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PN Junction Diodes

Outline of PN Junctions

• Formation of Junction: Understanding how a p-n junction is formed.

• Physical Operation of Diode: Overview of how diodes function.

• Contact Potential and Space Charge Phenomena: Explanation of the built-in potential and depletion regions.

• I-V Characteristics: Characteristics of current-voltage behavior in diodes.

• Zener Diodes: Specialized behavior of Zener diodes.

• Physical Operation of Special Diodes:

  • Tunnel Diode: Quantum tunneling effect in operation.

  • LED (Light Emitting Diode): Emission of light through electron recombination.

  • OLED (Organic LED): Use of organic compounds for light emission.

  • Photodiode: Conversion of light to electrical energy.

  • Varactor Diode: Voltage-variable capacitance behavior.

P-N Junctions

• Movement of carriers creates:

  • Space Charge Region: Depletion region forms at the junction, influenced by charge carriers.

  • Induced Electric Field: The electric field near x=0 and built-in potential voltage (vbi) develops across the junction.

Built-in Potential

• The space charge or depletion region creates an electric field that results in a potential voltage (vbi) across the junction.

Calculation of Built-in Potential

• Key components affecting built-in potential:

  • Energy Bands: EC, EV, Ei, EF and their relationships between n-region and p-region.

  • Depletion Region Analysis: Relationships and calculations involving charge densities (q, Nd, Na).

Forward Biased PN Junction

• Application of voltage (vD)

• Induces an electric field, EA, which opposes the original space-charge electric field.

• Results in a smaller net electric field and reduced barrier between n and p region.

Reverse Bias

• Characteristics under reverse bias conditions:

  • Increase in space-charge width (W).

  • Increases VR leading to additional fixed charges affecting junction capacitance.

Band Diagram: Forward and Reverse Biased

• Visual representation of energy bands and potential differences:

  • Movement of energy bands under forward and reverse conditions.

  • Visualization of electric fields and voltages across junctions.

Space Charge Region (Depletion Region)

• Concepts to Understand:

  • Charge Density: Distribution within depletion region and across p and n materials.

  • Poisson's Equation: Governing the electric field and charge distributions in semiconductor physics.

  • Electric Field Strength: Calculation of maximum electric field based on doping concentrations (Na, Nd).

Ideal Current-Voltage (I-V) Characteristics

• Diode conducts significant current only in forward bias,

• I-V curve modeled as exponential function in forward bias.

• Near-zero current in reverse bias region.

Ideal Diode Equation

• The fundamental equation relating current and voltage in diodes:

  • Describes temperature dependence (kT/q known as thermal voltage, VT).

  • VT = 25.9 mV at 300K.

Breakdown Voltage

• Breakdown voltage (BV) behavior in diodes:

  • Heavily doped diodes exhibit lower breakdown voltage.

  • Significant current increase upon breakdown occurrence.

Zener & Avalanche Breakdown

• Zener Effect: Electron tunneling in heavily doped diodes under reverse bias.

  • Diagrams demonstrating operational characteristics.

• Avalanche Breakdown: Explaining charge multiplication under reverse bias conditions.

Zener Diodes

• Breakdown diode designed for specific breakdown voltage (Zener diode).

Tunnel Diode

• Operates via quantum mechanical tunneling of electrons across potential barriers.

• Known for negative resistance characteristics useful in oscillators.

Tunnel Diode Characteristics

• V-I curves showcasing the distinct operating regions.

Varactor Diodes

• Characterized as voltage-variable capacitors.

• Capacitance changes with the applied voltage, affecting depletion region geometries.

Schottky Diode

• Describes metal-semiconductor contacts:

  • Characteristics of the Schottky barrier and its rectifying properties.

Ohmic Contact

• Details of Ohmic contact relationships based on metal-semiconductor interactions.

• Differentiation based on the potential barrier heights.

Photodiodes

• Photodiodes convert light into electrical current through the photovoltaic effect.

• Operate in reverse bias when illuminated.

Light Emitting Diodes (LEDs)

• Convert electrical energy into light (electroluminescence) during forward bias.

Organic LEDs (OLEDs)

• Utilize organic compounds for light-emitting capabilities; used in displays.

• Transparent electrodes are common in OLED technology.