Diodes: Lecture Notes

Introduction to Diodes

• Welcome to the YouTube channel ALL ABOUT ELECTRONICS.
• Overview of the video series:
  • What is a diode?
  • V-I characteristics of the diode.
  • Equivalent circuit for the diode.
  • Device physics and applications of diodes in subsequent videos.

What is a Diode?

• A diode is a two-terminal semiconductor device that allows current to flow in one direction only.
• Comparison with resistors:
  • Resistors allow current in both directions.
  • Diode allows current only in one direction (forward direction).
• Diode Symbol:
  • Arrow indicates the direction of current flow.
  • Anode (positive terminal) and Cathode (negative terminal).

Current Flow in Diodes

• Current flow depends on the polarity of the voltage applied:
  • Positive voltage: Allows current to flow (forward bias).
  • Negative voltage: Blocks current (reverse bias).

V-I Characteristics of Diodes

• Diodes are non-linear elements, unlike resistors which are linear (Ohm's law).
• Graph Description:
  • Symmetrical appearance but different scales for positive and negative axes.
  • Positive Y-axis: milli-amperes.
  • Negative Y-axis: micro-amperes.
  • X-axis ranges from 0.5V to -30V.
• In reverse direction, current is negligible.

Analyzing Diode Circuits

• Diode circuits can be analyzed using V-I characteristics.
• Ideal Diode Model:
  • Acts as a closed switch when forward biased (positive voltage).
  • Acts as an open switch when reverse biased (negative voltage).

Example Circuit Analysis

• Circuit with 10V and 100 ohm resistor:
  • When forward biased (10V), current = 10V / 100 ohm = 0.1A.
  • When reverse biased (-10V), current = 0A.

Threshold Voltage

• Actual diodes have a threshold voltage (Cut-in voltage) that must be surpassed to conduct:
  • Silicon Diode: Threshold voltage ~0.6V to 0.7V.
  • Germanium Diode: Threshold voltage ~0.3V.
• Current flows only after this voltage is crossed.

Approximations for Diode Characteristics

1. First Approximation:

   • Ideal diode with no resistance until threshold voltage is crossed.
   • Equivalent circuit changes based on threshold voltage.
   • Example calculation shows current as 0.093A when considering threshold (0.7V).

2. Second Approximation:

   • Introduces bulk resistance (Rb) after threshold is crossed.
   • Non-conducting until threshold and then shows finite resistance.
   • Second approximation allows calculation of more accurate current values.

Forward and Reverse Regions

• Forward Region:
  • Current increases exponentially after crossing threshold voltage.
• Reverse Region:
  • Current remains low (micro-amperes) until breakdown voltage.
• Breakdown Region:
  • Should be avoided in normal diodes; Zener diodes are designed for this region.

Key Parameters Found in Diode Datasheets

• Maximum breakdown voltage.
• Maximum allowable forward current.

Conclusion

• Summary of diode functionality and V-I characteristics discussed.
• Next video will cover diode resistance.
• Encouragement to ask questions in comments and to subscribe for more content.