Current Through a P-N Junction – An introduction

• What is a P-N Junction?
• Introduction to Current Through a P-N Junction
• Understanding the Theory of P-N Junctions
• Basic Structure of a P-N Junction
• Formation of Depletion Region
• Forward Bias and Reverse Bias
• Role of Electric Field in Current Flow
• Generation and Recombination of Charge Carriers
• Diffusion Current and Drift Current
• Concept of Minority and Majority Carriers

The P-N junction diode equation

• The current through a P-N junction diode can be described by the equation:
  • I = I0 * (e^((V/Q)*n) - 1)
    • I is the current
    • I0 is the reverse saturation current
    • V is the voltage across the junction
    • Q is the electron charge
    • n is the ideality factor

Characteristics of P-N junction diodes

• V-I characteristics:
  • For forward bias, the current increases with voltage
  • For reverse bias, the current is very small
• Rectification:
  • P-N junction diodes are used for rectification, converting alternating current (AC) to direct current (DC)
  • During forward bias, current flows easily, allowing only one half of the input AC cycle to pass through

Diode as a Switch

• P-N junction diodes can be used as switches in electrical circuits
• During forward bias:
  • The diode is in the “on” state and acts as a closed switch
  • Current flows easily through the diode
• During reverse bias:
  • The diode is in the “off” state and acts as an open switch
  • No current flows through the diode

Applications of P-N junction diodes

• Rectifiers: Convert AC to DC
• Voltage regulation: Stabilize voltage levels
• Clipping and clamping circuits: Modify waveforms
• Logic gates: Building blocks of digital circuits
• Signal detection and mixing: Enabling wireless communication

Diode Limitations

• Small forward voltage drop: Typical forward voltage drop of a diode is around 0.7V, which limits the voltage range it can handle
• Reverse leakage current: Even in reverse bias, a small amount of current (reverse leakage current) may flow
• Temperature sensitivity: Diode characteristics vary with temperature, affecting performance
• Power dissipation: Diodes have a limit on the amount of power they can dissipate without damage

P-N junction diode symbol

The symbol for a P-N junction diode is: `` Anode Cathode | | -|- | D |

`` The arrow points in the direction of the forward current flow.

Example: P-N junction diode forward bias circuit

Consider a circuit with a P-N junction diode connected in forward bias with a battery: `` + 10V - -|<|- | D |

``

• The positive terminal of the battery is connected to the anode of the diode.
• The negative terminal of the battery is connected to the cathode of the diode.

Example: P-N junction diode reverse bias circuit

Consider a circuit with a P-N junction diode connected in reverse bias with a battery: `` + 10V - -|>- | D |

``

• The positive terminal of the battery is connected to the cathode of the diode.
• The negative terminal of the battery is connected to the anode of the diode.

Calculation of P-N junction diode current

• The current through a P-N junction diode can be calculated using the diode equation.
• Example calculation:
  • Forward bias with V = 0.7V
  • I0 = 1μA
  • n = 2
  • Q = 1.6 x 10^-19 C
• Substitute these values in the diode equation to calculate the current.

Summary

• P-N junction diodes are crucial electronic components that can control the flow of current in electrical circuits.
• The current through a P-N junction diode can be described by the diode equation.
• P-N junction diodes have various applications, including rectification, voltage regulation, and signal detection.
• Diodes have limitations such as forward voltage drop, reverse leakage current, temperature sensitivity, and power dissipation.
• P-N junction diodes can be connected in forward or reverse bias, depending on the desired current flow.

Slide 21

• Factors Affecting P-N Junction Diode Behavior
  • Temperature: Diode characteristics change with temperature, affecting operation.
  • Intensity of Light: P-N junction diodes are responsive to light and their behavior can be manipulated.
  • Doping Concentration: The amount of doping in the P and N regions affects the behavior and performance of the diode.
  • Physical Size: The physical size of the diode can influence its power handling capabilities.
  • Breakdown Voltage: The maximum voltage that can be applied across the diode without causing breakdown is an important factor.

