Diode Lecture Notes

Introduction

• Topic of the video: Diode
• Series of topics to cover in subsequent videos including device physics and applications of diodes.

What is a Diode?

• A diode is a two-terminal semiconductor device that allows current to flow in one direction only.
• Symbol: An arrow indicates the direction of current flow.
  • Anode: Positive terminal
  • Cathode: Negative terminal

Current Flow in Diodes

• Current flow depends on the polarity of the voltage applied:
  • Positive voltage: Current flows (forward biased)
  • Negative voltage: Current is blocked (reverse biased)

V-I Characteristics of Diodes

• Diodes are non-linear elements unlike resistors which are linear.
• V-I characteristics graph:
  • Symmetric appearance but different scales on both axes (milliamperes for positive Y-axis, microamperes for negative Y-axis).
  • Current is negligible in reverse direction.

Ideal Diode Characteristics

• Ideal Model:
  • Acts as a closed switch when forward biased (positive voltage).
  • Acts as an open switch when reverse biased (negative voltage).
• V-I Characteristics:
  • Case 1: Positive voltage -> vertical line
  • Case 2: Negative voltage -> horizontal line

Forward and Reverse Bias

• Forward Bias: Diode allows current flow.
• Reverse Bias: Diode blocks current flow.

Analyzing Diode Circuits

• Example: Diode in series with a voltage source and resistor.
  • For 10V applied across an ideal diode:
    • Current through a 100-ohm resistor = 0.1A
  • For -10V applied:
    • Current = 0A (open switch).

Thevenin’s Equivalent Circuit

• To find voltage across the diode:
  • Remove the diode and calculate Thevenin's equivalent voltage.

Threshold Voltage (Cut-in Voltage)

• Actual diodes have a threshold voltage above which they start conducting:
  • Silicon: 0.6V to 0.7V
  • Germanium: Around 0.3V
• V-I Characteristics must reflect this threshold.

Second Approximation of Diode Characteristics

• Diode has finite resistance when conducting (bulk resistance).
• Characteristics:
  • Until threshold voltage, infinite resistance (open switch).
  • After threshold, finite resistance (bulk resistance) applies.

Including Diode Resistance

• Actual diode resistance can impact calculations.
• Example:
  • If diode resistance = 25 ohms and Thevenin's resistance is comparable:
    • Current through 100-ohm resistor = 10V - 0.7V / 125 ohms = 0.074A

Piecewise Linear Characteristics

• Diode characteristics segmented into linear pieces:
  • Non-conducting until threshold, then finite resistance.

Breakdown Region

• Reverse operation leads to a breakdown region which should be avoided:
  • Zener diodes allow operation in this region.
• Always check the maximum breakdown voltage in datasheets.

Forward Region

• Forward current increases exponentially as voltage increases.
• Maximum allowable forward current should not be exceeded.

Conclusion

• Understanding diodes and their V-I characteristics is crucial.
• Next video will cover diode resistance in detail.
• Questions or suggestions can be left in the comment section.