Module V: Special Purpose Diodes - Solved Questions

Characteristics

Forward Bias: Same as normal diode, conducts at ~0.7V

Reverse Bias (before V_Z): Very small leakage current

Breakdown (after V_Z): Current increases sharply, voltage remains constant at V_Z

Breakdown Mechanisms

Zener Breakdown	Avalanche Breakdown
VZ < 5V	VZ > 7V
High electric field	Impact ionization
Heavily doped	Lightly doped
-ve temp coefficient	+ve temp coefficient

Key: Designed to operate in reverse breakdown, maintains constant V_Z

Circuit

V_in → R_s (series resistor) → Zener (parallel with load R_L) → Ground

Working

• When V_in > V_Z, Zener operates in breakdown
• Output voltage = V_Z (constant)
• If V_in increases → excess dropped across R_s, I_Z increases
• If V_in decreases → I_Z decreases, output stays at V_Z
• If load changes → Zener adjusts its current to maintain V_Z

Equations

V_out = V_Z

I_s = (V_in - V_Z)/R_s

I_s = I_Z + I_L

Applications

Reference voltage, low power regulators, overvoltage protection, clipping circuits

Limitations: Low efficiency, limited current capacity, fixed output

Definition

LED = Light Emitting Diode. Converts electrical energy to light via electroluminescence.

Construction

• LED Chip: P-N junction where light is generated
• Epoxy Lens: Protects chip, focuses light
• Reflective Cavity: Reflects light upward
• Leads: Anode (+, longer) and Cathode (-, shorter with flat)

Working

Forward bias → electrons cross junction → recombine with holes → energy released as photons (light)

Materials & Colors

Color	Material
Red	GaAsP, AlGaAs
Green	GaP, AlGaInP
Blue	GaN, InGaN
White	Blue LED + Phosphor

Applications

• Indicators: Power ON, status displays
• Displays: 7-segment, dot matrix, digital clocks
• Lighting: Bulbs, street lights, automotive
• Backlighting: LCD TVs, monitors, mobiles
• Communication: Optical fiber, remotes, Li-Fi

Advantages: High efficiency, long life (50,000+ hrs), fast switching, compact, no mercury