In-Depth Notes on Semiconductor Devices: Diodes

P-N Junction Diode

  • Definition: A P-N junction diode is formed when a semiconductor (like silicon or germanium) is doped such that one side becomes p-type and the other side becomes n-type, creating a junction.
  • Junction Representation:
    • The circuit symbol shows the direction of conventional current via an arrow.

Formation of Depletion Layer and Barrier Potential

  • Diffusion Process:
    • Holes from the p-side diffuse to the n-side and combine with electrons, becoming neutral.
    • Electrons from the n-side diffuse to the p-side and also become neutral.
  • Charge Imbalance:
    • Electrons leaving the n-side create an immobile positive charge layer; holes leaving the p-side create an immobile negative charge layer.
  • Depletion Region:
    • The regions of positive and negative charges together form the depletion region, typically a few microns wide.
  • Electric Field Creation:
    • The immobile charges create an electric field (Potential Barrier, $V_B$) that prevents further diffusion of charge carriers.
    • The potential barrier depends on the crystal's nature, temperature, and doping levels.

Biasing Conditions

Forward Bias
  • Connection: Positive terminal of the battery is connected to the p-side, negative to the n-side.
  • Securing Motion: Electrons are repelled from the n-side and holes from the p-side toward the junction.
  • Effective Barrier Reduction:
    • When external voltage $V$ is applied: Effective barrier potential becomes $(V_B - V)$.
    • This reduces the width of the depletion layer, allowing current to flow.
Reverse Bias
  • Connection: Positive terminal connected to n-side, negative to p-side.
  • Securing Motion: Electrons are attracted away from the junction on the n-side, and holes are attracted away on the p-side.
  • Effective Barrier Increase:
    • Effective barrier potential becomes $(V + V_B)$.
    • This increases the width of the depletion layer, consequently inhibiting current flow.

Current-Voltage (I-V) Characteristics

  • The I-V curve depicts the relationship between the current through the diode and the voltage across it:
    • Zero Bias: No external voltage leads to no current flow (potential barrier intact).
    • Forward Bias:
    • Current flows after the potential barrier is overcome, typically around 0.7V for silicon and 0.3V for germanium.
    • As voltage increases, the current increases exponentially (non-linear region).
    • Reverse Bias:
    • Minor current called reverse saturation current flows, which can increase before breakdown occurs, potentially damaging the diode.

Applications of P-N Junction Diodes

  • Photodiode: Sensitive to light in reverse-biased configuration.
  • Solar Cells: Converts sunlight into electric energy.
  • LED Lighting: Function as light-emitting diodes when forward biased.
  • Rectifiers: Converts AC to DC in power supplies.
  • Voltage-Controlled Oscillators: Used in varactors for frequency control.

Light Emitting Diode (LED)

  • Definition: A semiconductor device emitting light when electric current flows through it.
  • Working Principle: Electrons recombine with holes, emitting light (photons) during the process.
  • Direction of Current: Allows current to flow forward and blocks reverse current.

Advantages of LED

  • Cost-effective and easy to handle.
  • Low power consumption with minimal operational voltage.
  • Fast response time with no warm-up needed.
  • Versatile applications in illumination and signaling.

Disadvantages of LED

  • Issues with light quality.
  • Sensitivity to electrical polarity and voltage fluctuations.
  • Efficiency droop and concerns regarding ecological impact on insects.

Liquid Crystal Display (LCD)

  • Definition: A flat-panel display using liquid crystals, primarily in TVs and computer monitors.
  • Construction: Consists of two polarized glass plates with a liquid crystal layer in between and electrodes for potential application.
  • Working Mechanism:
    • LCDs do not emit light but manipulate externally sourced light.
    • Light passes through one polarizer to the next by affecting the liquid crystal alignment based on applied bias.

Applications of LCD

  • Used in digital watches, calculators, TVs, and mobile screens.

Advantages of LCD

  • Compact and thin profile.
  • Low power consumption with less heat production.
  • Economical compared to alternative technologies.

Disadvantages of LCD

  • Slower operational speed.
  • Shorter lifespan in comparison to other technologies.
  • Limited viewing angles.

Photodiode

  • Definition: An optoelectronic device generating current from light through photoexcitation.
  • Construction: Formed by P-type and N-type semiconductors; operates under reverse bias.
  • Working: Light energy creates electron-hole pairs leading to a reverse current based on light intensity.

Uses of Photodiode

  • Useful in switching applications and demodulating optical signals.