Slide 22

• Role of Band Theory in P-N Junction Diodes
  • Valence Band: The band in an atom where valence electrons reside.
  • Conduction Band: The band in an atom where electrons are free to move and conduct electricity.
  • Forbidden Energy Gap: The energy range between the valence and conduction bands where electrons cannot exist.
  • Electron Excitation: Electrons can move from the valence band to the conduction band by acquiring sufficient energy.
  • Formation of the Depletion Region: The region at the junction where electron excitation and recombination occur.

Slide 23

• Generation and Recombination of Charge Carriers
  • Generation: When electrons move from the valence band to the conduction band by absorbing energy, creating electron-hole pairs.
  • Recombination: When an electron from the conduction band recombines with a hole from the valence band, releasing energy.
  • Minority Carriers: The type of charge carriers (electrons or holes) that are less in quantity compared to the majority carriers.
  • Majority Carriers: The type of charge carriers that are in abundance in the material, either electrons or holes.

Slide 24

• Diffusion Current in P-N Junction Diodes
  • Concentration Gradient: Difference in carrier concentrations at the junction causes diffusion movement.
  • Diffusion: Movement of charge carriers from an area of high concentration to an area of low concentration.
  • Diffusion Current: The current generated due to diffusion of charge carriers across the junction.
  • Drift Current: The current flow due to the motion of majority charge carriers under the influence of an electric field.
  • Total Current: The sum of diffusion current and drift current gives the total current flowing through the diode.

Slide 25

• Calculation of Diffusion Current and Drift Current
  • Diffusion Current: Calculated using the equation I_d = q * A * D * (dp/dx)
  • Drift Current: Calculated using the equation I_drift = q * A * μ * n * E
  • Total Current: Calculated by summing the diffusion current and drift current, I_total = I_diffusion + I_drift.
  • A: Cross-sectional area of the junction
  • D: Diffusion coefficient
  • dp/dx: Gradient of the concentration of minority carriers
  • μ: Mobility of the majority charge carriers
  • n: Concentration of majority charge carriers
  • E: Electric field strength

Slide 26

• Example Calculation of Diffusion Current and Drift Current
  • Given values:
    • A = 1 x 10^-4 m^2
    • D = 10^-4 m^2/s
    • dp/dx = 10^16 / m^3/m
    • μ = 0.1 m^2/Vs
    • n = 10^23 / m^3
    • E = 100 V/m
  • Calculate the diffusion current, drift current, and total current.

Slide 27

• P-N Junction Diode as a Rectifier
  • Rectification: The process of converting alternating current (AC) to direct current (DC) by allowing current flow in only one direction.
  • Half-wave Rectifier: A circuit utilizing a P-N junction diode that allows only one half of the input AC cycle to reach the output.
  • Full-wave Rectifier: A circuit with a bridge configuration that utilizes four diodes to allow both positive and negative half-cycles of the input AC signal to pass through.

Slide 28

• Applications of P-N Junction Diodes: Light Emitting Diodes (LEDs)
  • LEDs are a type of diode that emit light when forward biased.
  • Used in various applications, including indicator lights, display screens, and traffic signals.
  • Different LED colors are achieved by using different semiconductor materials.
  • Efficient and long-lasting compared to traditional incandescent bulbs.

Slide 29

• Applications of P-N Junction Diodes: Solar Cells
  • Solar Cells convert sunlight directly into electricity using P-N junction diodes.
  • Photovoltaic effect: The process by which solar cells convert light energy into electrical energy.
  • P-N junction diodes in solar cells absorb photons and generate an electric current.
  • Used in various applications such as residential and commercial solar power systems.

Slide 30

• Summary:
  • P-N junction diodes play a crucial role in modern electronics.
  • The behavior of P-N junction diodes is influenced by factors like temperature, light intensity, doping concentration, size, and breakdown voltage.
  • Band theory explains the behavior of electrons and holes in P-N junction diodes.
  • Generation and recombination of charge carriers occur at the junction.
  • Diffusion current and drift current contribute to the total current in P-N junction diodes.
  • P-N junction diodes are used in rectifier circuits, LEDs, solar cells, and various other applications.