definition: Applying a positive voltage to the P-side and a negative voltage to the N-side of a P-N junction is called forward biasing.
• Working principle: In forward biasing, the positive voltage repels the holes in the P-side, attracting them towards the N-side. Similarly, the negative voltage repels the electrons in the N-side, attracting them towards the P-side. This results in the formation of a depletion region.
• Characteristics of forward biased P-N junction:
  • The depletion region width decreases.
  • The electric field across the junction decreases.
  • The potential barrier decreases.
  • The current through the junction increases.
• Example: A forward biased diode in a circuit allows current to flow.

Reverse Biasing of P-N Junction

• Definition: Applying a negative voltage to the P-side and a positive voltage to the N-side of a P-N junction is called reverse biasing.
• Working principle: In reverse biasing, the negative voltage attracts the holes in the P-side, causing them to be pulled away from the N-side. Similarly, the positive voltage attracts the electrons in the N-side, causing them to be pulled away from the P-side. This widens the depletion region.
• Characteristics of reverse biased P-N junction:
  • The depletion region width increases.
  • The electric field across the junction increases.
  • The potential barrier increases.
  • The current through the junction is almost zero.
• Example: A reverse biased diode blocks the flow of current.

Current-Voltage Characteristics of P-N Junction

• Relationship between current and voltage:
  • Forward biased P-N junction: The current increases exponentially with the applied voltage due to the reduced potential barrier.
  • Reverse biased P-N junction: The current remains nearly constant at a very low value due to the increased potential barrier.
• Mathematically, the current-voltage relationship can be given by the diode equation:
  • Forward bias: I = I0 * (e^(V/Vt) - 1)
  • Reverse bias: I = I0 * e^(V/Vt) where I is the current, I0 is the reverse saturation current, V is the applied voltage, and Vt is the thermal voltage (≈ 26 mV at room temperature).
• Example: Plotting the current-voltage characteristics of a forward biased P-N junction.

Energy Band Diagram of P-N Junction under Biasing

• Energy band diagram represents the energy levels of electrons in a material.
• Forward bias: The energy bands in the P and N-sides shift closer together. The potential barrier reduces, allowing current flow.
• Reverse bias: The energy bands in the P and N-sides are pushed further apart. The potential barrier increases, restricting current flow.
• Diagram showing the energy band diagram of a P-N junction under different biasing conditions.
• Example: Analyzing the energy band diagram for a reverse biased P-N junction.

Effect of Temperature on P-N Junction

• The temperature affects the characteristics of a P-N junction.
• Increase in temperature:
  • Forward bias: The current through the junction increases due to the increased availability of carriers.
  • Reverse bias: The reverse saturation current increases exponentially with temperature due to the increased thermal energy of carriers.
• Decrease in temperature:
  • Forward bias: The current through the junction decreases due to the reduced availability of carriers.
  • Reverse bias: The reverse saturation current decreases, reducing the leakage current through the junction.
• Example: Investigating how temperature affects the IV characteristics of a P-N junction.

Capacitance of P-N Junction

• The P-N junction behaves like a capacitor when reverse biased.
• The depletion region acts as the dielectric and the P and N regions act as the plates.
• The capacitance of the P-N junction can be given by the relation: C = (εA) / W where C is the capacitance, ε is the permittivity of the material, A is the area of the junction, and W is the width of the depletion region.
• The capacitance increases with the decrease in the width of the depletion region.
• Example: Calculating the capacitance of a reverse biased P-N junction.

P-N Junction as a Photodiode

• A photodiode is a P-N junction operated under reverse bias.
• When light falls on the junction, electron-hole pairs are generated.
• The generated charge carriers contribute to the current flowing through the diode.
• The current in a photodiode is directly proportional to the intensity of incident light.
• Photodiodes are used in light sensors, solar cells, and optical communication systems.
• Example: Explaining the working of a photodiode and its applications.

Zener Diode and Zener Breakdown

• A Zener diode is a type of diode specifically designed to operate under reverse bias in the breakdown region.
• The reverse breakdown of a Zener diode occurs at a specific voltage called the Zener voltage.
• When the Zener voltage is reached, a sudden increase in current occurs due to the avalanche effect.
• Zener diodes are used for voltage regulation and as voltage references in various electronic circuits.
• Example: Understanding Zener breakdown in a Zener diode.

Avalanche Breakdown

• Avalanche breakdown is another type of breakdown that can occur in a reverse biased P-N junction.
• In avalanche breakdown, the electric field across the depletion region causes accelerated carriers to collide with other atoms, creating additional carriers.
• The process continues as a chain reaction, resulting in a large current flow through the junction.
• Avalanche breakdown can cause damage to the P-N junction if not controlled.
• Example: Explaining avalanche breakdown and its implications.

