Module V: Special Purpose Diodes

This module covers theory only - numerical problems are not expected.

1. Zener Diode

1.1 Introduction

What is a Zener Diode?

A Zener diode is a specially designed silicon diode that is optimized to operate in the reverse breakdown region. Unlike regular diodes that are damaged by reverse breakdown, Zener diodes are designed to operate reliably in this region.

The key feature of a Zener diode is that it maintains a nearly constant voltage across its terminals when reverse biased, even as the current through it varies.

Zener Diode Symbol

The bent ends on the cathode bar distinguish Zener diode symbol from regular diode

1.2 Characteristics and Operation

Zener Diode Characteristics

A Zener diode has two regions of operation:

Forward Bias Region:

When forward biased, a Zener diode behaves like a normal diode. It conducts current when the forward voltage exceeds approximately 0.7V (for silicon).

Reverse Bias Region:

This is where the Zener diode is special:

Below breakdown: Very small leakage current (like a normal diode)
At breakdown voltage (V_Z): Zener breakdown occurs
After breakdown: Current increases sharply but voltage remains nearly constant at V_Z

Zener Diode V-I Characteristics

Breakdown Mechanisms

Two mechanisms cause breakdown in reverse-biased Zener diodes:

Zener Breakdown

Occurs in heavily doped diodes with narrow depletion region.

Breakdown voltage: < 5V
High electric field ruptures covalent bonds
Electrons are pulled directly from valence band
Voltage decreases slightly with temperature

Avalanche Breakdown

Occurs in lightly doped diodes with wider depletion region.

Breakdown voltage: > 5V
High-speed electrons collide with atoms
Impact ionization creates more carriers (avalanche effect)
Voltage increases slightly with temperature

Diodes rated at exactly 5V have both mechanisms occurring, resulting in nearly zero temperature coefficient - ideal for reference applications.

1.3 Zener Diode as Voltage Regulator

Voltage Regulation Application

The most important application of a Zener diode is as a voltage regulator. It provides a constant output voltage despite variations in input voltage or load current.

Zener Diode Voltage Regulator Circuit

I_s = I_Z + I_L (Kirchhoff's Current Law)

How Voltage Regulation Works

The circuit maintains constant output voltage through the following mechanism:

When Input Voltage Increases:

Current through R_s tends to increase
Extra current flows through the Zener diode (I_Z increases)
Load current I_L remains same (voltage across R_L = V_Z = constant)
Output voltage stays constant at V_Z

When Load Resistance Decreases (Load Current Increases):

Load tries to draw more current
Zener current decreases to compensate
Total current through R_s remains nearly constant
Output voltage stays constant at V_Z

Key Points for Zener Voltage Regulator

V_in must be greater than V_Z for proper operation
R_s limits current and drops excess voltage (V_in − V_Z)
Zener diode is always reverse biased in this application
Minimum Zener current (I_Z(min)) required for regulation
Maximum power rating (P_Z = V_Z × I_Z) must not be exceeded
Output voltage = V_Z (constant)

2. Light Emitting Diode (LED)

2.1 Basic Structure

What is an LED?

An LED (Light Emitting Diode) is a semiconductor device that emits light when current flows through it in the forward direction. It converts electrical energy directly into light energy through a process called electroluminescence.

LED Symbol and Structure

LED Construction

Component	Description
LED Chip (Die)	The semiconductor material (P-N junction) that emits light
Anode Lead	Longer lead, connected to P-type material (positive terminal)
Cathode Lead	Shorter lead, connected to N-type material (negative terminal)
Reflector Cup	Directs light forward, improves efficiency
Epoxy Lens	Protects the chip and focuses/diffuses light
Flat Spot	On the body near cathode (helps identify polarity)

2.2 Working Principle

How LED Emits Light

When an LED is forward biased:

Electrons from N-region cross the junction to P-region
Holes from P-region cross to N-region
At the junction, electrons and holes recombine
During recombination, electrons fall from conduction band to valence band
The energy difference is released as a photon (light)
The color of light depends on the bandgap energy of the semiconductor

Energy Band Diagram - Light Emission

LED Colors and Materials

The color of light emitted depends on the semiconductor material used:

Color	Semiconductor Material	Forward Voltage
Red	GaAsP (Gallium Arsenide Phosphide)	1.8 - 2.1 V
Orange	GaAsP	2.0 - 2.2 V
Yellow	GaAsP / GaP	2.1 - 2.4 V
Green	GaP (Gallium Phosphide)	2.0 - 2.4 V
Blue	GaN (Gallium Nitride) / InGaN	3.0 - 3.5 V
White	Blue LED + Yellow phosphor	3.0 - 3.5 V

2.3 Applications of LED

Indicative Applications

LEDs used as visual indicators:

Power Indicators: On/off status of electronic devices
Status Indicators: Battery level, charging status, Wi-Fi connection
Panel Displays: Equipment status, control panels
Seven-Segment Displays: Digital clocks, calculators, meters
Traffic Signals: Red, yellow, green lights
Automotive: Brake lights, turn signals, dashboard indicators
Remote Controls: Infrared LEDs for TV/AC remotes

Lighting Applications

LEDs used for illumination:

General Lighting: LED bulbs, tube lights replacing incandescent/fluorescent
Street Lighting: Energy-efficient street lamps
Architectural Lighting: Decorative and accent lighting
Display Backlighting: TV, monitor, smartphone screens
Automotive Headlights: High-brightness LED headlamps
Flashlights/Torches: Portable LED lighting
Stage/Entertainment: Concert lighting, video walls

Advantages of LEDs

High Efficiency: Convert more electrical energy to light (80-90% efficient)
Long Life: 25,000 - 50,000 hours (vs 1,000 hours for incandescent)
Low Power Consumption: 80% less power than incandescent bulbs
Instant On: No warm-up time required
Compact Size: Small form factor allows flexible designs
Durability: No filament or glass envelope to break
Low Heat: Little heat generation compared to incandescent
Environmentally Friendly: No mercury, fully recyclable
Color Options: Available in many colors without filters

LED Display Types

Summary: Special Purpose Diodes

Feature	Zener Diode	LED
Operation	Reverse bias (breakdown region)	Forward bias
Key Property	Constant voltage at breakdown	Light emission
Main Application	Voltage regulation/reference	Indication and illumination
Material	Silicon	GaAs, GaP, GaN (compound semiconductors)
Symbol Feature	Bent cathode bar	Light arrows