Applications of P-N Junctions

• P-N junctions have various practical applications in electronic devices.
• Diodes: P-N junctions are the building blocks of diodes used in rectification and signal modulation.
• Transistors: Bipolar junction transistors (BJTs) and field-effect transistors (FETs) utilize P-N junctions for amplification and switching.
• Light-emitting diodes (LEDs): P-N junctions in LEDs emit light when forward biased.
• Photovoltaic cells: P-N junctions in solar cells convert light energy into electrical energy.
• Example: Discussing the applications of P-N junctions in different devices.

Field and Potential in P-N Junction - Biasing

• Recap: P-N junction biasing
• Field in a P-N junction
  • Definition and characteristics
  • Electric field direction
• Potential in a P-N junction
  • Definition and characteristics
  • Potential distribution across the junction
• Effect of biasing on electric field and potential
  • Forward biasing
  • Reverse biasing
• Example: Calculation of electric field and potential in a forward biased P-N junction

Field in a P-N Junction

• Definition: The field in a P-N junction refers to the electric field that exists within the depletion region.
• Characteristics of the field:
  • The field is caused by the presence of immobile ions in the depletion region.
  • It creates a potential barrier that opposes the flow of current.
  • The field strength is stronger in regions closer to the junction.
  • The direction of the field is from the N-side to the P-side in a forward biased junction.
• Example: Analyzing the field distribution in a P-N junction.

Potential in a P-N Junction

• Definition: The potential in a P-N junction refers to the potential difference across the depletion region.
• Characteristics of the potential:
  • The potential barrier exists due to the separation of immobile ions.
  • The potential barrier controls the flow of charge carriers through the junction.
  • The potential is higher on the P-side and lower on the N-side in a forward biased junction.
  • The potential distribution is not linear and depends on the width of the depletion region.
• Example: Understanding the potential distribution in a reverse biased P-N junction.

Effect of Biasing on Electric Field and Potential

• Forward biasing:
  • Reduces the potential barrier by decreasing the width of the depletion region.
  • Weakens the electric field across the junction.
  • Allows a higher current to flow through the junction.
• Reverse biasing:
  • Increases the potential barrier by widening the depletion region.
  • Strengthens the electric field across the junction.
  • Restricts the flow of current, resulting in leakage current.
• Example: Comparing the electric field and potential under different biasing conditions.

Capacitance of P-N Junction in Reverse Bias

• The reverse biased P-N junction acts as a capacitor.
• The capacitance is influenced by the width of the depletion region.
• The larger the depletion region, the higher the capacitance.
• The capacitance can be calculated using the formula: C = (εA) / W where C is the capacitance, ε is the permittivity, A is the cross-sectional area of the junction, and W is the width of the depletion region.
• Example: Determining the capacitance of a reverse biased P-N junction.

Effect of Temperature on P-N Junction Characteristics

• Temperature affects the behavior of a P-N junction.
• Increase in temperature:
  • Forward bias: Increases the current flow due to increased carrier concentration.
  • Reverse bias: Increases the reverse saturation current exponentially due to increased thermal energy.
• Decrease in temperature:
  • Forward bias: Decreases the current flow due to reduced carrier concentration.
  • Reverse bias: Decreases the reverse saturation current, reducing leakage current.
• Example: Analyzing the temperature effect on a reverse biased P-N junction.

Photodiode Application in Light Sensing

• Photodiodes are used as light sensors in various applications.
• When light falls on the photodiode, electron-hole pairs are generated and contribute to the current.
• The current through the photodiode is directly proportional to the intensity of the incident light.
• Photodiodes find applications in:
  • Automatic lighting control systems
  • Optical communication receivers
  • Light intensity meters
• Example: Explaining the working principle of a photodiode used in light sensing.

Zener Diode and Voltage Regulation

• Zener diodes are used for voltage regulation in electronic circuits.
• Zener diodes operate in the breakdown region under reverse bias.
• They maintain a constant voltage across their terminals, regardless of the current flowing through them.
• Zener diodes have a specific breakdown voltage (Zener voltage) that determines their operating characteristics.
• Example: Understanding the use of Zener diodes for voltage regulation.

Avalanche Breakdown and Implications

• Avalanche breakdown is a type of breakdown that occurs in reverse biased P-N junctions.
• It is caused by the generation of additional carriers due to high electric field strength.
• Avalanche breakdown can damage the junction if not controlled.
• Some applications utilize controlled avalanche breakdown, such as:
  • Avalanche photodiodes
  • Avalanche transistors
  • Gas discharge tubes
• Example: Discussing the implications of avalanche breakdown in P-N junctions.

Summary and Conclusion

• Recap of key points:
  • Biasing of P-N junction
  • Field and potential in a P-N junction
  • Effect of biasing on electric field and potential
  • Capacitance and temperature effects
  • Applications of P-N junctions
• Understanding the concepts covered today is crucial for understanding various electronic devices and their applications.
• Next lecture: Diode applications in rectification and signal modulation.
• Q&A session.